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Thu, 14 Aug 2025 13:12:15 -0700 (PDT) Date: Thu, 14 Aug 2025 16:12:11 -0400 From: Johannes Weiner To: Thadeu Lima de Souza Cascardo Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton , Vlastimil Babka , Suren Baghdasaryan , Michal Hocko , Brendan Jackman , Zi Yan , Mel Gorman , kernel-dev@igalia.com, Helen Koike , Matthew Wilcox , NeilBrown , Thierry Reding Subject: Re: [PATCH] mm/page_alloc: only set ALLOC_HIGHATOMIC for __GPF_HIGH allocations Message-ID: <20250814201211.GI115258@cmpxchg.org> References: <20250814172245.1259625-1-cascardo@igalia.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250814172245.1259625-1-cascardo@igalia.com> X-Rspamd-Queue-Id: D2B0318000D X-Stat-Signature: unxrsr9sm4dzf5c5wt7by63r5n9tigby X-Rspam-User: X-Rspamd-Server: rspam06 X-HE-Tag: 1755202336-399247 X-HE-Meta: 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 W66CiJ7j 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Hello Thadeu, On Thu, Aug 14, 2025 at 02:22:45PM -0300, Thadeu Lima de Souza Cascardo wrote: > Commit 524c48072e56 ("mm/page_alloc: rename ALLOC_HIGH to > ALLOC_MIN_RESERVE") is the start of a series that explains how __GFP_HIGH, > which implies ALLOC_MIN_RESERVE, is going to be used instead of > __GFP_ATOMIC for high atomic reserves. > > Commit eb2e2b425c69 ("mm/page_alloc: explicitly record high-order atomic > allocations in alloc_flags") introduced ALLOC_HIGHATOMIC for such > allocations of order higher than 0. It still used __GFP_ATOMIC, though. > > Then, commit 1ebbb21811b7 ("mm/page_alloc: explicitly define how __GFP_HIGH > non-blocking allocations accesses reserves") just turned that check for > !__GFP_DIRECT_RECLAIM, ignoring that high atomic reserves were expected to > test for __GFP_HIGH. It indeed looks accidental. From the cover letter, High-order atomic allocations are explicitly handled with the caveat that no __GFP_ATOMIC flag means that any high-order allocation that specifies GFP_HIGH and cannot enter direct reclaim will be treated as if it was GFP_ATOMIC. it sounds like the intent was what your patch does, and not to extend those privileges to anybody who is !gfp_direct_reclaim. > This leads to high atomic reserves being added for high-order GFP_NOWAIT > allocations and others that clear __GFP_DIRECT_RECLAIM, which is > unexpected. Later, those reserves lead to 0-order allocations going to the > slow path and starting reclaim. Can you please provide more background on the workload and the environment in which you observed this? Which GFP_NOWAIT requests you saw participating in the reserves etc. I would feel better with Mel or Vlastimil chiming in as well, but your fix looks correct to me.