From nobody Sat Feb 7 17:56:02 2026 Received: from mail-wr1-f51.google.com (mail-wr1-f51.google.com [209.85.221.51]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 642692F0C74 for ; Fri, 14 Nov 2025 20:18:08 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.221.51 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1763151490; cv=none; b=R4WUkU4T4akJJ1ZOF0wj2nf215Iz30HhEDfk22lUi03qNAA0pZIb+6f5Q5LFQiJKcS4U1IV9UfHf4Sz3DOgaLatTnpscIvxNG+NzFnCZ32KmBpwe4tc/OfMFU8eP/O8Pee0lboAgxvphDBICgKOm8kmpeCL1Rs7Syi+2IT64AcI= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1763151490; c=relaxed/simple; bh=kKvy37QW1/3BR8fdYquPuqP9EYS4mty39BS6D4AcBpA=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version; b=dJa0y7wivCqTze6uKDbHTs2X45ZXDaaMn6F93txXHcc42jHRVrinLUe2XJPI9w0uEsPVXeevGIbi2UCE2JehyVbjjmFFji6rPkVVCBKEOIqmMuJp2EM18KLKiqlIoHX1ucQdSNAPABvwqGgdyz3op1grqE4NynyTAOAAnIejDYM= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com; spf=pass smtp.mailfrom=gmail.com; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b=ggiVU6Pv; arc=none smtp.client-ip=209.85.221.51 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=gmail.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="ggiVU6Pv" Received: by mail-wr1-f51.google.com with SMTP id ffacd0b85a97d-42b566859ecso1642105f8f.2 for ; Fri, 14 Nov 2025 12:18:08 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20230601; t=1763151487; x=1763756287; darn=vger.kernel.org; h=content-transfer-encoding:mime-version:message-id:date:subject:cc :to:from:from:to:cc:subject:date:message-id:reply-to; bh=RH2DEqpNSuBdy+arsRdO9BnWsTVg860XrhzdhL1pOfA=; b=ggiVU6PvBcXszn/H13V2VpmhvGHUbeQX616doKn13MxiQgw7ADA09wcDBVl/m3kWgb SKcpiN4Abyyhn2n5AbgM1qlyGthi9TJXwRGhM7Q1QyoBXbDZMSI30ZJma85AqDFzIx9X tINJJotmWXeg0KQ9zbqEkT15JJr2OjbLlGeNBlMH+5494Ygn8anHqp8lY1GxFzddrjjz v8uYe2FTPBMQKVOLi6ACQo1wbh89o5becKU2D1NnQiTsbOcYj6ypWElBrRQrH+uHAIY5 +NjjbdkU8hGJSt95ma4OlLLa7FZKHo4wX20U99ASHsWHkcF+mzYhL0bZQNx8dNaJ2/kh mU1g== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1763151487; x=1763756287; h=content-transfer-encoding:mime-version:message-id:date:subject:cc :to:from:x-gm-gg:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to; bh=RH2DEqpNSuBdy+arsRdO9BnWsTVg860XrhzdhL1pOfA=; b=J1QO1MmzEI8PcWIpaQj+YWnCKSf1pu/l2HZPcSPDE4RAH5tQ5/xMoaB+rVWnbeZd4P Esgx7Kgz1IW1hXiSNGVVLOaXH5vvavn9vfyMWqCB9VV8/bIpLR2yua2bEnlBBG3mdcWk 0jeX7WpTCBVY0dpVViwLvKjZThdZ80xzXWLz9g4OFoai4QBPTyWw+z2sdWlKrm9FpgCo k2OUKiwoeb6S2KN2dIbMyI1Hn0Smx4c5jQAEeJX/q2eoaFd/tR3dlDk7aPC11HMGII4a WVjEh2uAKnmYaH0ihsmJtU+xNk0KbJwE6LqPsnkyo0/eBTqHelPnLjM89Wc/5D888SXH KT9Q== X-Forwarded-Encrypted: i=1; AJvYcCV3dtXfz5fnTXvzDLavudINtA6WASPOQp1LZdeiURmp/zI93I0bI10FD6lNYRN8v43MmyqW8o32fmGt/p4=@vger.kernel.org X-Gm-Message-State: AOJu0Yz6r7XToMEvLDu/mA4Ft/a/41GS1tSPdFAIxiDYD07MbwwHYnYP Kh4gffeAeS/lRIC5RA2x815EjvAcyRL8mrHPmfKahaF/6JhCnkJeyNh/ X-Gm-Gg: ASbGncsgPsHXpllJQZxVC2sygAOSxlSpD75hNtPwqMfUvVfEen+wWjE6QlqY/BDqcez 6B+L908pwXwmfUzSZJemd9Q9hf258KL8c8eWJD3Itnx38KFw1v0omVJRyKAY+EcCRCnJn8fak4U IxZONcQkwXBL5huI3clYZZctOFHiRS3+Rn+xiRQcZ30/gZOqV5yrVjdvRF2BEG0HEt5K2+ZkNVZ CpfTAms+ORajRRo+41GJju/K7nDo9Ofz/Jr8A8E/yU75ABGCLC4i7Eds1mTA1cebKzYOvjjhVJ/ WcJY0c+SgbF/eM1fgcEOusTSltt7FVxfhaAigz1xuU2UrfIcBTtkfPTh9KxkeRc4keZ/gCbgmNv uotV7dCGXCntMBWJe1tPqsN4CIz617lSqUjlVu2SaHsO4SQ+sAPeE52kNQizjgEaQETVGc1fSTA 3XcbDh9lMtON3WrE5yxS16IZNkxlJPtm9CZ856kRi+0kSj+b9a2zwzkUcp7HU= X-Google-Smtp-Source: AGHT+IEi/ByZwNTkmJ4EcrnRiPDFN+AfIVbFmG1jNcMXN07oSP3veXjXBSY+4y2d2Bmdx4mTbv6+1w== X-Received: by 2002:a05:6000:1842:b0:42b:3dfb:645f with SMTP id ffacd0b85a97d-42b59394e14mr4014548f8f.47.1763151486271; Fri, 14 Nov 2025 12:18:06 -0800 (PST) Received: from f.. (cst-prg-14-82.cust.vodafone.cz. [46.135.14.82]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-42b53e84a4fsm11436112f8f.11.2025.11.14.12.18.05 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 14 Nov 2025 12:18:05 -0800 (PST) From: Mateusz Guzik To: viro@zeniv.linux.org.uk Cc: brauner@kernel.org, jack@suse.cz, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, Mateusz Guzik Subject: [PATCH] fs: move mntput_no_expire() slowpath into a dedicated routine Date: Fri, 14 Nov 2025 21:18:03 +0100 Message-ID: <20251114201803.2183505-1-mjguzik@gmail.com> X-Mailer: git-send-email 2.43.0 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" In the stock variant the compiler spills several registers on the stack and employs stack smashing protection, adding even more code + a branch on exit.. The actual fast path is small enough that the compiler inlines it for all callers -- the symbol is no longer emitted. Forcing noinline on it just for code-measurement purposes shows the fast path dropping from 111 to 39 bytes. Signed-off-by: Mateusz Guzik --- fast path prior: call ffffffff81374630 <__fentry__> push %r15 push %r14 push %r13 push %r12 push %rbp push %rbx sub $0x18,%rsp mov %gs:0x2deef5d(%rip),%rbx # ffffffff8454f008 <__stack_chk_= guard> mov %rbx,0x10(%rsp) mov %rdi,%rbx mov %rsp,(%rsp) mov %rsp,0x8(%rsp) call ffffffff814615f0 <__rcu_read_lock> mov 0xe8(%rbx),%rax test %rax,%rax je ffffffff817600ff mov 0x58(%rbx),%rax decl %gs:(%rax) call ffffffff81466810 <__rcu_read_unlock> mov 0x10(%rsp),%rax sub %gs:0x2deef22(%rip),%rax # ffffffff8454f008 <__stack_chk_= guard> jne ffffffff8176030b add $0x18,%rsp pop %rbx pop %rbp pop %r12 pop %r13 pop %r14 pop %r15 jmp ffffffff823091f0 <__pi___x86_return_thunk> after (when forced to be out-of-line): call ffffffff81374630 <__fentry__> push %rbx mov %rdi,%rbx call ffffffff814615f0 <__rcu_read_lock> mov 0xe8(%rbx),%rax test %rax,%rax je ffffffff81760347 mov 0x58(%rbx),%rax decl %gs:(%rax) pop %rbx jmp ffffffff81466810 <__rcu_read_unlock> fs/namespace.c | 38 ++++++++++++++++++++++---------------- 1 file changed, 22 insertions(+), 16 deletions(-) diff --git a/fs/namespace.c b/fs/namespace.c index e8f1fe4bca06..6af6b082043c 100644 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -1346,26 +1346,12 @@ static void delayed_mntput(struct work_struct *unus= ed) } static DECLARE_DELAYED_WORK(delayed_mntput_work, delayed_mntput); =20 -static void mntput_no_expire(struct mount *mnt) +static void noinline mntput_no_expire_slowpath(struct mount *mnt) { LIST_HEAD(list); int count; =20 - rcu_read_lock(); - if (likely(READ_ONCE(mnt->mnt_ns))) { - /* - * Since we don't do lock_mount_hash() here, - * ->mnt_ns can change under us. However, if it's - * non-NULL, then there's a reference that won't - * be dropped until after an RCU delay done after - * turning ->mnt_ns NULL. So if we observe it - * non-NULL under rcu_read_lock(), the reference - * we are dropping is not the final one. - */ - mnt_add_count(mnt, -1); - rcu_read_unlock(); - return; - } + VFS_BUG_ON(mnt->mnt_ns); lock_mount_hash(); /* * make sure that if __legitimize_mnt() has not seen us grab @@ -1416,6 +1402,26 @@ static void mntput_no_expire(struct mount *mnt) cleanup_mnt(mnt); } =20 +static void mntput_no_expire(struct mount *mnt) +{ + rcu_read_lock(); + if (likely(READ_ONCE(mnt->mnt_ns))) { + /* + * Since we don't do lock_mount_hash() here, + * ->mnt_ns can change under us. However, if it's + * non-NULL, then there's a reference that won't + * be dropped until after an RCU delay done after + * turning ->mnt_ns NULL. So if we observe it + * non-NULL under rcu_read_lock(), the reference + * we are dropping is not the final one. + */ + mnt_add_count(mnt, -1); + rcu_read_unlock(); + return; + } + mntput_no_expire_slowpath(mnt); +} + void mntput(struct vfsmount *mnt) { if (mnt) { --=20 2.48.1