linux-kernel - Re: [PATCH v3] mm/rmap: do not add fully unmapped large folio to deferred split list

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Message-ID: <8BBD1B75-135D-42AA-8937-53B259803AA7@nvidia.com>
Date: Thu, 25 Apr 2024 10:53:43 -0400
From: Zi Yan <ziy@...dia.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
 "Matthew Wilcox (Oracle)" <willy@...radead.org>,
 Yang Shi <shy828301@...il.com>, Ryan Roberts <ryan.roberts@....com>,
 Barry Song <21cnbao@...il.com>, Lance Yang <ioworker0@...il.com>,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] mm/rmap: do not add fully unmapped large folio to
 deferred split list

On 25 Apr 2024, at 3:19, David Hildenbrand wrote:

> On 25.04.24 00:46, Zi Yan wrote:
>> From: Zi Yan <ziy@...dia.com>
>>
>> In __folio_remove_rmap(), a large folio is added to deferred split list
>> if any page in a folio loses its final mapping. It is possible that
>> the folio is unmapped fully, but it is unnecessary to add the folio
>> to deferred split list at all. Fix it by checking folio->_nr_pages_mapped
>> before adding a folio to deferred split list. If the folio is already
>> on the deferred split list, it will be skipped. This issue applies to
>> both PTE-mapped THP and mTHP.
>>
>> Commit 98046944a159 ("mm: huge_memory: add the missing
>> folio_test_pmd_mappable() for THP split statistics") tried to exclude
>> mTHP deferred split stats from THP_DEFERRED_SPLIT_PAGE, but it does not
>> fix the above issue. A fully unmapped PTE-mapped order-9 THP was still
>
> Once again: your patch won't fix it either.
>
>> added to deferred split list and counted as THP_DEFERRED_SPLIT_PAGE,
>> since nr is 512 (non zero), level is RMAP_LEVEL_PTE, and inside
>> deferred_split_folio() the order-9 folio is folio_test_pmd_mappable().
>> However, this miscount was present even earlier due to implementation,
>> since PTEs are unmapped individually and first PTE unmapping adds the THP
>> into the deferred split list.
>
> It will still be present. Just less frequently.

OK. Let me reread the email exchanges between you and Yang and clarify
the details in the commit log.

>
>>
>> With commit b06dc281aa99 ("mm/rmap: introduce
>> folio_remove_rmap_[pte|ptes|pmd]()"), kernel is able to unmap PTE-mapped
>> folios in one shot without causing the miscount, hence this patch.
>>
>> Signed-off-by: Zi Yan <ziy@...dia.com>
>> Reviewed-by: Yang Shi <shy828301@...il.com>
>> ---
>>   mm/rmap.c | 7 ++++---
>>   1 file changed, 4 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/rmap.c b/mm/rmap.c
>> index a7913a454028..2809348add7b 100644
>> --- a/mm/rmap.c
>> +++ b/mm/rmap.c
>> @@ -1553,9 +1553,10 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
>>   		 * page of the folio is unmapped and at least one page
>>   		 * is still mapped.
>>   		 */
>> -		if (folio_test_large(folio) && folio_test_anon(folio))
>> -			if (level == RMAP_LEVEL_PTE || nr < nr_pmdmapped)
>> -				deferred_split_folio(folio);
>> +		if (folio_test_large(folio) && folio_test_anon(folio) &&
>> +		    ((level == RMAP_LEVEL_PTE && atomic_read(mapped)) ||
>> +		     (level == RMAP_LEVEL_PMD && nr < nr_pmdmapped)))
>> +			deferred_split_folio(folio);
>
> Please refrain from posting a new patch before the discussion on the old one is done.
>
> See my comments on v2 why optimizing out the function call is a reasonable thing to do *where we cannot batch* and the misaccounting will still happen. But it can be done independently.

Got it. Will keep the deferred list checking here and send a new one with commit
log changes too.

Thank you for the reviews.


--
Best Regards,
Yan, Zi

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