linux-kernel - Re: NFS dentry caching regression? was Re: [GIT PULL] Please pull NFS client updates for Linux 6.19

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Message-ID: <aUJ9hliJJarv23Uj@infradead.org>
Date: Wed, 17 Dec 2025 01:53:10 -0800
From: Christoph Hellwig <hch@...radead.org>
To: Trond Myklebust <trondmy@...nel.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>,
	linux-fsdevel@...r.kernel.org,
	Anna Schumaker <anna.schumaker@...cle.com>
Subject: Re: NFS dentry caching regression? was Re: [GIT PULL] Please pull
 NFS client updates for Linux 6.19

On Wed, Dec 17, 2025 at 01:32:30AM -0800, Christoph Hellwig wrote:
> Hi all,
> 
> the merge of this branch causes NFS lookup operation to shoot up a lot
> for me.  And with merge I mean merge - both parent of the merge on their
> own are fine.
> 
> With the script below that simulates running python scripts with lots
> of imports that was created to benchmark delegation performance, the
> number of lookups in the measurement period shoots up from 4 to about
> 410000, which is a bit suboptimal.  I have no idea how this could
> happen, but it must be related to some sort of pathname lookup changes
> I guess.  Other operations looks roughly the same.

To further pin this down, I rebased the patches from the NFS pull request
on top of the baseline (d358e5254674b70f34c847715ca509e46eb81e6f) and
bisected that.  This ends up in:

NFS: Shortcut lookup revalidations if we have a directory delegation