linux-kernel - Re: [GIT PULL] fuse update for 6.19

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Message-ID: <20251206014242.GO1712166@ZenIV>
Date: Sat, 6 Dec 2025 01:42:42 +0000
From: Al Viro <viro@...iv.linux.org.uk>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Miklos Szeredi <miklos@...redi.hu>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [GIT PULL] fuse update for 6.19

On Fri, Dec 05, 2025 at 03:47:50PM -0800, Linus Torvalds wrote:
> On Thu, 4 Dec 2025 at 00:25, Miklos Szeredi <miklos@...redi.hu> wrote:
> >
> > The stale dentry cleanup has a patch touching dcache.c: this extracts
> > a helper from d_prune_aliases() that puts the unused dentry on a
> > dispose list.  Export this and shrink_dentry_list() to modules.
> 
> Is that
> 
>         spin_lock(&dentry->d_lock);
>         if (!dentry->d_lockref.count)
>                 to_shrink_list(dentry, dispose);
>         spin_unlock(&dentry->d_lock);
> 
> thing possibly hot, and count might be commonly non-zero?
> 
> Because it's possible that we could just make it a lockref operation
> where we atomically don't take the lock if the count is non-zero so
> that we don't unnecessarily move cachelines around...
> 
> IOW, some kind of "lockref_lock_if_zero()" pattern?
> 
> I have no idea what the fuse dentry lifetime patterns might be, maybe
> this is a complete non-issue...

Far more interesting question, IMO, is what's to prevent memory
pressure from evicting the damn argument right under us.

AFAICS, fuse_dentry_tree_work() calls that thing with no locks held.
The one and only reason why that's OK in d_prune_aliases() is ->i_lock
held over that thing - that's enough to prevent eviction.  I don't
see anything to serve the same purpose here.