From: Al Viro <viro@zeniv.linux.org.uk>
To: Breno Leitao <leitao@debian.org>
Cc: Christian Brauner <brauner@kernel.org>, Jan Kara <jack@suse.cz>,
linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
jlayton@kernel.org, rostedt@goodmis.org, kernel-team@meta.com
Subject: Re: [PATCH] fs/namei: Remove redundant DCACHE_MANAGED_DENTRY check in __follow_mount_rcu
Date: Mon, 5 Jan 2026 16:05:45 +0000 [thread overview]
Message-ID: <20260105160545.GK1712166@ZenIV> (raw)
In-Reply-To: <20260105-dcache-v1-1-f0d904b4a7c2@debian.org>
On Mon, Jan 05, 2026 at 07:10:27AM -0800, Breno Leitao wrote:
> The check for DCACHE_MANAGED_DENTRY at the start of __follow_mount_rcu()
> is redundant because the only caller (handle_mounts) already verifies
> d_managed(dentry) before calling this function, so, dentry in
> __follow_mount_rcu() has always DCACHE_MANAGED_DENTRY set.
... since 9d2a6211a7b9 "fs: tidy up step_into() & friends before inlining",
when the check got duplicated into the caller.
> This early-out optimization never fires in practice - but it is marking
> as likely().
>
> This was detected with branch profiling, which shows 100% misprediction
> in this likely.
>
> Remove the whole if clause instead of removing the likely, given we
> know for sure that dentry is not DCACHE_MANAGED_DENTRY.
AFAICS, that's OK, for the same reason why we didn't need barriers in
the original...
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
next prev parent reply other threads:[~2026-01-05 16:04 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-01-05 15:10 [PATCH] fs/namei: Remove redundant DCACHE_MANAGED_DENTRY check in __follow_mount_rcu Breno Leitao
2026-01-05 16:05 ` Al Viro [this message]
2026-01-06 22:51 ` Christian Brauner
2026-01-07 14:44 ` Mateusz Guzik
2026-01-07 16:07 ` Breno Leitao
2026-01-07 20:54 ` Al Viro
2026-01-08 9:42 ` Mateusz Guzik
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