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b=GDK4o+0Ip/5JV+tHpWOrpKA8LhGlkubU1oH0LvdADoYAmj522ddyfErcLtYdcOyp3 a/rlj6m5RDGC1DAm5L0c+kmLipV+wWSsOK29E4YGHxhPfYV6J527GVE6SIiHQxHIOp EKN5poE9LNIVtMZB5y4Jdg+2Nb+R9t4jEr6yJee7iBjHLu1jlF80gJtW8vXAeBvJRd BMgmYkjOCaPtoPHkuA2bkyASKzb/5fDjsfpZ73xpEzWYevp1LHoevAZFr4jxD8XmB4 mViEY8j74jENKP9RW4JCVihkPFf7pkruj3sGistboY3DAKdeJEiOVUZooiWtbuW3Ly 5cfBJ1A+hPlwA== Message-ID: Date: Tue, 10 Mar 2026 11:55:55 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird From: "Vlastimil Babka (SUSE)" Subject: Re: [PATCH] mm: optimize the implementation of WARN_ON_ONCE_GFP() Content-Language: en-US To: Xie Yuanbin , willy@infradead.org Cc: Liam.Howlett@oracle.com, akpm@linux-foundation.org, david.laight.linux@gmail.com, david@kernel.org, justinstitt@google.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, ljs@kernel.org, llvm@lists.linux.dev, mhocko@suse.com, morbo@google.com, nathan@kernel.org, nick.desaulniers+lkml@gmail.com, rppt@kernel.org, surenb@google.com References: <20260309155933.41179-1-qq570070308@gmail.com> In-Reply-To: <20260309155933.41179-1-qq570070308@gmail.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Queue-Id: 5517440007 X-Rspamd-Server: rspam08 X-Stat-Signature: piq1it13x45bcykz13jy8z88be6hwot3 X-HE-Tag: 1773140161-341463 X-HE-Meta: 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 L8EfLfi/ pfuB9vkU3gpgi895jgqEEo+sbkPht8Lu8nzWr31wtebTpAcm3A0pM72f6qBN8PeneonyiGePJsAxNkx8fyCyzvRLMi8org/qJaXkdMFdzJvKNh5UacpYasRswkp1fb+9AhvxLYjM5sbhXY/0zjlXruZAGeYNcoBgPpHA/u2TBj4YYmDGgbIP9x9u6S/ek5wv4/UBtEsRZJglFvaX4kfZoBd7fMiPOY5xcLOfoxnjYfMGiEfFZA31Tn5JGlhqOGAI6YCEma7y94qOkCsECw2MAQiON4T25TgwG/M4fYis3pJd9v8FHHhf5IjJQmXTQ7aMkhDSzOzHJzDqplj70P6ODg/6gpC2muKX6xRFSqe1WA9odhQgEkUfMIG129vgF2i3QTHnMFQxUWK0a/TywWvEKKg1Ihg== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 3/9/26 16:59, Xie Yuanbin wrote: > On Mon, 9 Mar 2026 15:40:13 +0000, Matthew Wilcox wrote: >>On Mon, Mar 09, 2026 at 11:38:11PM +0800, Xie Yuanbin wrote: >>> As shown in the commit message of commit 242b872239f6a7deacbc >>> ("include/linux/once_lite.h: fix judgment in WARN_ONCE with clang"), >>> the code "unlikely(a && b)" may generate poor assembly code if it is >>> actually "unlikely(a) && unlikely(b)" or "unlikely(a) && b". >> >> Why fix this in multiple places in the kernel instead of once in clang? > > If a and b is both unlikely, then "unlikely(a) && unlikely(b)" will > generate better code than "unlikely(a && b)". This is also true for gcc. What are the details of how it's better for gcc? > As for the issue of clang judging twice, I have already submitted it to > clang: > Link: https://github.com/llvm/llvm-project/issues/167117 > However, even if clang fixes it, this optimization will not be merged > back to the old version of clang. That's life and not worth complicating the kernel code for. This is not about making it functional, only about perf.