Re: [PATCH] mm: remove an avoidable load of page refcount in page_ref_add_unless

[David Hildenbrand <david@redhat.com>]

On 09.12.24 15:30, Mateusz Guzik wrote:
> On Mon, Dec 9, 2024 at 3:22 PM David Hildenbrand <david@redhat.com> wrote:
>>
>> On 09.12.24 13:33, Mateusz Guzik wrote:
>>> That is to say I think this thread just about exhausted the time
>>> warranted by this patch. No hard feelz if it gets dropped, but then I
>>> do strongly suggest adding a justification to the extra load.
>>
>> Maybe it's sufficient for now to simply do your change with a comment:
>>
>> diff --git a/include/linux/page_ref.h b/include/linux/page_ref.h
>> index 8c236c651d1d6..1efc992ad5687 100644
>> --- a/include/linux/page_ref.h
>> +++ b/include/linux/page_ref.h
>> @@ -234,7 +234,13 @@ static inline bool page_ref_add_unless(struct page *page, int nr, int u)
>>
>>           rcu_read_lock();
>>           /* avoid writing to the vmemmap area being remapped */
>> -       if (!page_is_fake_head(page) && page_ref_count(page) != u)
>> +       if (!page_is_fake_head(page))
>> +               /*
>> +                * atomic_add_unless() will currently never modify the value
>> +                * if it already is u. If that ever changes, we'd have to have
>> +                * a separate check here, such that we won't be writing to
>> +                * write-protected vmemmap areas.
>> +                */
>>                   ret = atomic_add_unless(&page->_refcount, nr, u);
>>           rcu_read_unlock();
>>
>>
>> It would bail out during testing ... hopefully, such that we can detect any such change.
>>
> 
> Not my call to make, but looks good. ;)
> 
> fwiw I don't need any credit and I would be more than happy if you
> just submitted the thing as your own without me being mentioned. *No*
> cc would also be appreciated.

Likely Andrew can add the comment as a fixup.

-- 
Cheers,

David / dhildenb