| Message ID | 20210417094039.51711-4-linmiaohe@huawei.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | close various race windows for swap | expand |
Miaohe Lin <linmiaohe@huawei.com> writes: > When I was investigating the swap code, I found the below possible race > window: > > CPU 1 CPU 2 > ----- ----- > do_swap_page This is OK for swap cache cases. So if (data_race(si->flags & SWP_SYNCHRONOUS_IO)) should be shown here. > swap_readpage(skip swap cache case) > if (data_race(sis->flags & SWP_FS_OPS)) { > swapoff > p->flags = &= ~SWP_VALID; > .. > synchronize_rcu(); > .. > p->swap_file = NULL; > struct file *swap_file = sis->swap_file; > struct address_space *mapping = swap_file->f_mapping;[oops!] > > Note that for the pages that are swapped in through swap cache, this isn't > an issue. Because the page is locked, and the swap entry will be marked > with SWAP_HAS_CACHE, so swapoff() can not proceed until the page has been > unlocked. > > Using current get/put_swap_device() to guard against concurrent swapoff for > swap_readpage() looks terrible because swap_readpage() may take really long > time. And this race may not be really pernicious because swapoff is usually > done when system shutdown only. To reduce the performance overhead on the > hot-path as much as possible, it appears we can use the percpu_ref to close > this race window(as suggested by Huang, Ying). I still suggest to squash PATCH 1-3, at least PATCH 1-2. That will change the relevant code together and make it easier to review. Best Regards, Huang, Ying > Fixes: 0bcac06f27d7 ("mm,swap: skip swapcache for swapin of synchronous device") > Reported-by: kernel test robot <lkp@intel.com> (auto build test ERROR) > Signed-off-by: Miaohe Lin <linmiaohe@huawei.com> > --- > include/linux/swap.h | 9 +++++++++ > mm/memory.c | 9 +++++++++ > 2 files changed, 18 insertions(+) > > diff --git a/include/linux/swap.h b/include/linux/swap.h > index 993693b38109..523c2411a135 100644 > --- a/include/linux/swap.h > +++ b/include/linux/swap.h > @@ -528,6 +528,15 @@ static inline struct swap_info_struct *swp_swap_info(swp_entry_t entry) > return NULL; > } > > +static inline struct swap_info_struct *get_swap_device(swp_entry_t entry) > +{ > + return NULL; > +} > + > +static inline void put_swap_device(struct swap_info_struct *si) > +{ > +} > + > #define swap_address_space(entry) (NULL) > #define get_nr_swap_pages() 0L > #define total_swap_pages 0L > diff --git a/mm/memory.c b/mm/memory.c > index 27014c3bde9f..7a2fe12cf641 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -3311,6 +3311,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) > { > struct vm_area_struct *vma = vmf->vma; > struct page *page = NULL, *swapcache; > + struct swap_info_struct *si = NULL; > swp_entry_t entry; > pte_t pte; > int locked; > @@ -3338,6 +3339,10 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) > goto out; > } > > + /* Prevent swapoff from happening to us. */ > + si = get_swap_device(entry); > + if (unlikely(!si)) > + goto out; > > delayacct_set_flag(current, DELAYACCT_PF_SWAPIN); > page = lookup_swap_cache(entry, vma, vmf->address); > @@ -3514,6 +3519,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) > unlock: > pte_unmap_unlock(vmf->pte, vmf->ptl); > out: > + if (si) > + put_swap_device(si); > return ret; > out_nomap: > pte_unmap_unlock(vmf->pte, vmf->ptl); > @@ -3525,6 +3532,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) > unlock_page(swapcache); > put_page(swapcache); > } > + if (si) > + put_swap_device(si); > return ret; > }
On 2021/4/19 10:23, Huang, Ying wrote: > Miaohe Lin <linmiaohe@huawei.com> writes: > >> When I was investigating the swap code, I found the below possible race >> window: >> >> CPU 1 CPU 2 >> ----- ----- >> do_swap_page > > This is OK for swap cache cases. So > > if (data_race(si->flags & SWP_SYNCHRONOUS_IO)) > > should be shown here. Ok. > >> swap_readpage(skip swap cache case) >> if (data_race(sis->flags & SWP_FS_OPS)) { >> swapoff >> p->flags = &= ~SWP_VALID; >> .. >> synchronize_rcu(); >> .. >> p->swap_file = NULL; >> struct file *swap_file = sis->swap_file; >> struct address_space *mapping = swap_file->f_mapping;[oops!] >> >> Note that for the pages that are swapped in through swap cache, this isn't >> an issue. Because the page is locked, and the swap entry will be marked >> with SWAP_HAS_CACHE, so swapoff() can not proceed until the page has been >> unlocked. >> >> Using current get/put_swap_device() to guard against concurrent swapoff for >> swap_readpage() looks terrible because swap_readpage() may take really long >> time. And this race may not be really pernicious because swapoff is usually >> done when system shutdown only. To reduce the performance overhead on the >> hot-path as much as possible, it appears we can use the percpu_ref to close >> this race window(as suggested by Huang, Ying). > > I still suggest to squash PATCH 1-3, at least PATCH 1-2. That will > change the relevant code together and make it easier to review. > Will squash PATCH 1-2. Thanks. > Best Regards, > Huang, Ying > >> Fixes: 0bcac06f27d7 ("mm,swap: skip swapcache for swapin of synchronous device") >> Reported-by: kernel test robot <lkp@intel.com> (auto build test ERROR) >> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com> >> --- >> include/linux/swap.h | 9 +++++++++ >> mm/memory.c | 9 +++++++++ >> 2 files changed, 18 insertions(+) >> >> diff --git a/include/linux/swap.h b/include/linux/swap.h >> index 993693b38109..523c2411a135 100644 >> --- a/include/linux/swap.h >> +++ b/include/linux/swap.h >> @@ -528,6 +528,15 @@ static inline struct swap_info_struct *swp_swap_info(swp_entry_t entry) >> return NULL; >> } >> >> +static inline struct swap_info_struct *get_swap_device(swp_entry_t entry) >> +{ >> + return NULL; >> +} >> + >> +static inline void put_swap_device(struct swap_info_struct *si) >> +{ >> +} >> + >> #define swap_address_space(entry) (NULL) >> #define get_nr_swap_pages() 0L >> #define total_swap_pages 0L >> diff --git a/mm/memory.c b/mm/memory.c >> index 27014c3bde9f..7a2fe12cf641 100644 >> --- a/mm/memory.c >> +++ b/mm/memory.c >> @@ -3311,6 +3311,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) >> { >> struct vm_area_struct *vma = vmf->vma; >> struct page *page = NULL, *swapcache; >> + struct swap_info_struct *si = NULL; >> swp_entry_t entry; >> pte_t pte; >> int locked; >> @@ -3338,6 +3339,10 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) >> goto out; >> } >> >> + /* Prevent swapoff from happening to us. */ >> + si = get_swap_device(entry); >> + if (unlikely(!si)) >> + goto out; >> >> delayacct_set_flag(current, DELAYACCT_PF_SWAPIN); >> page = lookup_swap_cache(entry, vma, vmf->address); >> @@ -3514,6 +3519,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) >> unlock: >> pte_unmap_unlock(vmf->pte, vmf->ptl); >> out: >> + if (si) >> + put_swap_device(si); >> return ret; >> out_nomap: >> pte_unmap_unlock(vmf->pte, vmf->ptl); >> @@ -3525,6 +3532,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) >> unlock_page(swapcache); >> put_page(swapcache); >> } >> + if (si) >> + put_swap_device(si); >> return ret; >> } > . >
diff --git a/include/linux/swap.h b/include/linux/swap.h index 993693b38109..523c2411a135 100644 --- a/include/linux/swap.h +++ b/include/linux/swap.h @@ -528,6 +528,15 @@ static inline struct swap_info_struct *swp_swap_info(swp_entry_t entry) return NULL; } +static inline struct swap_info_struct *get_swap_device(swp_entry_t entry) +{ + return NULL; +} + +static inline void put_swap_device(struct swap_info_struct *si) +{ +} + #define swap_address_space(entry) (NULL) #define get_nr_swap_pages() 0L #define total_swap_pages 0L diff --git a/mm/memory.c b/mm/memory.c index 27014c3bde9f..7a2fe12cf641 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -3311,6 +3311,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) { struct vm_area_struct *vma = vmf->vma; struct page *page = NULL, *swapcache; + struct swap_info_struct *si = NULL; swp_entry_t entry; pte_t pte; int locked; @@ -3338,6 +3339,10 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) goto out; } + /* Prevent swapoff from happening to us. */ + si = get_swap_device(entry); + if (unlikely(!si)) + goto out; delayacct_set_flag(current, DELAYACCT_PF_SWAPIN); page = lookup_swap_cache(entry, vma, vmf->address); @@ -3514,6 +3519,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) unlock: pte_unmap_unlock(vmf->pte, vmf->ptl); out: + if (si) + put_swap_device(si); return ret; out_nomap: pte_unmap_unlock(vmf->pte, vmf->ptl); @@ -3525,6 +3532,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) unlock_page(swapcache); put_page(swapcache); } + if (si) + put_swap_device(si); return ret; }
When I was investigating the swap code, I found the below possible race window: CPU 1 CPU 2 ----- ----- do_swap_page swap_readpage(skip swap cache case) if (data_race(sis->flags & SWP_FS_OPS)) { swapoff p->flags = &= ~SWP_VALID; .. synchronize_rcu(); .. p->swap_file = NULL; struct file *swap_file = sis->swap_file; struct address_space *mapping = swap_file->f_mapping;[oops!] Note that for the pages that are swapped in through swap cache, this isn't an issue. Because the page is locked, and the swap entry will be marked with SWAP_HAS_CACHE, so swapoff() can not proceed until the page has been unlocked. Using current get/put_swap_device() to guard against concurrent swapoff for swap_readpage() looks terrible because swap_readpage() may take really long time. And this race may not be really pernicious because swapoff is usually done when system shutdown only. To reduce the performance overhead on the hot-path as much as possible, it appears we can use the percpu_ref to close this race window(as suggested by Huang, Ying). Fixes: 0bcac06f27d7 ("mm,swap: skip swapcache for swapin of synchronous device") Reported-by: kernel test robot <lkp@intel.com> (auto build test ERROR) Signed-off-by: Miaohe Lin <linmiaohe@huawei.com> --- include/linux/swap.h | 9 +++++++++ mm/memory.c | 9 +++++++++ 2 files changed, 18 insertions(+)