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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Date:   Wed, 16 Aug 2023 07:30:35 +0800
From:   Yin Fengwei <fengwei.yin@...el.com>
To:     Daniel Gomez <da.gomez@...sung.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>,
        "david@...hat.com" <david@...hat.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 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.


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@...r.kernel.org
>> Signed-off-by: Yin Fengwei <fengwei.yin@...el.com>
>> Reviewed-by: Yu Zhao <yuzhao@...gle.com>
>> Reviewed-by: Ryan Roberts <ryan.roberts@....com>
>> ---
>>  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
>>