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2023 08:11:53 +0000 Message-ID: <2fa4d5ca-b486-b246-e4a8-531e322e86e9@intel.com> Date: Fri, 4 Aug 2023 16:11:43 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:102.0) Gecko/20100101 Firefox/102.0 Thunderbird/102.13.0 Subject: Re: [PATCH 0/2] don't use mapcount() to check large folio sharing Content-Language: en-US To: David Hildenbrand , Yu Zhao CC: Ryan Roberts , , , , , , , , , References: <20230728161356.1784568-1-fengwei.yin@intel.com> <3bbfde16-ced1-dca8-6a3f-da893e045bc5@arm.com> <56c8f4f9-b54b-b0bb-250c-ec8643accfc7@intel.com> <3541d2de-5cf8-2f84-8153-277e2bfc0101@arm.com> <5f98748a-97ca-6426-1e24-a5675da75381@intel.com> <837ba176-c97f-f81b-c044-eb6aa3d88bb7@intel.com> <40d49276-fae9-e538-61cf-64bb79233bc3@intel.com> <45457815-66c4-029f-42f9-6c377e4eb1e3@intel.com> <75996f6b-63fe-4878-c19d-bf35ee2ad20b@redhat.com> From: "Yin, Fengwei" In-Reply-To: <75996f6b-63fe-4878-c19d-bf35ee2ad20b@redhat.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 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>>>>> On Wed, Aug 2, 2023 at 6:56 AM Yin, Fengwei wrote: >>>>>> >>>>>> " >>>>>> >>>>>> On 8/2/2023 8:49 PM, Ryan Roberts wrote: >>>>>>> On 02/08/2023 13:42, Yin, Fengwei wrote: >>>>>>>> >>>>>>>> >>>>>>>> On 8/2/2023 8:40 PM, Ryan Roberts wrote: >>>>>>>>> On 02/08/2023 13:35, Yin, Fengwei wrote: >>>>>>>>>> >>>>>>>>>> >>>>>>>>>> On 8/2/2023 6:27 PM, Ryan Roberts wrote: >>>>>>>>>>> On 28/07/2023 17:13, Yin Fengwei wrote: >>>>>>>>>>>> In madvise_cold_or_pageout_pte_range() and madvise_free_pte_range(), >>>>>>>>>>>> folio_mapcount() is used to check whether the folio is shared. But it's >>>>>>>>>>>> not correct as folio_mapcount() returns total mapcount of large folio. >>>>>>>>>>>> >>>>>>>>>>>> Use folio_estimated_sharers() here as the estimated number is enough. >>>>>>>>>>>> >>>>>>>>>>>> Yin Fengwei (2): >>>>>>>>>>>>    madvise: don't use mapcount() against large folio for sharing check >>>>>>>>>>>>    madvise: don't use mapcount() against large folio for sharing check >>>>>>>>>>>> >>>>>>>>>>>>   mm/huge_memory.c | 2 +- >>>>>>>>>>>>   mm/madvise.c     | 6 +++--- >>>>>>>>>>>>   2 files changed, 4 insertions(+), 4 deletions(-) >>>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>> As a set of fixes, I agree this is definitely an improvement, so: >>>>>>>>>>> >>>>>>>>>>> Reviewed-By: Ryan Roberts >>>>>>>>>> Thanks. >>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>> But I have a couple of comments around further improvements; >>>>>>>>>>> >>>>>>>>>>> Once we have the scheme that David is working on to be able to provide precise >>>>>>>>>>> exclusive vs shared info, we will probably want to move to that. Although that >>>>>>>>>>> scheme will need access to the mm_struct of a process known to be mapping the >>>>>>>>>>> folio. We have that info, but its not passed to folio_estimated_sharers() so we >>>>>>>>>>> can't just reimplement folio_estimated_sharers() - we will need to rework these >>>>>>>>>>> call sites again. >>>>>>>>>> Yes. This could be extra work. Maybe should delay till David's work is done. >>>>>>>>> >>>>>>>>> What you have is definitely an improvement over what was there before. And is >>>>>>>>> probably the best we can do without David's scheme. So I wouldn't delay this. >>>>>>>>> Just pointing out that we will be able to make it even better later on (if >>>>>>>>> David's stuff goes in). >>>>>>>> Yes. I agree that we should wait for David's work ready and do fix based on that. >>>>>>> >>>>>>> I was suggesting the opposite - not waiting. Then we can do separate improvement >>>>>>> later. >>>>>> Let's wait for David's work ready. >>>>> >>>>> Waiting is fine as long as we don't miss the next merge window -- we >>>>> don't want these two bugs to get into another release. Also I think we >>>>> should cc stable, since as David mentioned, they have been causing >>>>> selftest failures. >>>> >>>> Stable was CCed. >>> >>> Need to add the "Cc: stable@vger.kernel.org" tag: >>> Documentation/process/stable-kernel-rules.rst >> OK. Thanks for clarification. I totally mis-understanded this. :). >> >> I'd like to wait for answer from Andrew whether these patches are suitable >> for stable (I suppose you think so) branch. > > Note that the COW test does not fail -- it skips -- but the behavir changed: > > $ ./cow > # [INFO] detected THP size: 2048 KiB > # [INFO] detected hugetlb page size: 2048 KiB > # [INFO] detected hugetlb page size: 1048576 KiB > # [INFO] huge zeropage is enabled > TAP version 13 > 1..190 > # [INFO] Anonymous memory tests in private mappings > # [RUN] Basic COW after fork() ... with base page > ok 1 No leak from parent into child > # [RUN] Basic COW after fork() ... with swapped out base page > ok 2 No leak from parent into child > # [RUN] Basic COW after fork() ... with THP > ok 3 No leak from parent into child > # [RUN] Basic COW after fork() ... with swapped-out THP > ok 4 No leak from parent into child > # [RUN] Basic COW after fork() ... with PTE-mapped THP > ok 5 No leak from parent into child > # [RUN] Basic COW after fork() ... with swapped-out, PTE-mapped THP > ok 6 # SKIP MADV_PAGEOUT did not work, is swap enabled? > # [RUN] Basic COW after fork() ... with single PTE of THP > ok 7 No leak from parent into child > # [RUN] Basic COW after fork() ... with single PTE of swapped-out THP > ok 8 No leak from parent into child > # [RUN] Basic COW after fork() ... with partially mremap()'ed THP > ok 9 No leak from parent into child > # [RUN] Basic COW after fork() ... with partially shared THP > ok 10 No leak from parent into child > ... > > Observe how patch #6 skips because the MADV_PAGEOUT was not effective (which might have happened due to other reasons as well, thus no failure). > > The code that broke it is > > commit 07e8c82b5eff8ef34b74210eacb8d9c4a2886b82 > Author: Vishal Moola (Oracle) > Date:   Wed Dec 21 10:08:46 2022 -0800 > >     madvise: convert madvise_cold_or_pageout_pte_range() to use folios >         This change removes a number of calls to compound_head(), and saves >     1729 bytes of kernel text. >         Link: https://lkml.kernel.org/r/20221221180848.20774-3-vishal.moola@gmail.com >     Signed-off-by: Vishal Moola (Oracle) >     Reviewed-by: Matthew Wilcox (Oracle) >     Cc: SeongJae Park >     Signed-off-by: Andrew Morton > > > Ever since v6.3. > > The simplest way to fix it would be to revert the page_mapcount() -> folio_mapcount(), > conversion. Confirmed this patchset also can make swapped-out, PTE-mapped THP related tests from skip to pass. Regards Yin, Fengwei > > > Probably all that is information worth having in the patch description. >