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

[Al Viro]

[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.  No idea about quota
situation.