[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.
