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

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Message-ID: <2b6cc6b6-8fcb-35ff-3d5b-e4a6068847d9@redhat.com>
Date:   Wed, 16 Aug 2023 17:11:54 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Daniel Gomez <da.gomez@...sung.com>,
        "Yin, Fengwei" <fengwei.yin@...el.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "willy@...radead.org" <willy@...radead.org>,
        "vishal.moola@...il.com" <vishal.moola@...il.com>,
        "wangkefeng.wang@...wei.com" <wangkefeng.wang@...wei.com>,
        "minchan@...nel.org" <minchan@...nel.org>,
        "yuzhao@...gle.com" <yuzhao@...gle.com>,
        "ryan.roberts@....com" <ryan.roberts@....com>,
        "shy828301@...il.com" <shy828301@...il.com>
Subject: Re: [PATCH v2 3/3] madvise:madvise_free_pte_range(): don't use
 mapcount() against large folio for sharing check

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.

-- 
Cheers,

David / dhildenb