On Thu, Dec 17, 2020 at 10:57:28AM +0100, Jan Kara wrote: > On Thu 17-12-20 00:49:35, 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. No idea about quota > > situation. > > WRT quota situation: All the ->sync_fs() calls there are due to cache > coherency reasons (we need to get quota changes to disk, then prune quota > files's page cache, and then userspace can read current quota structures > from the disk). We don't want to fail dquot_disable() just because caches > might be incoherent so ignoring ->sync_fs() return value there is fine. > With dquot_quota_sync() it might make some sense to return the error - > that's just a backend for Q_SYNC quotactl(2). OTOH I'm not sure anybody > really cares - Q_SYNC is rarely used. Thanks Jan. May be I will leave dquot_quota_sync() untouched for now. When somebody has a need to capture return code from ->sync_fs() there, it can be easily added. Vivek
