Re: [RFC PATCH v2 1/4] madvise: not use mapcount() against large folio for sharing check

["Yin, Fengwei" <fengwei.yin@intel.com>]

On 7/22/2023 2:57 AM, Yu Zhao wrote:
> On Fri, Jul 21, 2023 at 3:41 AM Yin Fengwei <fengwei.yin@intel.com> wrote:
>>
>> The commit
>> 07e8c82b5eff ("madvise: convert madvise_cold_or_pageout_pte_range() to
>> use folios") replaced the page_mapcount() with folio_mapcount() to
>> check whether the folio is shared by other mapping.
>>
>> But it's not correct for large folio. 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. But it should be OK for
>> madvise case here.
>>
>> Signed-off-by: Yin Fengwei <fengwei.yin@intel.com>
> 
> Fixes:
> Cc: stable
OK

> 
>> @@ -383,7 +383,7 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
>>                 folio = pfn_folio(pmd_pfn(orig_pmd));
>>
>>                 /* Do not interfere with other mappings of this folio */
>> -               if (folio_mapcount(folio) != 1)
>> +               if (folio_estimated_sharers(folio) != 1)
> 
> Strictly speaking, this isn't a bug. But it may be ok to include it in
> the same patch.
OK. I will drop the change for pmd.

> 
>>                         goto huge_unlock;
>>
>>                 if (pageout_anon_only_filter && !folio_test_anon(folio))
>> @@ -459,7 +459,7 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
>>                 if (folio_test_large(folio)) {
>>                         int err;
>>
>> -                       if (folio_mapcount(folio) != 1)
>> +                       if (folio_estimated_sharers(folio) != 1)
>>                                 break;
>>                         if (pageout_anon_only_filter && !folio_test_anon(folio))
>>                                 break;
>> @@ -682,7 +682,7 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
> 
> What about madvise_free_huge_pmd()? Should it be changed as well so
> that it's consistent with the first change? Either change both or neither.
> 
>>                 if (folio_test_large(folio)) {
>>                         int err;
>>
>> -                       if (folio_mapcount(folio) != 1)
>> +                       if (folio_estimated_sharers(folio) != 1)
> 
> This is another bug fix and should be in a separate patch.
OK. Will split to two patches.

> 
>>                                 break;
>>                         if (!folio_trylock(folio))
>>                                 break;
> 
> Please send two separate fixes, and then:
> 
> Reviewed-by: Yu Zhao <yuzhao@google.com>
Thanks a lot. I will drop the mapcount() change for pmd and sent to patches
for madvise_cold_or_pageout_pte_range() and madvise_free_pte_range().


Regards
Yin, Fengwei