On Sun, Apr 12, 2020 at 1:29 AM Dave Chinner <david@xxxxxxxxxxxxx> wrote: > > On Thu, Apr 09, 2020 at 11:29:47AM +0300, Konstantin Khlebnikov wrote: > > Stacked filesystems like overlayfs has no own writeback, but they have to > > forward syncfs() requests to backend for keeping data integrity. > > > > During global sync() each overlayfs instance calls method ->sync_fs() > > for backend although it itself is in global list of superblocks too. > > As a result one syscall sync() could write one superblock several times > > and send multiple disk barriers. > > > > This patch adds flag SB_I_SKIP_SYNC into sb->sb_iflags to avoid that. > > Why wouldn't you just remove the ->sync_fs method from overlay? > > I mean, if you don't need the filesystem to do anything special for > one specific data integrity sync_fs call, you don't need it for any > of them, yes? > No, but I understand the confusion. Say you have 1000 overlay sb's all of them using upper directories from a single xfs sb (quite common for containers). syncfs(2) of each overlay, must call sync_fs of xfs (see ovl_sync_fs) sync(2) will call xfs sync_fs anyway, so there is no point in calling ovl_sync_fs => xfs sync_fs 1000 more times. Thanks, Amir.
