Re: [PATCH v2] mm: hold PTL from the first PTE while reclaiming a large folio

[Ryan Roberts]

On 26/03/2024 16:10, David Hildenbrand wrote:
> On 06.03.24 10:52, Barry Song wrote:
>> From: Barry Song <v-songbaohua@xxxxxxxx>
>>
>> Within try_to_unmap_one(), page_vma_mapped_walk() races with other
>> PTE modifications preceded by pte clear. While iterating over PTEs
>> of a large folio, it only starts acquiring PTL from the first valid
>> (present) PTE. PTE modifications can temporarily set PTEs to
>> pte_none. Consequently, the initial PTEs of a large folio might
>> be skipped in try_to_unmap_one().
>> For example, for an anon folio, if we skip PTE0, we may have PTE0
>> which is still present, while PTE1 ~ PTE(nr_pages - 1) are swap
>> entries after try_to_unmap_one().
>> So folio will be still mapped, the folio fails to be reclaimed
>> and is put back to LRU in this round.
>> This also breaks up PTEs optimization such as CONT-PTE on this
>> large folio and may lead to accident folio_split() afterwards.
>> And since a part of PTEs are now swap entries, accessing those
>> parts will introduce overhead - do_swap_page.
>> Although the kernel can withstand all of the above issues, the
>> situation still seems quite awkward and warrants making it more
>> ideal.
>> The same race also occurs with small folios, but they have only
>> one PTE, thus, it won't be possible for them to be partially
>> unmapped.
>> This patch holds PTL from PTE0, allowing us to avoid reading PTE
>> values that are in the process of being transformed. With stable
>> PTE values, we can ensure that this large folio is either
>> completely reclaimed or that all PTEs remain untouched in this
>> round.
>> A corner case is that if we hold PTL from PTE0 and most initial
>> PTEs have been really unmapped before that, we may increase the
>> duration of holding PTL. Thus we only apply this optimization to
>> folios which are still entirely mapped (not in deferred_split
>> list).
>>
>> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
>> Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
>> ---
>>   v2:
>>    * Refine commit message and code comment after reading all comments
>>      from Ryan and David, thanks!
>>    * Avoid increasing the duration of PTL by applying optimization
>>      on folios not in deferred_split_list with respect to Ying's
>>      comment, thanks!
>>
>>   mm/vmscan.c | 12 ++++++++++++
>>   1 file changed, 12 insertions(+)
>>
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index 0b888a2afa58..7106741387e8 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -1270,6 +1270,18 @@ static unsigned int shrink_folio_list(struct list_head
>> *folio_list,
>>                 if (folio_test_pmd_mappable(folio))
>>                   flags |= TTU_SPLIT_HUGE_PMD;
>> +            /*
>> +             * Without TTU_SYNC, try_to_unmap will only begin to hold PTL
>> +             * from the first present PTE within a large folio. Some initial
>> +             * PTEs might be skipped due to races with parallel PTE writes
>> +             * in which PTEs can be cleared temporarily before being written
>> +             * new present values. This will lead to a large folio is still
>> +             * mapped while some subpages have been partially unmapped after
>> +             * try_to_unmap; TTU_SYNC helps try_to_unmap acquire PTL from the
>> +             * first PTE, eliminating the influence of temporary PTE values.
>> +             */
>> +            if (folio_test_large(folio) && list_empty(&folio->_deferred_list))
>> +                flags |= TTU_SYNC;
>>                 try_to_unmap(folio, flags);
>>               if (folio_mapped(folio)) {
> 
> Hopefully this won't have unexpected performance "surprises".
> 
> I do wonder if we should really care about the "_deferred_list" optimization
> here, though, I'd just drop it.

I also concluded that we do need the data_race() annotation around list_empty()
if you do wind up keeping it. But I agree with David about dropping it.

> 
> In any case
> 
> Acked-by: David Hildenbrand <david@xxxxxxxxxx>
>