linux-kernel - Re: [PATCH v3 20/21] __dentry

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Message-ID: <CAKPOu+9=AV-NxJYXjwiUL4iXPH=oUSF25+6t25M8ujfj2OvHVQ@mail.gmail.com>
Date: Tue, 8 Jul 2025 06:45:14 +0200
From: Max Kellermann <max.kellermann@...os.com>
To: Al Viro <viro@...iv.linux.org.uk>
Cc: linux-fsdevel@...r.kernel.org, Christian Brauner <brauner@...nel.org>, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 20/21] __dentry_kill(): new locking scheme

On Tue, Jul 8, 2025 at 12:37 AM Al Viro <viro@...iv.linux.org.uk> wrote:
> You are asked to evict everything evictable in there.  It would be rather odd
> if you ended up with some dentries sticking around (_still_ with refcounts
> equal to the number of their surviving children) just because in the middle
> of your work a memory pressure had been applied and started evicting one of
> the leaves in that tree (none of them busy, all leaves have refcount 0, so
> all of them are evictable).

They are not evictable, or else you'd be evicting them, but you do not.
Instead, you busy-wait for the dying dentry to disappear. (Which can
take a loooong time)

Your explanations do make sense, I understand them, and I think I'm
getting a slight understanding of the dcache code. But you haven't
even tried to argue why you implemented busy-waiting in this patch.
I believe the busy-wait was accidental.
I've been trying to make you aware that this is effectively a
busy-wait, one that can take a long time burning CPU cycles, but I
have a feeling I can't reach you.

Al, please confirm that it was your intention to busy-wait until dying
dentries disappear!