| Message ID | 20240205110959.4021-1-ryncsn@gmail.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | mm/swap: fix race condition in direct swapin path | expand |
On Mon, Feb 5, 2024 at 7:10 PM Kairui Song <ryncsn@gmail.com> wrote: > > From: Kairui Song <kasong@tencent.com> > > In the direct swapin path, when two or more threads swapin the same entry > at the same time, they get different pages (A, B) because swap cache is > skipped. Before one thread (T0) finishes the swapin and installs page (A) > to the PTE, another thread (T1) could finish swapin of page (B), > swap_free the entry, then modify and swap-out the page again, using the Even if T0's swap_read_folio is later than T1, problems can still happen. after T1 swaps in and sets ptes, then frees the swap entry. T0 reads zRAM later. it will get zero as zRAM will fill zero for freed slot, static int zram_read_from_zspool(struct zram *zram, struct page *page, u32 index) { ... value = handle ? zram_get_element(zram, index) : 0; mem = kmap_local_page(page); zram_fill_page(mem, PAGE_SIZE, value); kunmap_local(mem); return 0; } } Even though nobody modifies the data before the page is swapped out to the same swap offset as before tT0's orig_pte, T0's pte_same check is still true and T0 will map filled zeroed page to pte. so there is more than one risk besides modified data losses. > same entry. It break the pte_same check because PTE value is unchanged, > causing ABA problem. Then thread (T0) will then install the stalled page > (A) into the PTE so new data in page (B) is lost, one possible callstack > is like this: > > CPU0 CPU1 > ---- ---- > do_swap_page() do_swap_page() with same entry > <direct swapin path> <direct swapin path> > <alloc page A> <alloc page B> > swap_readpage() <- read to page A swap_readpage() <- read to page B > <slow on later locks or interrupt> <finished swapin first> > .. set_pte_at() > swap_free() <- Now the entry is freed. > <write to page B, now page A stalled> > <swap out page B using same swap entry> > pte_same() <- Check pass, PTE seems > unchanged, but page A > is stalled! > swap_free() <- page B content lost! > set_pte_at() <- staled page A installed! > > To fix this, reuse swapcache_prepare which will pin the swap entry using > the cache flag, and allow only one thread to pin it. Release the pin > after PT unlocked. Racers will simply busy wait since it's a rare > and very short event. > > Other methods like increasing the swap count don't seem to be a good > idea after some tests, that will cause racers to fall back to the > cached swapin path, two swapin path being used at the same time > leads to a much more complex scenario. > > Reproducer: > > This race issue can be triggered easily using a well constructed > reproducer and patched brd (with a delay in read path) [1]: > > With latest 6.8 mainline, race caused data loss can be observed easily: > $ gcc -g -lpthread test-thread-swap-race.c && ./a.out > Polulating 32MB of memory region... > Keep swapping out... > Starting round 0... > Spawning 65536 workers... > 32746 workers spawned, wait for done... > Round 0: Error on 0x5aa00, expected 32746, got 32743, 3 data loss! > Round 0: Error on 0x395200, expected 32746, got 32743, 3 data loss! > Round 0: Error on 0x3fd000, expected 32746, got 32737, 9 data loss! > Round 0 Failed, 15 data loss! i am also reading these codes recently. It is quite unbelievable this is really happening now. as freeing swaps is returning slot to slots_ret, but allocating swap is from slots. so if swapfile is large, the chance that the newly allocated swap was a recently freed swap is close to 0%. but yes, the code does have the risk. > > This reproducer spawns multiple threads sharing the same memory region > using a small swap device. Every two threads updates mapped pages one by > one in opposite direction trying to create a race, with one dedicated > thread keep swapping out the data out using madvise. > > The reproducer created a reproduce rate of about once every 5 minutes, > so the race should be totally possible in production. > > After this patch, I ran the reproducer for over a few hundred rounds > and no data loss observed. > > Performance overhead is minimal, microbenchmark swapin 10G from 32G > zram: > > Before: 10934698 us > After: 11157121 us > Non-direct: 13155355 us (Dropping SWP_SYNCHRONOUS_IO flag) > > Fixes: 0bcac06f27d7 ("mm, swap: skip swapcache for swapin of synchronous device") > Link: https://github.com/ryncsn/emm-test-project/tree/master/swap-stress-race [1] > Signed-off-by: Kairui Song <kasong@tencent.com> I will also run your patch on my problem I reported today[1]. will update the result to you this week. [1] https://lore.kernel.org/linux-mm/d4f602db-403b-4b1f-a3de-affeb40bc499@arm.com/T/#m41701d0c0e127cdae636e97a13ab521364a810f4 > --- > Huge thanks to Huang Ying and Chris Li for help finding this issue! > > mm/memory.c | 19 +++++++++++++++++++ > mm/swap.h | 5 +++++ > mm/swapfile.c | 16 ++++++++++++++++ > 3 files changed, 40 insertions(+) > > diff --git a/mm/memory.c b/mm/memory.c > index 7e1f4849463a..fd7c55a292f1 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -3867,6 +3867,20 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) > if (!folio) { > if (data_race(si->flags & SWP_SYNCHRONOUS_IO) && > __swap_count(entry) == 1) { > + /* > + * With swap count == 1, after we read the entry, > + * other threads could finish swapin first, free > + * the entry, then swapout the modified page using > + * the same entry. Now the content we just read is > + * stalled, and it's undetectable as pte_same() > + * returns true due to entry reuse. > + * > + * So pin the swap entry using the cache flag even > + * cache is not used. > + */ > + if (swapcache_prepare(entry)) > + goto out; > + > /* skip swapcache */ > folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0, > vma, vmf->address, false); > @@ -4116,6 +4130,9 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) > unlock: > if (vmf->pte) > pte_unmap_unlock(vmf->pte, vmf->ptl); > + /* Clear the swap cache pin for direct swapin after PTL unlock */ > + if (folio && !swapcache) > + swapcache_clear(si, entry); > out: > if (si) > put_swap_device(si); > @@ -4124,6 +4141,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) > if (vmf->pte) > pte_unmap_unlock(vmf->pte, vmf->ptl); > out_page: > + if (!swapcache) > + swapcache_clear(si, entry); > folio_unlock(folio); > out_release: > folio_put(folio); > diff --git a/mm/swap.h b/mm/swap.h > index 758c46ca671e..fc2f6ade7f80 100644 > --- a/mm/swap.h > +++ b/mm/swap.h > @@ -41,6 +41,7 @@ void __delete_from_swap_cache(struct folio *folio, > void delete_from_swap_cache(struct folio *folio); > void clear_shadow_from_swap_cache(int type, unsigned long begin, > unsigned long end); > +void swapcache_clear(struct swap_info_struct *si, swp_entry_t entry); > struct folio *swap_cache_get_folio(swp_entry_t entry, > struct vm_area_struct *vma, unsigned long addr); > struct folio *filemap_get_incore_folio(struct address_space *mapping, > @@ -97,6 +98,10 @@ static inline int swap_writepage(struct page *p, struct writeback_control *wbc) > return 0; > } > > +static inline void swapcache_clear(struct swap_info_struct *si, swp_entry_t entry) > +{ > +} > + > static inline struct folio *swap_cache_get_folio(swp_entry_t entry, > struct vm_area_struct *vma, unsigned long addr) > { > diff --git a/mm/swapfile.c b/mm/swapfile.c > index 556ff7347d5f..f7d4ad152a7f 100644 > --- a/mm/swapfile.c > +++ b/mm/swapfile.c > @@ -3365,6 +3365,22 @@ int swapcache_prepare(swp_entry_t entry) > return __swap_duplicate(entry, SWAP_HAS_CACHE); > } > > +/* > + * Clear the cache flag and release pinned entry. > + */ > +void swapcache_clear(struct swap_info_struct *si, swp_entry_t entry) > +{ > + struct swap_cluster_info *ci; > + unsigned long offset = swp_offset(entry); > + unsigned char usage; > + > + ci = lock_cluster_or_swap_info(si, offset); > + usage = __swap_entry_free_locked(si, offset, SWAP_HAS_CACHE); > + unlock_cluster_or_swap_info(si, ci); > + if (!usage) > + free_swap_slot(entry); > +} > + > struct swap_info_struct *swp_swap_info(swp_entry_t entry) > { > return swap_type_to_swap_info(swp_type(entry)); > -- > 2.43.0 > > Thanks Barry
On Mon, Feb 5, 2024 at 8:25 PM Barry Song <21cnbao@gmail.com> wrote: > Hi, Barry Thanks for the comments. > On Mon, Feb 5, 2024 at 7:10 PM Kairui Song <ryncsn@gmail.com> wrote: > > > > From: Kairui Song <kasong@tencent.com> > > > > In the direct swapin path, when two or more threads swapin the same entry > > at the same time, they get different pages (A, B) because swap cache is > > skipped. Before one thread (T0) finishes the swapin and installs page (A) > > to the PTE, another thread (T1) could finish swapin of page (B), > > swap_free the entry, then modify and swap-out the page again, using the > > Even if T0's swap_read_folio is later than T1, problems can still happen. > after T1 swaps in and sets ptes, then frees the swap entry. T0 reads zRAM > later. it will get zero as zRAM will fill zero for freed slot, > > static int zram_read_from_zspool(struct zram *zram, struct page *page, > u32 index) > { > ... > > > value = handle ? zram_get_element(zram, index) : 0; > mem = kmap_local_page(page); > zram_fill_page(mem, PAGE_SIZE, value); > kunmap_local(mem); > return 0; > } > } > > Even though nobody modifies the data before the page is swapped out to the > same swap offset as before tT0's orig_pte, T0's pte_same check is still true > and T0 will map filled zeroed page to pte. > > so there is more than one risk besides modified data losses. Thanks for the complement, I think this is true, and it shares the same problem of the entry reuse, so this patch also covered this potential race. I can add more words later to cover this case as well. > > > same entry. It break the pte_same check because PTE value is unchanged, > > causing ABA problem. Then thread (T0) will then install the stalled page > > (A) into the PTE so new data in page (B) is lost, one possible callstack > > is like this: > > > > CPU0 CPU1 > > ---- ---- > > do_swap_page() do_swap_page() with same entry > > <direct swapin path> <direct swapin path> > > <alloc page A> <alloc page B> > > swap_readpage() <- read to page A swap_readpage() <- read to page B > > <slow on later locks or interrupt> <finished swapin first> > > .. set_pte_at() > > swap_free() <- Now the entry is freed. > > <write to page B, now page A stalled> > > <swap out page B using same swap entry> > > pte_same() <- Check pass, PTE seems > > unchanged, but page A > > is stalled! > > swap_free() <- page B content lost! > > set_pte_at() <- staled page A installed! > > > > To fix this, reuse swapcache_prepare which will pin the swap entry using > > the cache flag, and allow only one thread to pin it. Release the pin > > after PT unlocked. Racers will simply busy wait since it's a rare > > and very short event. > > > > Other methods like increasing the swap count don't seem to be a good > > idea after some tests, that will cause racers to fall back to the > > cached swapin path, two swapin path being used at the same time > > leads to a much more complex scenario. > > > > Reproducer: > > > > This race issue can be triggered easily using a well constructed > > reproducer and patched brd (with a delay in read path) [1]: > > > > With latest 6.8 mainline, race caused data loss can be observed easily: > > $ gcc -g -lpthread test-thread-swap-race.c && ./a.out > > Polulating 32MB of memory region... > > Keep swapping out... > > Starting round 0... > > Spawning 65536 workers... > > 32746 workers spawned, wait for done... > > Round 0: Error on 0x5aa00, expected 32746, got 32743, 3 data loss! > > Round 0: Error on 0x395200, expected 32746, got 32743, 3 data loss! > > Round 0: Error on 0x3fd000, expected 32746, got 32737, 9 data loss! > > Round 0 Failed, 15 data loss! > > i am also reading these codes recently. It is quite unbelievable this > is really happening > now. as freeing swaps is returning slot to slots_ret, but allocating > swap is from slots. > so if swapfile is large, the chance that the newly allocated swap was > a recently freed swap > is close to 0%. but yes, the code does have the risk. Indeed, for reproducing I used a 32M swap device, and the data being swapped in/out is large enough to make full use of it. So the reproduce rate is increased by a lot. It's not a completely fictional test as some low end device do have smaller swaps, and real world race could happen in many strange ways. > > > > This reproducer spawns multiple threads sharing the same memory region > > using a small swap device. Every two threads updates mapped pages one by > > one in opposite direction trying to create a race, with one dedicated > > thread keep swapping out the data out using madvise. > > > > The reproducer created a reproduce rate of about once every 5 minutes, > > so the race should be totally possible in production. > > > > After this patch, I ran the reproducer for over a few hundred rounds > > and no data loss observed. > > > > Performance overhead is minimal, microbenchmark swapin 10G from 32G > > zram: > > > > Before: 10934698 us > > After: 11157121 us > > Non-direct: 13155355 us (Dropping SWP_SYNCHRONOUS_IO flag) > > > > Fixes: 0bcac06f27d7 ("mm, swap: skip swapcache for swapin of synchronous device") > > Link: https://github.com/ryncsn/emm-test-project/tree/master/swap-stress-race [1] > > Signed-off-by: Kairui Song <kasong@tencent.com> > > I will also run your patch on my problem I reported today[1]. will update > the result to you this week. > > [1] https://lore.kernel.org/linux-mm/d4f602db-403b-4b1f-a3de-affeb40bc499@arm.com/T/#m41701d0c0e127cdae636e97a13ab521364a810f4 > Thanks!
Kairui Song <ryncsn@gmail.com> writes: > From: Kairui Song <kasong@tencent.com> > > In the direct swapin path, when two or more threads swapin the same entry > at the same time, they get different pages (A, B) because swap cache is > skipped. Before one thread (T0) finishes the swapin and installs page (A) > to the PTE, another thread (T1) could finish swapin of page (B), > swap_free the entry, then modify and swap-out the page again, using the > same entry. It break the pte_same check because PTE value is unchanged, > causing ABA problem. Then thread (T0) will then install the stalled page > (A) into the PTE so new data in page (B) is lost, one possible callstack > is like this: > > CPU0 CPU1 > ---- ---- > do_swap_page() do_swap_page() with same entry > <direct swapin path> <direct swapin path> > <alloc page A> <alloc page B> > swap_readpage() <- read to page A swap_readpage() <- read to page B > <slow on later locks or interrupt> <finished swapin first> > ... set_pte_at() > swap_free() <- Now the entry is freed. > <write to page B, now page A stalled> > <swap out page B using same swap entry> > pte_same() <- Check pass, PTE seems > unchanged, but page A > is stalled! > swap_free() <- page B content lost! > set_pte_at() <- staled page A installed! > > To fix this, reuse swapcache_prepare which will pin the swap entry using > the cache flag, and allow only one thread to pin it. Release the pin > after PT unlocked. Racers will simply busy wait since it's a rare > and very short event. > > Other methods like increasing the swap count don't seem to be a good > idea after some tests, that will cause racers to fall back to the > cached swapin path, two swapin path being used at the same time > leads to a much more complex scenario. > > Reproducer: > > This race issue can be triggered easily using a well constructed > reproducer and patched brd (with a delay in read path) [1]: > > With latest 6.8 mainline, race caused data loss can be observed easily: > $ gcc -g -lpthread test-thread-swap-race.c && ./a.out > Polulating 32MB of memory region... > Keep swapping out... > Starting round 0... > Spawning 65536 workers... > 32746 workers spawned, wait for done... > Round 0: Error on 0x5aa00, expected 32746, got 32743, 3 data loss! > Round 0: Error on 0x395200, expected 32746, got 32743, 3 data loss! > Round 0: Error on 0x3fd000, expected 32746, got 32737, 9 data loss! > Round 0 Failed, 15 data loss! > > This reproducer spawns multiple threads sharing the same memory region > using a small swap device. Every two threads updates mapped pages one by > one in opposite direction trying to create a race, with one dedicated > thread keep swapping out the data out using madvise. > > The reproducer created a reproduce rate of about once every 5 minutes, > so the race should be totally possible in production. > > After this patch, I ran the reproducer for over a few hundred rounds > and no data loss observed. > > Performance overhead is minimal, microbenchmark swapin 10G from 32G > zram: > > Before: 10934698 us > After: 11157121 us > Non-direct: 13155355 us (Dropping SWP_SYNCHRONOUS_IO flag) > > Fixes: 0bcac06f27d7 ("mm, swap: skip swapcache for swapin of synchronous device") > Link: https://github.com/ryncsn/emm-test-project/tree/master/swap-stress-race [1] > Signed-off-by: Kairui Song <kasong@tencent.com> Reported-by: "Huang, Ying" <ying.huang@intel.com> > --- > Huge thanks to Huang Ying and Chris Li for help finding this issue! > > mm/memory.c | 19 +++++++++++++++++++ > mm/swap.h | 5 +++++ > mm/swapfile.c | 16 ++++++++++++++++ > 3 files changed, 40 insertions(+) > > diff --git a/mm/memory.c b/mm/memory.c > index 7e1f4849463a..fd7c55a292f1 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -3867,6 +3867,20 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) > if (!folio) { > if (data_race(si->flags & SWP_SYNCHRONOUS_IO) && > __swap_count(entry) == 1) { > + /* > + * With swap count == 1, after we read the entry, > + * other threads could finish swapin first, free > + * the entry, then swapout the modified page using > + * the same entry. Now the content we just read is > + * stalled, and it's undetectable as pte_same() > + * returns true due to entry reuse. > + * > + * So pin the swap entry using the cache flag even "pin" doesn't sound intuitive here. I know that the swap entry will not be freed with this. But now, the parallel swap in will busy waiting. So, I suggest to say, Prevent parallel swapin from proceeding with the cache flag. Otherwise, it may swapin first, free the entry, then swapout the modified page using the same entry ... > + * cache is not used. > + */ > + if (swapcache_prepare(entry)) > + goto out; > + > /* skip swapcache */ > folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0, > vma, vmf->address, false); > @@ -4116,6 +4130,9 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) > unlock: > if (vmf->pte) > pte_unmap_unlock(vmf->pte, vmf->ptl); > + /* Clear the swap cache pin for direct swapin after PTL unlock */ > + if (folio && !swapcache) > + swapcache_clear(si, entry); > out: > if (si) > put_swap_device(si); > @@ -4124,6 +4141,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) > if (vmf->pte) > pte_unmap_unlock(vmf->pte, vmf->ptl); > out_page: > + if (!swapcache) > + swapcache_clear(si, entry); > folio_unlock(folio); > out_release: > folio_put(folio); > diff --git a/mm/swap.h b/mm/swap.h > index 758c46ca671e..fc2f6ade7f80 100644 > --- a/mm/swap.h > +++ b/mm/swap.h > @@ -41,6 +41,7 @@ void __delete_from_swap_cache(struct folio *folio, > void delete_from_swap_cache(struct folio *folio); > void clear_shadow_from_swap_cache(int type, unsigned long begin, > unsigned long end); > +void swapcache_clear(struct swap_info_struct *si, swp_entry_t entry); > struct folio *swap_cache_get_folio(swp_entry_t entry, > struct vm_area_struct *vma, unsigned long addr); > struct folio *filemap_get_incore_folio(struct address_space *mapping, > @@ -97,6 +98,10 @@ static inline int swap_writepage(struct page *p, struct writeback_control *wbc) > return 0; > } > > +static inline void swapcache_clear(struct swap_info_struct *si, swp_entry_t entry) > +{ > +} > + > static inline struct folio *swap_cache_get_folio(swp_entry_t entry, > struct vm_area_struct *vma, unsigned long addr) > { > diff --git a/mm/swapfile.c b/mm/swapfile.c > index 556ff7347d5f..f7d4ad152a7f 100644 > --- a/mm/swapfile.c > +++ b/mm/swapfile.c > @@ -3365,6 +3365,22 @@ int swapcache_prepare(swp_entry_t entry) > return __swap_duplicate(entry, SWAP_HAS_CACHE); > } > > +/* > + * Clear the cache flag and release pinned entry. Even if we will keep "pin" in above comments, this is hard to be understood for reader. Need a little more explanation like "release pinned entry for device with SWP_SYNCHRONOUS_IO. Or, just remove the comments. We have comments in calling site already. > + */ > +void swapcache_clear(struct swap_info_struct *si, swp_entry_t entry) > +{ > + struct swap_cluster_info *ci; > + unsigned long offset = swp_offset(entry); > + unsigned char usage; > + > + ci = lock_cluster_or_swap_info(si, offset); > + usage = __swap_entry_free_locked(si, offset, SWAP_HAS_CACHE); > + unlock_cluster_or_swap_info(si, ci); > + if (!usage) > + free_swap_slot(entry); > +} > + > struct swap_info_struct *swp_swap_info(swp_entry_t entry) > { > return swap_type_to_swap_info(swp_type(entry)); Otherwise it looks good for me, Thanks! Reviewed-by: "Huang, Ying" <ying.huang@intel.com>
Hi Kairui, On Mon, 5 Feb 2024 19:09:59 +0800 Kairui Song <ryncsn@gmail.com> wrote: [...] > mm/memory.c | 19 +++++++++++++++++++ > mm/swap.h | 5 +++++ > mm/swapfile.c | 16 ++++++++++++++++ > 3 files changed, 40 insertions(+) > > diff --git a/mm/memory.c b/mm/memory.c > index 7e1f4849463a..fd7c55a292f1 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -3867,6 +3867,20 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) > if (!folio) { > if (data_race(si->flags & SWP_SYNCHRONOUS_IO) && > __swap_count(entry) == 1) { > + /* > + * With swap count == 1, after we read the entry, > + * other threads could finish swapin first, free > + * the entry, then swapout the modified page using > + * the same entry. Now the content we just read is > + * stalled, and it's undetectable as pte_same() > + * returns true due to entry reuse. > + * > + * So pin the swap entry using the cache flag even > + * cache is not used. > + */ > + if (swapcache_prepare(entry)) > + goto out; > + I'm getting below build error after this patch. I guess maybe the code need to take care of CONFIG_SWAP unset case? .../mm/memory.c: In function 'do_swap_page': .../mm/memory.c:4004:8: error: implicit declaration of function 'swapcache_prepare'; did you mean 'swapcache_clear'? [-Werror=implicit-function-declaration] 4004 | if (swapcache_prepare(entry)) | ^~~~~~~~~~~~~~~~~ | swapcache_clear Thanks, SJ [...]
On Tue, Feb 6, 2024 at 10:24 AM SeongJae Park <sj@kernel.org> wrote: > > Hi Kairui, > > On Mon, 5 Feb 2024 19:09:59 +0800 Kairui Song <ryncsn@gmail.com> wrote: > > [...] > > mm/memory.c | 19 +++++++++++++++++++ > > mm/swap.h | 5 +++++ > > mm/swapfile.c | 16 ++++++++++++++++ > > 3 files changed, 40 insertions(+) > > > > diff --git a/mm/memory.c b/mm/memory.c > > index 7e1f4849463a..fd7c55a292f1 100644 > > --- a/mm/memory.c > > +++ b/mm/memory.c > > @@ -3867,6 +3867,20 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) > > if (!folio) { > > if (data_race(si->flags & SWP_SYNCHRONOUS_IO) && > > __swap_count(entry) == 1) { > > + /* > > + * With swap count == 1, after we read the entry, > > + * other threads could finish swapin first, free > > + * the entry, then swapout the modified page using > > + * the same entry. Now the content we just read is > > + * stalled, and it's undetectable as pte_same() > > + * returns true due to entry reuse. > > + * > > + * So pin the swap entry using the cache flag even > > + * cache is not used. > > + */ > > + if (swapcache_prepare(entry)) > > + goto out; > > + > > I'm getting below build error after this patch. I guess maybe the code need to > take care of CONFIG_SWAP unset case? > > .../mm/memory.c: In function 'do_swap_page': > .../mm/memory.c:4004:8: error: implicit declaration of function 'swapcache_prepare'; did you mean 'swapcache_clear'? [-Werror=implicit-function-declaration] > 4004 | if (swapcache_prepare(entry)) > | ^~~~~~~~~~~~~~~~~ > | swapcache_clear > > Ah, right. Thanks for the feedback. For the CONFIG_SWAP unset case I added an empty function for swapcache_clear, but the original swapcache_prepare also needs an empty placeholder now. Will add that in V2.
On Tue, Feb 6, 2024 at 9:35 AM Huang, Ying <ying.huang@intel.com> wrote: > > Kairui Song <ryncsn@gmail.com> writes: > > > From: Kairui Song <kasong@tencent.com> > > > > In the direct swapin path, when two or more threads swapin the same entry > > at the same time, they get different pages (A, B) because swap cache is > > skipped. Before one thread (T0) finishes the swapin and installs page (A) > > to the PTE, another thread (T1) could finish swapin of page (B), > > swap_free the entry, then modify and swap-out the page again, using the > > same entry. It break the pte_same check because PTE value is unchanged, > > causing ABA problem. Then thread (T0) will then install the stalled page > > (A) into the PTE so new data in page (B) is lost, one possible callstack > > is like this: > > > > CPU0 CPU1 > > ---- ---- > > do_swap_page() do_swap_page() with same entry > > <direct swapin path> <direct swapin path> > > <alloc page A> <alloc page B> > > swap_readpage() <- read to page A swap_readpage() <- read to page B > > <slow on later locks or interrupt> <finished swapin first> > > ... set_pte_at() > > swap_free() <- Now the entry is freed. > > <write to page B, now page A stalled> > > <swap out page B using same swap entry> > > pte_same() <- Check pass, PTE seems > > unchanged, but page A > > is stalled! > > swap_free() <- page B content lost! > > set_pte_at() <- staled page A installed! > > > > To fix this, reuse swapcache_prepare which will pin the swap entry using > > the cache flag, and allow only one thread to pin it. Release the pin > > after PT unlocked. Racers will simply busy wait since it's a rare > > and very short event. > > > > Other methods like increasing the swap count don't seem to be a good > > idea after some tests, that will cause racers to fall back to the > > cached swapin path, two swapin path being used at the same time > > leads to a much more complex scenario. > > > > Reproducer: > > > > This race issue can be triggered easily using a well constructed > > reproducer and patched brd (with a delay in read path) [1]: > > > > With latest 6.8 mainline, race caused data loss can be observed easily: > > $ gcc -g -lpthread test-thread-swap-race.c && ./a.out > > Polulating 32MB of memory region... > > Keep swapping out... > > Starting round 0... > > Spawning 65536 workers... > > 32746 workers spawned, wait for done... > > Round 0: Error on 0x5aa00, expected 32746, got 32743, 3 data loss! > > Round 0: Error on 0x395200, expected 32746, got 32743, 3 data loss! > > Round 0: Error on 0x3fd000, expected 32746, got 32737, 9 data loss! > > Round 0 Failed, 15 data loss! > > > > This reproducer spawns multiple threads sharing the same memory region > > using a small swap device. Every two threads updates mapped pages one by > > one in opposite direction trying to create a race, with one dedicated > > thread keep swapping out the data out using madvise. > > > > The reproducer created a reproduce rate of about once every 5 minutes, > > so the race should be totally possible in production. > > > > After this patch, I ran the reproducer for over a few hundred rounds > > and no data loss observed. > > > > Performance overhead is minimal, microbenchmark swapin 10G from 32G > > zram: > > > > Before: 10934698 us > > After: 11157121 us > > Non-direct: 13155355 us (Dropping SWP_SYNCHRONOUS_IO flag) > > > > Fixes: 0bcac06f27d7 ("mm, swap: skip swapcache for swapin of synchronous device") > > Link: https://github.com/ryncsn/emm-test-project/tree/master/swap-stress-race [1] > > Signed-off-by: Kairui Song <kasong@tencent.com> > > Reported-by: "Huang, Ying" <ying.huang@intel.com> Of course :), wasn't sure about how to add your credit, will add this to V2. > > --- > > Huge thanks to Huang Ying and Chris Li for help finding this issue! > > > > mm/memory.c | 19 +++++++++++++++++++ > > mm/swap.h | 5 +++++ > > mm/swapfile.c | 16 ++++++++++++++++ > > 3 files changed, 40 insertions(+) > > > > diff --git a/mm/memory.c b/mm/memory.c > > index 7e1f4849463a..fd7c55a292f1 100644 > > --- a/mm/memory.c > > +++ b/mm/memory.c > > @@ -3867,6 +3867,20 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) > > if (!folio) { > > if (data_race(si->flags & SWP_SYNCHRONOUS_IO) && > > __swap_count(entry) == 1) { > > + /* > > + * With swap count == 1, after we read the entry, > > + * other threads could finish swapin first, free > > + * the entry, then swapout the modified page using > > + * the same entry. Now the content we just read is > > + * stalled, and it's undetectable as pte_same() > > + * returns true due to entry reuse. > > + * > > + * So pin the swap entry using the cache flag even > > "pin" doesn't sound intuitive here. I know that the swap entry will not > be freed with this. But now, the parallel swap in will busy waiting. > So, I suggest to say, > > Prevent parallel swapin from proceeding with the cache flag. Otherwise, > it may swapin first, free the entry, then swapout the modified page > using the same entry ... Good suggestion. > > > + * cache is not used. > > + */ > > + if (swapcache_prepare(entry)) > > + goto out; > > + > > /* skip swapcache */ > > folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0, > > vma, vmf->address, false); > > @@ -4116,6 +4130,9 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) > > unlock: > > if (vmf->pte) > > pte_unmap_unlock(vmf->pte, vmf->ptl); > > + /* Clear the swap cache pin for direct swapin after PTL unlock */ > > + if (folio && !swapcache) > > + swapcache_clear(si, entry); > > out: > > if (si) > > put_swap_device(si); > > @@ -4124,6 +4141,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) > > if (vmf->pte) > > pte_unmap_unlock(vmf->pte, vmf->ptl); > > out_page: > > + if (!swapcache) > > + swapcache_clear(si, entry); > > folio_unlock(folio); > > out_release: > > folio_put(folio); > > diff --git a/mm/swap.h b/mm/swap.h > > index 758c46ca671e..fc2f6ade7f80 100644 > > --- a/mm/swap.h > > +++ b/mm/swap.h > > @@ -41,6 +41,7 @@ void __delete_from_swap_cache(struct folio *folio, > > void delete_from_swap_cache(struct folio *folio); > > void clear_shadow_from_swap_cache(int type, unsigned long begin, > > unsigned long end); > > +void swapcache_clear(struct swap_info_struct *si, swp_entry_t entry); > > struct folio *swap_cache_get_folio(swp_entry_t entry, > > struct vm_area_struct *vma, unsigned long addr); > > struct folio *filemap_get_incore_folio(struct address_space *mapping, > > @@ -97,6 +98,10 @@ static inline int swap_writepage(struct page *p, struct writeback_control *wbc) > > return 0; > > } > > > > +static inline void swapcache_clear(struct swap_info_struct *si, swp_entry_t entry) > > +{ > > +} > > + > > static inline struct folio *swap_cache_get_folio(swp_entry_t entry, > > struct vm_area_struct *vma, unsigned long addr) > > { > > diff --git a/mm/swapfile.c b/mm/swapfile.c > > index 556ff7347d5f..f7d4ad152a7f 100644 > > --- a/mm/swapfile.c > > +++ b/mm/swapfile.c > > @@ -3365,6 +3365,22 @@ int swapcache_prepare(swp_entry_t entry) > > return __swap_duplicate(entry, SWAP_HAS_CACHE); > > } > > > > +/* > > + * Clear the cache flag and release pinned entry. > > Even if we will keep "pin" in above comments, this is hard to be > understood for reader. Need a little more explanation like "release > pinned entry for device with SWP_SYNCHRONOUS_IO. > > Or, just remove the comments. We have comments in calling site already. Then I prefer to remove this, there is only one caller, it should be easy to understand. > > + */ > > +void swapcache_clear(struct swap_info_struct *si, swp_entry_t entry) > > +{ > > + struct swap_cluster_info *ci; > > + unsigned long offset = swp_offset(entry); > > + unsigned char usage; > > + > > + ci = lock_cluster_or_swap_info(si, offset); > > + usage = __swap_entry_free_locked(si, offset, SWAP_HAS_CACHE); > > + unlock_cluster_or_swap_info(si, ci); > > + if (!usage) > > + free_swap_slot(entry); > > +} > > + > > struct swap_info_struct *swp_swap_info(swp_entry_t entry) > > { > > return swap_type_to_swap_info(swp_type(entry)); > > Otherwise it looks good for me, Thanks! > > Reviewed-by: "Huang, Ying" <ying.huang@intel.com> Thanks for the review.
On Mon, Feb 5, 2024 at 4:10 AM Kairui Song <ryncsn@gmail.com> wrote: > > From: Kairui Song <kasong@tencent.com> > > In the direct swapin path, when two or more threads swapin the same entry There is no other places referring to that path as "direct" swapin. I'd rephrase it as: "When skipping swapcache for SWP_SYNCHRONOUS_IO, ...", and similarly for the subject: "mm: fix race when skipping swapcache". > at the same time, they get different pages (A, B) because swap cache is > skipped. Before one thread (T0) finishes the swapin and installs page (A) > to the PTE, another thread (T1) could finish swapin of page (B), > swap_free the entry, then modify and swap-out the page again, using the > same entry. It break the pte_same check because PTE value is unchanged, > causing ABA problem. Then thread (T0) will then install the stalled page > (A) into the PTE so new data in page (B) is lost, one possible callstack > is like this: > > CPU0 CPU1 > ---- ---- > do_swap_page() do_swap_page() with same entry > <direct swapin path> <direct swapin path> > <alloc page A> <alloc page B> > swap_readpage() <- read to page A swap_readpage() <- read to page B > <slow on later locks or interrupt> <finished swapin first> > ... set_pte_at() > swap_free() <- Now the entry is freed. > <write to page B, now page A stalled> > <swap out page B using same swap entry> > pte_same() <- Check pass, PTE seems > unchanged, but page A > is stalled! > swap_free() <- page B content lost! > set_pte_at() <- staled page A installed! > > To fix this, reuse swapcache_prepare which will pin the swap entry using > the cache flag, and allow only one thread to pin it. Release the pin > after PT unlocked. Racers will simply busy wait since it's a rare > and very short event. > > Other methods like increasing the swap count don't seem to be a good > idea after some tests, that will cause racers to fall back to the > cached swapin path, two swapin path being used at the same time > leads to a much more complex scenario. > > Reproducer: > > This race issue can be triggered easily using a well constructed > reproducer and patched brd (with a delay in read path) [1]: > > With latest 6.8 mainline, race caused data loss can be observed easily: > $ gcc -g -lpthread test-thread-swap-race.c && ./a.out > Polulating 32MB of memory region... > Keep swapping out... > Starting round 0... > Spawning 65536 workers... > 32746 workers spawned, wait for done... > Round 0: Error on 0x5aa00, expected 32746, got 32743, 3 data loss! > Round 0: Error on 0x395200, expected 32746, got 32743, 3 data loss! > Round 0: Error on 0x3fd000, expected 32746, got 32737, 9 data loss! > Round 0 Failed, 15 data loss! > > This reproducer spawns multiple threads sharing the same memory region > using a small swap device. Every two threads updates mapped pages one by > one in opposite direction trying to create a race, with one dedicated > thread keep swapping out the data out using madvise. > > The reproducer created a reproduce rate of about once every 5 minutes, > so the race should be totally possible in production. > > After this patch, I ran the reproducer for over a few hundred rounds > and no data loss observed. > > Performance overhead is minimal, microbenchmark swapin 10G from 32G > zram: > > Before: 10934698 us > After: 11157121 us > Non-direct: 13155355 us (Dropping SWP_SYNCHRONOUS_IO flag) > > Fixes: 0bcac06f27d7 ("mm, swap: skip swapcache for swapin of synchronous device") > Link: https://github.com/ryncsn/emm-test-project/tree/master/swap-stress-race [1] > Signed-off-by: Kairui Song <kasong@tencent.com> Cc: stable@vger.kernel.org Acked-by: Yu Zhao <yuzhao@google.com>
On Tue, Feb 6, 2024 at 2:03 PM Yu Zhao <yuzhao@google.com> wrote: > > On Mon, Feb 5, 2024 at 4:10 AM Kairui Song <ryncsn@gmail.com> wrote: > > > > From: Kairui Song <kasong@tencent.com> > > > > In the direct swapin path, when two or more threads swapin the same entry > > There is no other places referring to that path as "direct" swapin. > > I'd rephrase it as: "When skipping swapcache for SWP_SYNCHRONOUS_IO, > ...", and similarly for the subject: "mm: fix race when skipping > swapcache". Good suggestion. > > > at the same time, they get different pages (A, B) because swap cache is > > skipped. Before one thread (T0) finishes the swapin and installs page (A) > > to the PTE, another thread (T1) could finish swapin of page (B), > > swap_free the entry, then modify and swap-out the page again, using the > > same entry. It break the pte_same check because PTE value is unchanged, > > causing ABA problem. Then thread (T0) will then install the stalled page > > (A) into the PTE so new data in page (B) is lost, one possible callstack > > is like this: > > > > CPU0 CPU1 > > ---- ---- > > do_swap_page() do_swap_page() with same entry > > <direct swapin path> <direct swapin path> > > <alloc page A> <alloc page B> > > swap_readpage() <- read to page A swap_readpage() <- read to page B > > <slow on later locks or interrupt> <finished swapin first> > > ... set_pte_at() > > swap_free() <- Now the entry is freed. > > <write to page B, now page A stalled> > > <swap out page B using same swap entry> > > pte_same() <- Check pass, PTE seems > > unchanged, but page A > > is stalled! > > swap_free() <- page B content lost! > > set_pte_at() <- staled page A installed! > > > > To fix this, reuse swapcache_prepare which will pin the swap entry using > > the cache flag, and allow only one thread to pin it. Release the pin > > after PT unlocked. Racers will simply busy wait since it's a rare > > and very short event. > > > > Other methods like increasing the swap count don't seem to be a good > > idea after some tests, that will cause racers to fall back to the > > cached swapin path, two swapin path being used at the same time > > leads to a much more complex scenario. > > > > Reproducer: > > > > This race issue can be triggered easily using a well constructed > > reproducer and patched brd (with a delay in read path) [1]: > > > > With latest 6.8 mainline, race caused data loss can be observed easily: > > $ gcc -g -lpthread test-thread-swap-race.c && ./a.out > > Polulating 32MB of memory region... > > Keep swapping out... > > Starting round 0... > > Spawning 65536 workers... > > 32746 workers spawned, wait for done... > > Round 0: Error on 0x5aa00, expected 32746, got 32743, 3 data loss! > > Round 0: Error on 0x395200, expected 32746, got 32743, 3 data loss! > > Round 0: Error on 0x3fd000, expected 32746, got 32737, 9 data loss! > > Round 0 Failed, 15 data loss! > > > > This reproducer spawns multiple threads sharing the same memory region > > using a small swap device. Every two threads updates mapped pages one by > > one in opposite direction trying to create a race, with one dedicated > > thread keep swapping out the data out using madvise. > > > > The reproducer created a reproduce rate of about once every 5 minutes, > > so the race should be totally possible in production. > > > > After this patch, I ran the reproducer for over a few hundred rounds > > and no data loss observed. > > > > Performance overhead is minimal, microbenchmark swapin 10G from 32G > > zram: > > > > Before: 10934698 us > > After: 11157121 us > > Non-direct: 13155355 us (Dropping SWP_SYNCHRONOUS_IO flag) > > > > Fixes: 0bcac06f27d7 ("mm, swap: skip swapcache for swapin of synchronous device") > > Link: https://github.com/ryncsn/emm-test-project/tree/master/swap-stress-race [1] > > Signed-off-by: Kairui Song <kasong@tencent.com> > > Cc: stable@vger.kernel.org > > Acked-by: Yu Zhao <yuzhao@google.com> Thanks!
diff --git a/mm/memory.c b/mm/memory.c index 7e1f4849463a..fd7c55a292f1 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -3867,6 +3867,20 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) if (!folio) { if (data_race(si->flags & SWP_SYNCHRONOUS_IO) && __swap_count(entry) == 1) { + /* + * With swap count == 1, after we read the entry, + * other threads could finish swapin first, free + * the entry, then swapout the modified page using + * the same entry. Now the content we just read is + * stalled, and it's undetectable as pte_same() + * returns true due to entry reuse. + * + * So pin the swap entry using the cache flag even + * cache is not used. + */ + if (swapcache_prepare(entry)) + goto out; + /* skip swapcache */ folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0, vma, vmf->address, false); @@ -4116,6 +4130,9 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) unlock: if (vmf->pte) pte_unmap_unlock(vmf->pte, vmf->ptl); + /* Clear the swap cache pin for direct swapin after PTL unlock */ + if (folio && !swapcache) + swapcache_clear(si, entry); out: if (si) put_swap_device(si); @@ -4124,6 +4141,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) if (vmf->pte) pte_unmap_unlock(vmf->pte, vmf->ptl); out_page: + if (!swapcache) + swapcache_clear(si, entry); folio_unlock(folio); out_release: folio_put(folio); diff --git a/mm/swap.h b/mm/swap.h index 758c46ca671e..fc2f6ade7f80 100644 --- a/mm/swap.h +++ b/mm/swap.h @@ -41,6 +41,7 @@ void __delete_from_swap_cache(struct folio *folio, void delete_from_swap_cache(struct folio *folio); void clear_shadow_from_swap_cache(int type, unsigned long begin, unsigned long end); +void swapcache_clear(struct swap_info_struct *si, swp_entry_t entry); struct folio *swap_cache_get_folio(swp_entry_t entry, struct vm_area_struct *vma, unsigned long addr); struct folio *filemap_get_incore_folio(struct address_space *mapping, @@ -97,6 +98,10 @@ static inline int swap_writepage(struct page *p, struct writeback_control *wbc) return 0; } +static inline void swapcache_clear(struct swap_info_struct *si, swp_entry_t entry) +{ +} + static inline struct folio *swap_cache_get_folio(swp_entry_t entry, struct vm_area_struct *vma, unsigned long addr) { diff --git a/mm/swapfile.c b/mm/swapfile.c index 556ff7347d5f..f7d4ad152a7f 100644 --- a/mm/swapfile.c +++ b/mm/swapfile.c @@ -3365,6 +3365,22 @@ int swapcache_prepare(swp_entry_t entry) return __swap_duplicate(entry, SWAP_HAS_CACHE); } +/* + * Clear the cache flag and release pinned entry. + */ +void swapcache_clear(struct swap_info_struct *si, swp_entry_t entry) +{ + struct swap_cluster_info *ci; + unsigned long offset = swp_offset(entry); + unsigned char usage; + + ci = lock_cluster_or_swap_info(si, offset); + usage = __swap_entry_free_locked(si, offset, SWAP_HAS_CACHE); + unlock_cluster_or_swap_info(si, ci); + if (!usage) + free_swap_slot(entry); +} + struct swap_info_struct *swp_swap_info(swp_entry_t entry) { return swap_type_to_swap_info(swp_type(entry));