linux-kernel - Re: [PATCH 1/2] kernfs: add kernfs_ops.free operation to free resources tied to the file

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Message-ID: <ZJseFr/BHh+bZ7NM@casper.infradead.org>
Date:   Tue, 27 Jun 2023 18:36:22 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     Greg KH <gregkh@...uxfoundation.org>, tj@...nel.org,
        peterz@...radead.org, lujialin4@...wei.com,
        lizefan.x@...edance.com, hannes@...xchg.org, mingo@...hat.com,
        ebiggers@...nel.org, oleg@...hat.com, akpm@...ux-foundation.org,
        viro@...iv.linux.org.uk, brauner@...nel.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, vschneid@...hat.com,
        linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH 1/2] kernfs: add kernfs_ops.free operation to free
 resources tied to the file

On Tue, Jun 27, 2023 at 10:03:15AM -0700, Suren Baghdasaryan wrote:
> On Mon, Jun 26, 2023 at 11:25 PM Greg KH <gregkh@...uxfoundation.org> wrote:
> >
> > On Mon, Jun 26, 2023 at 01:17:12PM -0700, Suren Baghdasaryan wrote:
> > > kernfs_ops.release operation can be called from kernfs_drain_open_files
> > > which is not tied to the file's real lifecycle. Introduce a new kernfs_ops
> > > free operation which is called only when the last fput() of the file is
> > > performed and therefore is strictly tied to the file's lifecycle. This
> > > operation will be used for freeing resources tied to the file, like
> > > waitqueues used for polling the file.
> >
> > This is confusing, shouldn't release be the "last" time the file is
> > handled and then all resources attached to it freed?  Why do we need
> > another callback, shouldn't release handle this?
> 
> That is what I thought too but apparently kernfs_drain_open_files()
> can also cause ops->release to be called while the file keeps on
> living (see details here:
> https://lore.kernel.org/all/CAJuCfpFZ3B4530TgsSHqp5F_gwfrDujwRYewKReJru==MdEHQg@mail.gmail.com/#t).

If we're splitting these two functions apart, can we use the same naming
as the VFS please?

``flush``
        called by the close(2) system call to flush a file

``release``
        called when the last reference to an open file is closed