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Date: Wed, 13 Dec 2023 15:01:28 +0000
From: Matthew Wilcox <willy@infradead.org>
To: Yosry Ahmed <yosryahmed@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Michal Hocko <mhocko@kernel.org>,
	Roman Gushchin <roman.gushchin@linux.dev>,
	Shakeel Butt <shakeelb@google.com>,
	Muchun Song <muchun.song@linux.dev>, cgroups@vger.kernel.org,
	linux-mm@kvack.org
Subject: Re: [PATCH] mm: memcg: remove direct use of
 __memcg_kmem_uncharge_page
Message-ID: <ZXnHSPuaVW913iVZ@casper.infradead.org>
References: <20231213130414.353244-1-yosryahmed@google.com>
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In-Reply-To: <20231213130414.353244-1-yosryahmed@google.com>

On Wed, Dec 13, 2023 at 01:04:14PM +0000, Yosry Ahmed wrote:
> memcg_kmem_uncharge_page() is an inline wrapper around
> __memcg_kmem_uncharge_page() that checks memcg_kmem_online() before
> making the function call. Internally, __memcg_kmem_uncharge_page() has a
> folio_memcg_kmem() check.
> 
> The only direct user of __memcg_kmem_uncharge_page(),
> free_pages_prepare(), checks PageMemcgKmem() before calling it to avoid
> the function call if possible. Move the folio_memcg_kmem() check from
> __memcg_kmem_uncharge_page() to memcg_kmem_uncharge_page() as
> PageMemcgKmem() -- which does the same thing under the hood. Now
> free_pages_prepare() can also use memcg_kmem_uncharge_page().

I think you've just pessimised all the other places which call
memcg_kmem_uncharge_page().  It's a matter of probabilities.  In
free_pages_prepare(), most of the pages being freed are not accounted
to memcg.  Whereas in fork() we are absolutely certain that the pages
were accounted because we accounted them.

I think this is a bad change.