Re: [PATCH v2 3/3] madvise:madvise_free_pte_range(): don't use mapcount() against large folio for sharing check

[Daniel Gomez]

On Wed, Aug 16, 2023 at 07:30:35AM +0800, Yin Fengwei wrote:
>
>
> On 8/15/23 21:25, Daniel Gomez wrote:
> > Hi Yin,
> > On Tue, Aug 08, 2023 at 10:09:17AM +0800, Yin Fengwei wrote:
> >> Commit 98b211d6415f ("madvise: convert madvise_free_pte_range() to use a
> >> folio") replaced the page_mapcount() with folio_mapcount() to check
> >> whether the folio is shared by other mapping.
> >>
> >> It's not correct for large folios. folio_mapcount() returns the total
> >> mapcount of large folio which is not suitable to detect whether the folio
> >> is shared.
> >>
> >> Use folio_estimated_sharers() which returns a estimated number of shares.
> >> That means it's not 100% correct. It should be OK for madvise case here.
> >
> > I'm trying to understand why it should be ok for madvise this change, so
> > I hope it's okay to ask you few questions.
> >
> > folio_mapcount() calculates the total maps for all the subpages of a
> > folio. However, the folio_estimated_sharers does it only for the first
> > subpage making it not true for large folios. Then, wouldn't this change
> > drop support for large folios?
> I saw David explained this very well in another mail.
>
> >
> > Seems like folio_entire_mapcount() is not accurate either because of it
> > does not inclue PTE-mapped sub-pages which I think we need here. Hence,
> > the folio_mapcount(). Could this be something missing in the test side?
> >
> > I tried to replicate the setup with CONFIG_TRANSPARENT_HUGEPAGE but
> > seems like I'm not able to do it:
> >
> > ./cow
> > # [INFO] detected THP size: 2048 KiB
> > # [INFO] detected hugetlb size: 2048 KiB
> > # [INFO] detected hugetlb size: 1048576 KiB
> > # [INFO] huge zeropage is enabled
> > TAP version 13
> > 1..166
> > # [INFO] Anonymous memory tests in private mappings
> > # [RUN] Basic COW after fork() ... with base page
> > not ok 1 MADV_NOHUGEPAGE failed
> > # [RUN] Basic COW after fork() ... with swapped out base page
> > not ok 2 MADV_NOHUGEPAGE failed
> > # [RUN] Basic COW after fork() ... with THP
> > not ok 3 MADV_HUGEPAGE failed
> > # [RUN] Basic COW after fork() ... with swapped-out THP
> > not ok 4 MADV_HUGEPAGE failed
> > # [RUN] Basic COW after fork() ... with PTE-mapped THP
> > not ok 5 MADV_HUGEPAGE failed
> > # [RUN] Basic COW after fork() ... with swapped-out, PTE-mapped THP
> > not ok 6 MADV_HUGEPAGE failed
> > ...
> Can you post the MADV_PAGEOUT and PTE-mapped THP related testing result?
> And I suppose swap need be enabled also for the testing.

You may find a dump of the logs in the link below with system information. Let me
know if you find something wrong in my setup or if you need something else.
Besides CONFIG_TRANSPARENT_HUGEPAGE, CONFIG_SWAP is also enabled in the kernel.

https://gitlab.com/-/snippets/2584135

Also, strace reports ENOSYS for MADV_*:
madvise(0x7f2912465000, 4096, MADV_NOHUGEPAGE) = -1 ENOSYS (Function not implemented)
madvise(0x7f2912000000, 2097152, MADV_HUGEPAGE) = -1 ENOSYS (Function not implemented)


>
>
> Regards
> Yin, Fengwei
>
> >
> >
> > Daniel
> >>
> >> User-visible effects is that the THP is skipped when user call madvise.
> >> But the correct behavior is THP should be split and processed then.
> >>
> >> NOTE: this change is a temporary fix to reduce the user-visible effects
> >> before the long term fix from David is ready.
> >>
> >> Fixes: 98b211d6415f ("madvise: convert madvise_free_pte_range() to use a folio")
> >> Cc: stable@xxxxxxxxxxxxxxx
> >> Signed-off-by: Yin Fengwei <fengwei.yin@xxxxxxxxx>
> >> Reviewed-by: Yu Zhao <yuzhao@xxxxxxxxxx>
> >> Reviewed-by: Ryan Roberts <ryan.roberts@xxxxxxx>
> >> ---
> >>  mm/madvise.c | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/mm/madvise.c b/mm/madvise.c
> >> index 49af35e2d99a..4dded5d27e7e 100644
> >> --- a/mm/madvise.c
> >> +++ b/mm/madvise.c
> >> @@ -683,7 +683,7 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
> >>  		if (folio_test_large(folio)) {
> >>  			int err;
> >>
> >> -			if (folio_mapcount(folio) != 1)
> >> +			if (folio_estimated_sharers(folio) != 1)
> >>  				break;
> >>  			if (!folio_trylock(folio))
> >>  				break;
> >> --
> >> 2.39.2
> >>