linux-kernel - Re: [PATCH v3] fs: try an opportunistic lookup for O

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Message-ID: <20240807142428.GC242945@perftesting>
Date: Wed, 7 Aug 2024 10:24:28 -0400
From: Josef Bacik <josef@...icpanda.com>
To: Jeff Layton <jlayton@...nel.org>
Cc: Alexander Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <ak@...ux.intel.com>, Mateusz Guzik <mjguzik@...il.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] fs: try an opportunistic lookup for O_CREAT opens too

On Wed, Aug 07, 2024 at 08:10:27AM -0400, Jeff Layton wrote:
> Today, when opening a file we'll typically do a fast lookup, but if
> O_CREAT is set, the kernel always takes the exclusive inode lock. I
> assume this was done with the expectation that O_CREAT means that we
> always expect to do the create, but that's often not the case. Many
> programs set O_CREAT even in scenarios where the file already exists.
> 
> This patch rearranges the pathwalk-for-open code to also attempt a
> fast_lookup in certain O_CREAT cases. If a positive dentry is found, the
> inode_lock can be avoided altogether, and if auditing isn't enabled, it
> can stay in rcuwalk mode for the last step_into.
> 
> One notable exception that is hopefully temporary: if we're doing an
> rcuwalk and auditing is enabled, skip the lookup_fast. Legitimizing the
> dentry in that case is more expensive than taking the i_rwsem for now.
> 
> Signed-off-by: Jeff Layton <jlayton@...nel.org>

Reviewed-by: Josef Bacik <josef@...icpanda.com>

Thanks,

Josef