linux-kernel - Re: [PATCH] mm: remove an avoidable load of page refcount in page_ref_add

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Message-ID: <94d0dcbe-2001-4a9c-a767-b337b688b616@redhat.com>
Date: Mon, 9 Dec 2024 10:28:46 +0100
From: David Hildenbrand <david@...hat.com>
To: Mateusz Guzik <mjguzik@...il.com>, yuzhao@...gle.com
Cc: akpm@...ux-foundation.org, willy@...radead.org,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm: remove an avoidable load of page refcount in
 page_ref_add_unless

On 07.12.24 09:29, Mateusz Guzik wrote:
> Explicitly pre-checking the count adds nothing as atomic_add_unless
> starts with doing the same thing. iow no functional changes.

I recall that we added that check because with the hugetlb vmemmap 
optimization, some of the tail pages we don't ever expect to be modified 
  (because they are fake-duplicated) might be mapped R/O.

If the arch implementation of atomic_add_unless() would trigger an 
unconditional write fault, we'd be in trouble. That would likely only be 
the case if the arch provides a dedicate instruction.

atomic_add_unless()->raw_atomic_add_unless()

Nobody currently defines arch_atomic_add_unless().

raw_atomic_fetch_add_unless()->arch_atomic_fetch_add_unless() is defined 
on some architectures.

I scanned some of the inline-asm, and I think most of them perform a 
check first.

So this currently looks good to me, but we'll rely on the fact that 
atomic_add_unless() will never trigger a write fault if the values 
match. Which makes me wonder if we should document that behavior of 
atomic_add_unless().

-- 
Cheers,

David / dhildenb