linux-kernel - Re: [PATCH v2] mm/rmap: do not add fully unmapped large folio to deferred split list

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Message-ID: <815e2114-f739-4f2f-b09f-a23a2fc3214b@redhat.com>
Date: Thu, 25 Apr 2024 09:15:46 +0200
From: David Hildenbrand <david@...hat.com>
To: Zi Yan <ziy@...dia.com>, Yang Shi <shy828301@...il.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
 "Matthew Wilcox (Oracle)" <willy@...radead.org>,
 Ryan Roberts <ryan.roberts@....com>, Barry Song <21cnbao@...il.com>,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm/rmap: do not add fully unmapped large folio to
 deferred split list

On 25.04.24 00:39, Zi Yan wrote:
> On 24 Apr 2024, at 18:32, Yang Shi wrote:
> 
>> On Wed, Apr 24, 2024 at 2:10 PM Zi Yan <zi.yan@...t.com> 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.
>>>
>>> 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 everything. A fully unmapped PTE-mapped order-9 THP was also 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.
>>
>> Shall you mention the miscounting for mTHP too? There is another patch
>> series adding the counter support for mTHP.
> 
> OK, will add it.

I thought I made it clear: this patch won't "fix" it. Misaccounting will 
still happen. Just less frequently.

Please spell that out.

>>
>>>
>>> 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>
>>> ---
>>>   mm/rmap.c | 8 +++++---
>>>   1 file changed, 5 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/mm/rmap.c b/mm/rmap.c
>>> index a7913a454028..220ad8a83589 100644
>>> --- a/mm/rmap.c
>>> +++ b/mm/rmap.c
>>> @@ -1553,9 +1553,11 @@ 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) &&
>>> +                   list_empty(&folio->_deferred_list) &&
>>
>> Do we really need this check? deferred_split_folio() does the same
>> check too. Bailing out earlier sounds ok too, but there may not be too
>> much gain.
> 
> Sure, I can remove it.

Please leave it. It's a function call that cannot be optimized out 
otherwise.



-- 
Cheers,

David / dhildenb