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 05:11:54PM +0200, David Hildenbrand wrote:
> On 16.08.23 16:13, Daniel Gomez wrote:
> > On Wed, Aug 16, 2023 at 08:04:11PM +0800, Yin, Fengwei wrote:
> > >
> > >
> > > On 8/16/2023 7:44 PM, Daniel Gomez wrote:
> > > > 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)
> > > O. The problem here is MADV_HUGEPAGE/MADV_NOHUGEPAGE doesn't work.
> > > Do you have CONFIG_ADVISE_SYSCALLS enabled?
> > It worked after I enabled the conf. Some tests failed and some were
> > skipped. But I managed to reproduce the issue now, thanks Yin!
> >
> > Bail out! 4 out of 166 tests failed
> > # Totals: pass:146 fail:4 xfail:0 xpass:0 skip:16 error:0
> >
>
> These hugetlb that are failing are known failures.

Hi David, thanks for letting me know. Also, thanks for the description
of the folio mapping in the other mail.
>
> --
> Cheers,
>
> David / dhildenb
>