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(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-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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 *unused) } static DECLARE_DELAYED_WORK(delayed_mntput_work, delayed_mntput); -static void mntput_no_expire(struct mount *mnt) +static void noinline mntput_no_expire_slowpath(struct mount *mnt) { LIST_HEAD(list); int count; - 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); } +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) { -- 2.48.1