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[109.81.87.110]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-48e6db01122sm87807305e9.6.2026.05.11.06.00.12 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 11 May 2026 06:00:13 -0700 (PDT) Date: Mon, 11 May 2026 15:00:11 +0200 From: Michal Hocko To: Muchun Song Cc: Muchun Song , Andrew Morton , Vlastimil Babka , linux-mm@kvack.org, Suren Baghdasaryan , Brendan Jackman , Johannes Weiner , Zi Yan , zihan zhou <15645113830zzh@gmail.com>, yaowenchao , linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm/page_alloc: Fix zone reserve update serialization Message-ID: References: <53EC8695-885F-4C47-A9E9-89CCF963F0F3@linux.dev> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <53EC8695-885F-4C47-A9E9-89CCF963F0F3@linux.dev> X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 321621C0022 X-Stat-Signature: 9bijqt74fop4piupf4hbc893oxrjdn6w X-Rspam-User: X-HE-Tag: 1778504416-295439 X-HE-Meta: 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 HPbY9r/a 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon 11-05-26 20:53:56, Muchun Song wrote: > > > > On May 11, 2026, at 20:33, Michal Hocko wrote: > > > > On Mon 11-05-26 20:04:09, Muchun Song wrote: > >> Serialize lowmem reserve and watermark updates with the same lock so > >> calculate_totalreserve_pages() cannot observe partially updated zone > >> reserve state. > > > > Could you describe the problem you are facing? > > To be more precise, commit 9726891fe753 moved > the call to setup_per_zone_lowmem_reserve into > adjust_managed_page_count. Since adjust_managed_page_count > can be executed concurrently across multiple CPUs > (especially during memory hotplug or parallel initialization), > I am concerned that this might lead to inconsistent updates for > the following counters: > > zone->lowmem_reserve > pgdat->totalreserve_pages > The global totalreserve_pages > > If these updates are not atomic or properly synchronized, > the resulting values could be inaccurate. This inconsistency > might cause issues for other kernel subsystems that rely on > these reserve counts for memory allocation and reclamation > decisions. > > Just to clarify, I noticed this potential issue while reviewing > the source code; it is not a bug I have encountered in a production > environment yet. This is important part that should be part of the changelog. Theoretical issue observed when reading the code. While it is really trivial to see that there is a race condition. It is much less obvious whether the race condition actually matters and worth fixing by introducing a new lock. So this needs much more explanation. I am not against the patch but the changelog is quite underdocumented. -- Michal Hocko SUSE Labs