| Message ID | cover.1651998586.git.baolin.wang@linux.alibaba.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | Introduce new huge_ptep_get_access_flags() interface | expand |
On Sun, May 08, 2022 at 04:58:51PM +0800, Baolin Wang wrote: > Hi, > > As Mike pointed out [1], the huge_ptep_get() will only return one specific > pte value for the CONT-PTE or CONT-PMD size hugetlb on ARM64 system, which > will not take into account the subpages' dirty or young bits of a CONT-PTE/PMD > size hugetlb page. That will make us miss dirty or young flags of a CONT-PTE/PMD > size hugetlb page for those functions that want to check the dirty or > young flags of a hugetlb page. For example, the gather_hugetlb_stats() will > get inaccurate dirty hugetlb page statistics, and the DAMON for hugetlb monitoring > will also get inaccurate access statistics. > > To fix this issue, one approach is that we can define an ARM64 specific huge_ptep_get() > implementation, which will take into account any subpages' dirty or young bits. IIUC, we could get the page size by page_size(pte_page(pte)). So, how about the following implementation of huge_ptep_get()? Does this work for you? pte_t huge_ptep_get(pte_t *ptep) { int ncontig, i; size_t pgsize; pte_t orig_pte = ptep_get(ptep); if (!pte_present(orig_pte) || !pte_cont(orig_pte)) return orig_pte; ncontig = num_contig_ptes(page_size(pte_page(orig_pte)), &pgsize); for (i = 0; i < ncontig; i++, ptep++) { pte_t pte = ptep_get(ptep); if (pte_dirty(pte)) orig_pte = pte_mkdirty(orig_pte); if (pte_young(pte)) orig_pte = pte_mkyoung(orig_pte); } return orig_pte; } > However we should add a new parameter for ARM64 specific huge_ptep_get() to check > how many continuous PTEs or PMDs in this CONT-PTE/PMD size hugetlb, that means we > should convert all the places using huge_ptep_get(), meanwhile most places using > huge_ptep_get() did not care about the dirty or young flags at all. > > So instead of changing the prototype of huge_ptep_get(), this patch set introduces > a new huge_ptep_get_access_flags() interface and define an ARM64 specific implementation, > that will take into account any subpages' dirty or young bits for CONT-PTE/PMD size > hugetlb page. And we can only change to use huge_ptep_get_access_flags() for those > functions that care about the dirty or young flags of a hugetlb page. > > [1] https://lore.kernel.org/linux-mm/85bd80b4-b4fd-0d3f-a2e5-149559f2f387@oracle.com/ > > Baolin Wang (3): > arm64/hugetlb: Introduce new huge_ptep_get_access_flags() interface > fs/proc/task_mmu: Change to use huge_ptep_get_access_flags() > mm/damon/vaddr: Change to use huge_ptep_get_access_flags() > > arch/arm64/include/asm/hugetlb.h | 2 ++ > arch/arm64/mm/hugetlbpage.c | 24 ++++++++++++++++++++++++ > fs/proc/task_mmu.c | 3 ++- > include/asm-generic/hugetlb.h | 7 +++++++ > mm/damon/vaddr.c | 5 +++-- > 5 files changed, 38 insertions(+), 3 deletions(-) > > -- > 1.8.3.1 > >
On Sun, May 08, 2022 at 04:58:51PM +0800, Baolin Wang wrote: > As Mike pointed out [1], the huge_ptep_get() will only return one specific > pte value for the CONT-PTE or CONT-PMD size hugetlb on ARM64 system, which > will not take into account the subpages' dirty or young bits of a CONT-PTE/PMD > size hugetlb page. That will make us miss dirty or young flags of a CONT-PTE/PMD > size hugetlb page for those functions that want to check the dirty or > young flags of a hugetlb page. For example, the gather_hugetlb_stats() will > get inaccurate dirty hugetlb page statistics, and the DAMON for hugetlb monitoring > will also get inaccurate access statistics. > > To fix this issue, one approach is that we can define an ARM64 specific huge_ptep_get() > implementation, which will take into account any subpages' dirty or young bits. > However we should add a new parameter for ARM64 specific huge_ptep_get() to check > how many continuous PTEs or PMDs in this CONT-PTE/PMD size hugetlb, that means we > should convert all the places using huge_ptep_get(), meanwhile most places using > huge_ptep_get() did not care about the dirty or young flags at all. > > So instead of changing the prototype of huge_ptep_get(), this patch set introduces > a new huge_ptep_get_access_flags() interface and define an ARM64 specific implementation, > that will take into account any subpages' dirty or young bits for CONT-PTE/PMD size > hugetlb page. And we can only change to use huge_ptep_get_access_flags() for those > functions that care about the dirty or young flags of a hugetlb page. I question whether this is the right approach. I understand that different hardware implementations have different requirements here, but at least one that I'm aware of (AMD Zen 2/3) requires that all PTEs that are part of a contig PTE must have identical A/D bits. Now, you could say that's irrelevant because it's x86 and we don't currently support contPTE on x86, but I wouldn't be surprised to see that other hardware has the same requirement. So what if we make that a Linux requirement? Setting a contPTE dirty or accessed becomes a bit more expensive (although still one/two cachelines, so not really much more expensive than a single write). Then there's no need to change the "get" side of things because they're always identical. It does mean that we can't take advantage of hardware setting A/D bits, unless hardware can be persuaded to behave this way. I don't have any ARM specs in front of me to check. I don't have a hard objection to your approach, I just want to discuss other possibilities.
On 5/8/2022 11:26 PM, Muchun Song wrote: > On Sun, May 08, 2022 at 04:58:51PM +0800, Baolin Wang wrote: >> Hi, >> >> As Mike pointed out [1], the huge_ptep_get() will only return one specific >> pte value for the CONT-PTE or CONT-PMD size hugetlb on ARM64 system, which >> will not take into account the subpages' dirty or young bits of a CONT-PTE/PMD >> size hugetlb page. That will make us miss dirty or young flags of a CONT-PTE/PMD >> size hugetlb page for those functions that want to check the dirty or >> young flags of a hugetlb page. For example, the gather_hugetlb_stats() will >> get inaccurate dirty hugetlb page statistics, and the DAMON for hugetlb monitoring >> will also get inaccurate access statistics. >> >> To fix this issue, one approach is that we can define an ARM64 specific huge_ptep_get() >> implementation, which will take into account any subpages' dirty or young bits. > > IIUC, we could get the page size by page_size(pte_page(pte)). > So, how about the following implementation of huge_ptep_get()? > Does this work for you? > > pte_t huge_ptep_get(pte_t *ptep) > { > int ncontig, i; > size_t pgsize; > pte_t orig_pte = ptep_get(ptep); > > if (!pte_present(orig_pte) || !pte_cont(orig_pte)) > return orig_pte; > > ncontig = num_contig_ptes(page_size(pte_page(orig_pte)), &pgsize); > > for (i = 0; i < ncontig; i++, ptep++) { > pte_t pte = ptep_get(ptep); > > if (pte_dirty(pte)) > orig_pte = pte_mkdirty(orig_pte); > > if (pte_young(pte)) > orig_pte = pte_mkyoung(orig_pte); > } > > return orig_pte; > } Thanks for your suggestion, and I think this works for me and looks more straight forward in case some functions using huge_ptep_get() will care about the young or dirty bits in future. My only concern is that all the functions using huge_ptep_get() will set a contPTE dirty or accessed bit, however most functions do not care about the dirty and accessed bit, which becomes a bit more expensive for them? Also mentioned by Matthew in his comments. Anyway, I still think your suggestion is straight forward and I can change in next version if no other objections.
On 5/9/2022 1:08 AM, Matthew Wilcox wrote: > On Sun, May 08, 2022 at 04:58:51PM +0800, Baolin Wang wrote: >> As Mike pointed out [1], the huge_ptep_get() will only return one specific >> pte value for the CONT-PTE or CONT-PMD size hugetlb on ARM64 system, which >> will not take into account the subpages' dirty or young bits of a CONT-PTE/PMD >> size hugetlb page. That will make us miss dirty or young flags of a CONT-PTE/PMD >> size hugetlb page for those functions that want to check the dirty or >> young flags of a hugetlb page. For example, the gather_hugetlb_stats() will >> get inaccurate dirty hugetlb page statistics, and the DAMON for hugetlb monitoring >> will also get inaccurate access statistics. >> >> To fix this issue, one approach is that we can define an ARM64 specific huge_ptep_get() >> implementation, which will take into account any subpages' dirty or young bits. >> However we should add a new parameter for ARM64 specific huge_ptep_get() to check >> how many continuous PTEs or PMDs in this CONT-PTE/PMD size hugetlb, that means we >> should convert all the places using huge_ptep_get(), meanwhile most places using >> huge_ptep_get() did not care about the dirty or young flags at all. >> >> So instead of changing the prototype of huge_ptep_get(), this patch set introduces >> a new huge_ptep_get_access_flags() interface and define an ARM64 specific implementation, >> that will take into account any subpages' dirty or young bits for CONT-PTE/PMD size >> hugetlb page. And we can only change to use huge_ptep_get_access_flags() for those >> functions that care about the dirty or young flags of a hugetlb page. > > I question whether this is the right approach. I understand that > different hardware implementations have different requirements here, > but at least one that I'm aware of (AMD Zen 2/3) requires that all > PTEs that are part of a contig PTE must have identical A/D bits. Now, > you could say that's irrelevant because it's x86 and we don't currently > support contPTE on x86, but I wouldn't be surprised to see that other > hardware has the same requirement. Yes, so on x86, we can use the default huge_ptep_get(). But for ARM64, unfortunately the A/D bits of a contig PTE is independent, that's why we want a ARM64 specific huge_ptep_get(). > So what if we make that a Linux requirement? Setting a contPTE dirty or > accessed becomes a bit more expensive (although still one/two cachelines, > so not really much more expensive than a single write). Then there's no > need to change the "get" side of things because they're always identical. > > It does mean that we can't take advantage of hardware setting A/D bits, > unless hardware can be persuaded to behave this way. I don't have any > ARM specs in front of me to check. I hope the hardware can make sure the contPTE are always identical, however in fact like I said the A/D bits setting of a contig PTE by hardware is independent in a contig-PTE size hugetlb page, they are not always identical. From my testing, if I monitored a contig-PTE size hugetlb page with DAMON, and I only modified the subpages of the contig-PTE size hugetlb page. The result is I can not monitor any accesses, but actually there are. So I think an ARM64 specific huge_ptep_get() implementation seems the right way as Muchun suggested? Thanks.
On Sun, May 08, 2022 at 06:08:18PM +0100, Matthew Wilcox wrote: > On Sun, May 08, 2022 at 04:58:51PM +0800, Baolin Wang wrote: > > As Mike pointed out [1], the huge_ptep_get() will only return one specific > > pte value for the CONT-PTE or CONT-PMD size hugetlb on ARM64 system, which > > will not take into account the subpages' dirty or young bits of a CONT-PTE/PMD > > size hugetlb page. That will make us miss dirty or young flags of a CONT-PTE/PMD > > size hugetlb page for those functions that want to check the dirty or > > young flags of a hugetlb page. For example, the gather_hugetlb_stats() will > > get inaccurate dirty hugetlb page statistics, and the DAMON for hugetlb monitoring > > will also get inaccurate access statistics. > > > > To fix this issue, one approach is that we can define an ARM64 specific huge_ptep_get() > > implementation, which will take into account any subpages' dirty or young bits. > > However we should add a new parameter for ARM64 specific huge_ptep_get() to check > > how many continuous PTEs or PMDs in this CONT-PTE/PMD size hugetlb, that means we > > should convert all the places using huge_ptep_get(), meanwhile most places using > > huge_ptep_get() did not care about the dirty or young flags at all. > > > > So instead of changing the prototype of huge_ptep_get(), this patch set introduces > > a new huge_ptep_get_access_flags() interface and define an ARM64 specific implementation, > > that will take into account any subpages' dirty or young bits for CONT-PTE/PMD size > > hugetlb page. And we can only change to use huge_ptep_get_access_flags() for those > > functions that care about the dirty or young flags of a hugetlb page. > > I question whether this is the right approach. I understand that > different hardware implementations have different requirements here, > but at least one that I'm aware of (AMD Zen 2/3) requires that all > PTEs that are part of a contig PTE must have identical A/D bits. Now, > you could say that's irrelevant because it's x86 and we don't currently > support contPTE on x86, but I wouldn't be surprised to see that other > hardware has the same requirement. > > So what if we make that a Linux requirement? Setting a contPTE dirty or > accessed becomes a bit more expensive (although still one/two cachelines, > so not really much more expensive than a single write). Then there's no > need to change the "get" side of things because they're always identical. > > It does mean that we can't take advantage of hardware setting A/D bits, > unless hardware can be persuaded to behave this way. I don't have any > ARM specs in front of me to check. > I have looked at the comments in get_clear_flush() (in arch/arm64/mm/hugetlbpage.c). That says: /* * If HW_AFDBM is enabled, then the HW could turn on * the dirty or accessed bit for any page in the set, * so check them all. */ Unfortunately, the AD bits are not identical in all subpages. Thanks.