Re: [PATCH 1/3] vfs: add new f_op->syncfs vector

[Vivek Goyal <vgoyal@redhat.com>]

On Thu, Dec 17, 2020 at 12:49:35AM +0000, Al Viro wrote:
> [Christoph added to Cc...]
> On Wed, Dec 16, 2020 at 06:31:47PM -0500, Vivek Goyal wrote:
> > Current implementation of __sync_filesystem() ignores the return code
> > from ->sync_fs(). I am not sure why that's the case. There must have
> > been some historical reason for this.
> > 
> > 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).
> > 
> > If we modify existing implementation, there is a concern that it will
> > lead to user space visible behavior changes and break things. So
> > instead implement a new file_operations->syncfs() call which will
> > be called in syncfs() syscall path. Return code from this new
> > call will be captured. And all the writeback error detection
> > logic can go in there as well. Only filesystems which implement
> > this call get affected by this change. Others continue to fallback
> > to existing mechanism.
> 
> That smells like a massive source of confusion down the road.  I'd just
> looked through the existing instances; many always return 0, but quite
> a few sometimes try to return an error:
> fs/btrfs/super.c:2412:  .sync_fs        = btrfs_sync_fs,
> fs/exfat/super.c:204:   .sync_fs        = exfat_sync_fs,
> fs/ext4/super.c:1674:   .sync_fs        = ext4_sync_fs,
> fs/f2fs/super.c:2480:   .sync_fs        = f2fs_sync_fs,
> fs/gfs2/super.c:1600:   .sync_fs                = gfs2_sync_fs,
> fs/hfsplus/super.c:368: .sync_fs        = hfsplus_sync_fs,
> fs/nilfs2/super.c:689:  .sync_fs        = nilfs_sync_fs,
> fs/ocfs2/super.c:139:   .sync_fs        = ocfs2_sync_fs,
> fs/overlayfs/super.c:399:       .sync_fs        = ovl_sync_fs,
> fs/ubifs/super.c:2052:  .sync_fs       = ubifs_sync_fs,
> is the list of such.  There are 4 method callers:
> dquot_quota_sync(), dquot_disable(), __sync_filesystem() and
> sync_fs_one_sb().  For sync_fs_one_sb() we want to ignore the
> return value; for __sync_filesystem() we almost certainly
> do *not* - it ends with return __sync_blockdev(sb->s_bdev, wait),
> after all.  The question for that one is whether we want
> __sync_blockdev() called even in case of ->sync_fs() reporting
> a failure, and I suspect that it's safer to call it anyway and
> return the first error value we'd got.

I posted V1 patch to do exactly above. In __sync_filesystem(), capture
return code from ->sync_fs() but continue to call __sync_blockdev() and
and return error code from ->sync_fs() if there is one otherwise
return error code from __sync_blockdev().

https://lore.kernel.org/linux-fsdevel/20201216143802.GA10550@redhat.com/

Thanks
Vivek

> No idea about quota situation.
>