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

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Date:   Tue, 5 Dec 2023 07:01:04 +0200
From:   Amir Goldstein <amir73il@...il.com>
To:     Bert Karwatzki <spasswolf@....de>,
        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

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

View attachment "0001-fs-fix-vfs_copy_file_range-for-files-on-same-sb.patch" of type "text/x-patch" (1837 bytes)