linux-kernel - Re: [PATCH] fs: read_write: make default in vfs_copy_file

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Message-ID: <b0803db66dfd4816b21d78bcbccefdb75318444d.camel@web.de>
Date:   Tue, 05 Dec 2023 10:50:20 +0100
From:   Bert Karwatzki <spasswolf@....de>
To:     Amir Goldstein <amir73il@...il.com>,
        Christian Brauner <brauner@...nel.org>
Cc:     Jan Kara <jack@...e.cz>, axboe@...nel.dk, dhowells@...hat.com,
        hch@....de, jlayton@...nel.org, josef@...icpanda.com,
        linux-fsdevel@...r.kernel.org, miklos@...redi.hu,
        viro@...iv.linux.org.uk, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs: read_write: make default in vfs_copy_file_range()
 reachable

Am Dienstag, dem 05.12.2023 um 07:01 +0200 schrieb Amir Goldstein:
> On Tue, Dec 5, 2023 at 5:45 AM Amir Goldstein <amir73il@...il.com> wrote:
> >
> > On Tue, Dec 5, 2023 at 2:16 AM Bert Karwatzki <spasswolf@....de> wrote:
> > >
> > > If vfs_copy_file_range() is called with (flags & COPY_FILE_SPLICE == 0)
> > > and both file_out->f_op->copy_file_range and file_in->f_op-
> > > >remap_file_range
> > > are NULL, too, the default call to do_splice_direct() cannot be reached.
> > > This patch adds an else clause to make the default reachable in all
> > > cases.
> > >
> > > Signed-off-by: Bert Karwatzki <spasswolf@....de>
> >
> > Hi Bert,
> >
> > Thank you for testing and reporting this so early!!
> >
> > I would edit the commit message differently, but anyway, I think that
> > the fix should be folded into commit 05ee2d85cd4a ("fs: use
> > do_splice_direct() for nfsd/ksmbd server-side-copy").
> >
> > Since I end up making a mistake every time I touch this code,
> > I also added a small edit to your patch below, that should make the logic
> > more clear to readers. Hopefully, that will help me avoid making a mistake
> > the next time I touch this code...
> >
> > Would you mind testing my revised fix, so we can add:
> >   Tested-by: Bert Karwatzki <spasswolf@....de>
> > when folding it into the original patch?
> >
>
> Attached an even cleaner version of the fix patch for you to test.
> I tested fstests check -g copy_range on ext4.
> My fault was that I had tested earlier only on xfs and overlayfs
> (the two other cases in the if/else if statement).
>
> Thanks,
> Amir.
>
> > > ---
> > >  fs/read_write.c | 2 ++
> > >  1 file changed, 2 insertions(+)
> > >
> > > diff --git a/fs/read_write.c b/fs/read_write.c
> > > index e0c2c1b5962b..3599c54bd26d 100644
> > > --- a/fs/read_write.c
> > > +++ b/fs/read_write.c
> > > @@ -1554,6 +1554,8 @@ ssize_t vfs_copy_file_range(struct file *file_in,
> > > loff_t pos_in,
> > >                 /* fallback to splice */
> > >                 if (ret <= 0)
> > >                         splice = true;
> > > +       } else {
> >
> > This is logically correct because of the earlier "same sb" check in
> > generic_copy_file_checks(), but we better spell out the logic here as well:
> >
> > +            } else if (file_inode(file_in)->i_sb ==
> > file_inode(file_out)->i_sb) {
> > +                    /* Fallback to splice for same sb copy for
> > backward compat */
> >
> > > +               splice = true;
> > >         }
> > >
> > >         file_end_write(file_out);
> > > --
> > > 2.39.2
> > >
> > > Since linux-next-20231204 I noticed that it was impossible to start the
> > > game Path of Exile (using the steam client). I bisected the error to
> > > commit 05ee2d85cd4ace5cd37dc24132e3fd7f5142ebef. Reverting this commit
> > > in linux-next-20231204 made the game start again and after inserting
> > > printks into vfs_copy_file_range() I found that steam (via python3)
> > > calls this function with (flags & COPY_FILE_SPLICE == 0),
> > > file_out->f_op->copy_file_range == NULL and
> > > file_in->f_op->remap_file_range == NULL so the default is never reached.
> > > This patch adds a catch all else clause so the default is reached in
> > > all cases. This patch fixes the describe issue with steam and Path of
> > > Exile.
> > >
> > > Bert Karwatzki

Your new patch works fine (I applied it to linux-next-20231204), again tested by
starting Path of Exile via steam from an ext4 filesystem.

Bert Karwatzki