On Wed, Dec 16, 2020 at 09:57:49AM -0500, Jeff Layton wrote:
> On Wed, 2020-12-16 at 09:38 -0500, Vivek Goyal wrote:
> > I see that current implementation of __sync_filesystem() ignores the
> > return code from ->sync_fs(). I am not sure why that's the case.
> >
> > Ignoring ->sync_fs() return code is problematic for overlayfs where
> > it can return error if sync_filesystem() on upper super block failed.
> > That error will simply be lost and sycnfs(overlay_fd), will get
> > success (despite the fact it failed).
> >
> > I am assuming that we want to continue to call __sync_blockdev()
> > despite the fact that there have been errors reported from
> > ->sync_fs(). So I wrote this simple patch which captures the
> > error from ->sync_fs() but continues to call __sync_blockdev()
> > and returns error from sync_fs() if there is one.
> >
> > There might be some very good reasons to not capture ->sync_fs()
> > return code, I don't know. Hence thought of proposing this patch.
> > Atleast I will get to know the reason. I still need to figure
> > a way out how to propagate overlay sync_fs() errors to user
> > space.
> >
> > Signed-off-by: Vivek Goyal <vgoyal@xxxxxxxxxx>
> > ---
> > fs/sync.c | 8 ++++++--
> > 1 file changed, 6 insertions(+), 2 deletions(-)
> >
> > Index: redhat-linux/fs/sync.c
> > ===================================================================
> > --- redhat-linux.orig/fs/sync.c 2020-12-16 09:15:49.831565653 -0500
> > +++ redhat-linux/fs/sync.c 2020-12-16 09:23:42.499853207 -0500
> > @@ -30,14 +30,18 @@
> > */
> > static int __sync_filesystem(struct super_block *sb, int wait)
> > {
> > + int ret, ret2;
> > +
> > if (wait)
> > sync_inodes_sb(sb);
> > else
> > writeback_inodes_sb(sb, WB_REASON_SYNC);
> >
> >
> > if (sb->s_op->sync_fs)
> > - sb->s_op->sync_fs(sb, wait);
> > - return __sync_blockdev(sb->s_bdev, wait);
> > + ret = sb->s_op->sync_fs(sb, wait);
> > + ret2 = __sync_blockdev(sb->s_bdev, wait);
> > +
> > + return ret ? ret : ret2;
> > }
> >
> >
> > /*
> >
>
> I posted a patchset that took a similar approach a couple of years ago,
> and we decided not to go with it [1].
>
> While it's not ideal to ignore the error here, I think this is likely to
> break stuff.
So one side affect I see is that syncfs() might start returning errors
in some cases which were not reported at all. I am wondering will that
count as breakage.
> What may be better is to just make sync_fs void return, so
> people don't think that returned errors there mean anything.
May be.
But then question remains that how do we return error to user space
in syncfs(fd) for overlayfs. I will not be surprised if other
filesystems want to return errors as well.
Shall I create new helpers and call these in case of syncfs(). But
that too will start returning new errors on syncfs(). So it does
not solve that problem (if it is a problem).
Or we can define a new super block op say ->sync_fs2() and call that
first and in that case capture return code. That way it will not
impact existing cases and overlayfs can possibly make use of
->sync_fs2() and return error. IOW, impact will be limited to
only file systems which chose to implement ->sync_fs2().
Thanks
Vivek
>
> [1]: https://lore.kernel.org/linux-fsdevel/20180518123415.28181-1-jlayton@xxxxxxxxxx/
> --
> Jeff Layton <jlayton@xxxxxxxxxx>
>
