linux-kernel - Re: [PATCH] vfs: avoid spurious dentry ref/unref cycle on open

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Message-ID: <CAGudoHE1dPb4m=FsTPeMBiqittNOmFrD-fJv9CmX8Nx8_=njcQ@mail.gmail.com>
Date: Wed, 7 Aug 2024 09:59:33 +0200
From: Mateusz Guzik <mjguzik@...il.com>
To: Al Viro <viro@...iv.linux.org.uk>
Cc: brauner@...nel.org, jack@...e.cz, linux-kernel@...r.kernel.org, 
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] vfs: avoid spurious dentry ref/unref cycle on open

On Wed, Aug 7, 2024 at 9:52 AM Al Viro <viro@...iv.linux.org.uk> wrote:
>
> On Wed, Aug 07, 2024 at 09:22:59AM +0200, Mateusz Guzik wrote:
>
> > Well it's your call, you wrote the thing and I need the problem out of
> > the way, so I'm not going to argue about the patchset.
> >
> > I verified it boots and provides the expected perf win [I have to
> > repeat it is highly variable between re-runs because of ever-changing
> > offsets between different inode allocations resulting in different
> > false-sharing problems; i'm going to separately mail about that]
> >
> > I think it will be fine to copy the result from my commit message and
> > denote it's from a different variant achieving the same goal.
> >
> > That said feel free to use my commit message in whatever capacity,
> > there is no need to mention me.
>
> Original analysis had been yours, same for "let's change the calling
> conventions for do_dentry_open() wrt path refcounting", same for
> the version I'd transformed into that...  FWIW, my approach to
> that had been along the lines of "how do we get it consistently,
> whether we go through vfs_open() or finish_open()", which pretty
> much required keeping hold on the path until just before
> terminate_walk().  This "transfer from nd->path to whatever borrowed
> it" was copied from path_openat() (BTW, might be worth an inlined helper
> next to terminate_walk(), just to document that it's not an accidental
> property of terminate_walk()) and that was pretty much it.
>
> Co-developed-by: seems to be the usual notation these days for
> such situations - that really had been incremental changes.
>
> Anyway, I really need to get some sleep before writing something
> usable as commit messages...

Nobody is getting a Turing award for noticing the extra ref trip and eliding it.

Co-developed-by is fine with me if you insist on sharing credit.

My only objective here is to expedite the fix so that I can get on
with speeding up refcount management. :)
-- 
Mateusz Guzik <mjguzik gmail.com>