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Received: from willy by casper.infradead.org with local (Exim 4.98.2 #2 (Red Hat Linux)) id 1vneKw-00000002EFm-1nY5; Wed, 04 Feb 2026 14:58:34 +0000 Date: Wed, 4 Feb 2026 14:58:34 +0000 From: Matthew Wilcox To: Aswin Kumar Cc: linux-mm@kvack.org, andrew.morton@linux-foundation.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm/userfaultfd: fix likely/unlikely annotation in move_pages() Message-ID: References: <20260204142350.1642-1-aswinkumar3301@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260204142350.1642-1-aswinkumar3301@gmail.com> X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 3BF8318000A X-Stat-Signature: gpjqu98feebdncah913xc7s3apbtdpci X-Rspam-User: X-HE-Tag: 1770217116-150181 X-HE-Meta: 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 kHp41byj bcKhDoffwBxoWhSOOrUKJ1mDD6nOZEEVhTTWgNmTeM4PvKd3dYLheq8N9Iy0rISfSaVJ5S57vTwAOkpHZRMS979Z7nm19xfmdk2Tu7BAZpV6+C7MTRBfYCqS9B6pAiFUecBeU+GC2Ct0daddDPiWw00m8//T/QfC8F8oGnq/hH5Cf53SrOXvUHEa0daIScbGWA5Acv7QiKz7tVfVkBB+D2rt58cj+PUruWqoVVg0rGxrd+kbZnKy13lOYCXiC205nE7YVSGO0z4Mk1yArhu5wGSNscLSEWvsd6vOAH8r3Bc7AEZA= 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: On Wed, Feb 04, 2026 at 02:23:50PM +0000, Aswin Kumar wrote: > All other userfaultfd paths use unlikely() for the mmap_changing check, No they don't? $ git grep mmap_changing mm mm/userfaultfd.c: if (atomic_read(&ctx->mmap_changing)) mm/userfaultfd.c: if (atomic_read(&ctx->mmap_changing)) mm/userfaultfd.c: if (atomic_read(&ctx->mmap_changing)) mm/userfaultfd.c: if (likely(atomic_read(&ctx->mmap_changing))) > This is a performance-only fix - the logic is correct but the branch > prediction annotation is wrong, potentially causing a minor performance > penalty on the fast path. Can you measure it? Bet you can't. Honestly, I'd just remove the likely() annotation, not change it to unlikely().