Re: [PATCH] madvise:madvise_cold_or_pageout_pte_range(): allow split while folio_estimated_sharers = 0

[Ryan Roberts <ryan.roberts@arm.com>]

On 26/02/2024 21:17, Barry Song wrote:
> On Tue, Feb 27, 2024 at 2:46 AM Ryan Roberts <ryan.roberts@arm.com> wrote:
>>
>> On 21/02/2024 08:50, Barry Song wrote:
>>> From: Barry Song <v-songbaohua@oppo.com>
>>>
>>> The purpose is stopping splitting large folios whose mapcount are 2 or
>>> above. Folios whose estimated_shares = 0 should be still perfect and
>>> even better candidates than estimated_shares = 1.
>>>
>>> Consider a pte-mapped large folio with 16 subpages, if we unmap 1-15,
>>> the current code will split folios and reclaim them while madvise goes
>>> on this folio; but if we unmap subpage 0, we will keep this folio and
>>> break. This is weird.
>>>
>>> For pmd-mapped large folios, we can still use "= 1" as the condition
>>> as anyway we have the entire map for it. So this patch doesn't change
>>> the condition for pmd-mapped large folios.
>>> This also explains why we had been using "= 1" for both pmd-mapped and
>>> pte-mapped large folios before commit 07e8c82b5eff ("madvise: convert
>>> madvise_cold_or_pageout_pte_range() to use folios"), because in the
>>> past, we used the mapcount of the specific subpage, since the subpage
>>> had pte present, its mapcount wouldn't be 0.
>>>
>>> The problem can be quite easily reproduced by writing a small program,
>>> unmapping the first subpage of a pte-mapped large folio vs. unmapping
>>> anyone other than the first subpage.
>>>
>>> Fixes: 2f406263e3e9 ("madvise:madvise_cold_or_pageout_pte_range(): don't use mapcount() against large folio for sharing check")
>>> Cc: Yin Fengwei <fengwei.yin@intel.com>
>>> Cc: Yu Zhao <yuzhao@google.com>
>>> Cc: Ryan Roberts <ryan.roberts@arm.com>
>>> Cc: David Hildenbrand <david@redhat.com>
>>> Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
>>> Cc: Matthew Wilcox <willy@infradead.org>
>>> Cc: Minchan Kim <minchan@kernel.org>
>>> Cc: Vishal Moola (Oracle) <vishal.moola@gmail.com>
>>> Cc: Yang Shi <shy828301@gmail.com>
>>> Signed-off-by: Barry Song <v-songbaohua@oppo.com>
>>> ---
>>>  mm/madvise.c | 2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/mm/madvise.c b/mm/madvise.c
>>> index cfa5e7288261..abde3edb04f0 100644
>>> --- a/mm/madvise.c
>>> +++ b/mm/madvise.c
>>> @@ -453,7 +453,7 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
>>>               if (folio_test_large(folio)) {
>>>                       int err;
>>>
>>> -                     if (folio_estimated_sharers(folio) != 1)
>>> +                     if (folio_estimated_sharers(folio) > 1)
>>>                               break;
>>>                       if (pageout_anon_only_filter && !folio_test_anon(folio))
>>>                               break;
>>
>> I wonder if we should change all the instances:
>>
>> folio_estimated_sharers() != 1   ->   folio_estimated_sharers() > 1
>> folio_estimated_sharers() == 1   ->   folio_estimated_sharers() <= 1
>>
>> It shouldn't cause a problem for the pmd case, and there are definitely other
>> cases where it will help. e.g. madvise_free_pte_range().
> 
> right. My test case covered PAGEOUT only and I agree madvise_free and
> others have
> exactly the same issue. for pmd case, it doesn't matter whether we
> change the condition
> or not because we have already pmd-mapped in the page table.
> 
> And good to know David will have a wrapper in folio_mapped_shared()  to more
> widely address this issue.
> 
>>
>> Regardless:
>>
>> Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
>>
> 
> Thanks though we might have missed your tag as this one has been
> in mm-stable.

No problem! I've been out on holiday so a bit behind on where everything is.

> 
> Best regards,
> Barry