linux-kernel - Re: [PATCH v2 0/3] don't use mapcount() to check large folio sharing

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Date:   Tue, 8 Aug 2023 12:10:55 +0800
From:   "Yin, Fengwei" <fengwei.yin@...el.com>
To:     Yu Zhao <yuzhao@...gle.com>, <akpm@...ux-foundation.org>
CC:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        <stable@...r.kernel.org>, <willy@...radead.org>,
        <vishal.moola@...il.com>, <wangkefeng.wang@...wei.com>,
        <minchan@...nel.org>, <david@...hat.com>, <ryan.roberts@....com>,
        <shy828301@...il.com>
Subject: Re: [PATCH v2 0/3] don't use mapcount() to check large folio sharing



On 8/8/2023 10:43 AM, Yu Zhao wrote:
> On Mon, Aug 7, 2023 at 8:10 PM Yin Fengwei <fengwei.yin@...el.com> 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.
>>
>>
>> This patchset will fix the cases:
>> User space application call madvise() with MADV_FREE, MADV_COLD and
>> MADV_PAGEOUT for specific address range. There are THP mapped to the
>> range. Without the patchset, the THP is skipped. With the patch, the
>> THP will be split and handled accordingly.
>>
>> David reported the cow self test skip some cases because of
>> MADV_PAGEOUT skip THP:
>> https://lore.kernel.org/linux-mm/9e92e42d-488f-47db-ac9d-75b24cd0d037@intel.com/T/#mbf0f2ec7fbe45da47526de1d7036183981691e81
>> and I confirmed this patchset make it work again.
>>
>>
>> Changelog from v1:
>>   - Avoid two Fixes tags make backport harder. Thank Andrew for pointing
>>     this out.
>>
>>   - Add note section to mention this is a temporary fix which is fine
>>     to reduce user-visble effects. For long term fix, we should wait for
>>     David's solution. Thank Ryan and David for pointing this out.
>>
>>   - Spell user-visible effects out. Then people could decide whether
>>     these patches are necessary for stable branch. Thank Andrew for
>>     pointing this out.
> 
> LGTM, thank you.
Thanks a lot for looking at this patchset.


Regards
Yin, Fengwei