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Mon, 09 Mar 2026 08:38:25 -0700 (PDT) Received: from DESKTOP-3LEPQG8.localdomain ([119.28.20.50]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-829a48a54cesm10516486b3a.48.2026.03.09.08.38.18 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 09 Mar 2026 08:38:25 -0700 (PDT) From: Xie Yuanbin To: rppt@kernel.org, david.laight.linux@gmail.com, akpm@linux-foundation.org, david@kernel.org, ljs@kernel.org, Liam.Howlett@oracle.com, vbabka@kernel.org, surenb@google.com, mhocko@suse.com, nathan@kernel.org, nick.desaulniers+lkml@gmail.com, morbo@google.com, justinstitt@google.com Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, llvm@lists.linux.dev, Xie Yuanbin Subject: [PATCH] mm: optimize the implementation of WARN_ON_ONCE_GFP() Date: Mon, 9 Mar 2026 23:38:11 +0800 Message-ID: <20260309153811.40958-1-qq570070308@gmail.com> X-Mailer: git-send-email 2.53.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" 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". WARN_ON_ONCE_GFP() may be used in the hot path code: 1. The argument cond shoud be unlikely. 2. When "1." is true, !(gfp & __GFP_NOWARN) is unlikely, otherwise, a WARN may be triggered, which is a very unlikely case. 3. When "1. && 2." is true, just like the implementation of WARN_ONCE(), !__warned can be unlikely. Reorder __ret_warn_once judgement to first and split out the unlikely() in WARN_ON_ONCE_GFP() to optimize performance. Cc: Mike Rapoport Cc: David Laight Cc: Andrew Morton Signed-off-by: Xie Yuanbin --- mm/internal.h | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/mm/internal.h b/mm/internal.h index 6e1162e13289..49d2b7a270d3 100644 --- a/mm/internal.h +++ b/mm/internal.h @@ -93,7 +93,8 @@ struct pagetable_move_control { static bool __section(".data..once") __warned; \ int __ret_warn_once =3D !!(cond); \ \ - if (unlikely(!(gfp & __GFP_NOWARN) && __ret_warn_once && !__warned)) { \ + if (unlikely(__ret_warn_once) && \ + unlikely(!(gfp & __GFP_NOWARN)) && unlikely(!__warned)) { \ __warned =3D true; \ WARN_ON(1); \ } \ --=20 2.53.0