linux-kernel - Re: [PATCH v1] mm/rmap: cleanup partially-mapped handling in __folio_remove

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Message-ID: <25a87009-5318-457b-926b-69185767b274@redhat.com>
Date: Wed, 10 Jul 2024 23:40:16 +0200
From: David Hildenbrand <david@...hat.com>
To: Yosry Ahmed <yosryahmed@...gle.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v1] mm/rmap: cleanup partially-mapped handling in
 __folio_remove_rmap()

On 24.06.24 13:48, Yosry Ahmed wrote:
> On Tue, Jun 18, 2024 at 8:10 AM David Hildenbrand <david@...hat.com> wrote:
>>
>> Let's simplify and reduce code indentation. In the RMAP_LEVEL_PTE case, we
>> already check for nr when computing partially_mapped.
>>
>> For RMAP_LEVEL_PMD, it's a bit more confusing. Likely, we don't need the
>> "nr" check, but we could have "nr < nr_pmdmapped" also if we stumbled
>> into the "/* Raced ahead of another remove and an add? */" case. So
>> let's simply move the nr check in there.
>>
>> Note that partially_mapped is always false for small folios.
>>
>> No functional change intended.
>>
>> Cc: Yosry Ahmed <yosryahmed@...gle.com>
>> Signed-off-by: David Hildenbrand <david@...hat.com>
> 
> Makes sense to me. FWIW:
> 
> Reviewed-by: Yosry Ahmed <yosryahmed@...gle.com>
> 
>> ---
>>   mm/rmap.c | 10 ++++------
>>   1 file changed, 4 insertions(+), 6 deletions(-)
>>
>> diff --git a/mm/rmap.c b/mm/rmap.c
>> index afec4b6800caf..aa900e46cdf82 100644
>> --- a/mm/rmap.c
>> +++ b/mm/rmap.c
>> @@ -1551,11 +1551,12 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
>>                          }
>>                  }
>>
>> -               partially_mapped = nr < nr_pmdmapped;
>> +               partially_mapped = nr && nr < nr_pmdmapped;
>>                  break;
>>          }
>>
>> -       if (nr) {
>> +       if (partially_mapped && folio_test_anon(folio) &&
>> +           list_empty(&folio->_deferred_list))
>>                  /*
>>                   * Queue anon large folio for deferred split if at least one
>>                   * page of the folio is unmapped and at least one page
>> @@ -1563,10 +1564,7 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
>>                   *
>>                   * Check partially_mapped first to ensure it is a large folio.
>>                   */
>> -               if (folio_test_anon(folio) && partially_mapped &&
>> -                   list_empty(&folio->_deferred_list))
>> -                       deferred_split_folio(folio);
>> -       }
>> +               deferred_split_folio(folio);
> 
> FWIW, I prefer moving the comment out of the one-line if block as you
> originally suggested in [1].
> 
> [1]https://lore.kernel.org/lkml/1a408ed1-7e81-457e-a205-db274b4d6b78@redhat.com/

Sorry for the late reply, crazy times here.

Let me do that and resend, thanks!

-- 
Cheers,

David / dhildenb