On Thu, Apr 9, 2020 at 2:28 PM Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx> wrote: > > On 09/04/2020 13.23, Amir Goldstein wrote: > > On Thu, Apr 9, 2020 at 11:30 AM Konstantin Khlebnikov > > <khlebnikov@xxxxxxxxxxxxxx> 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. > >> > >> Reported-by: Dmitry Monakhov <dmtrmonakhov@xxxxxxxxxxxxxx> > >> Signed-off-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx> > >> --- > > > > Seems reasonable. > > You may add: > > Reviewed-by: Amir Goldstein <amir73il@xxxxxxxxx> > > > > +CC: containers list > > Thanks > > > > > This bring up old memories. > > I posted this way back to fix handling of emergency_remount() in the > > presence of loop mounted fs: > > https://lore.kernel.org/linux-ext4/CAA2m6vfatWKS1CQFpaRbii2AXiZFvQUjVvYhGxWTSpz+2rxDyg@xxxxxxxxxxxxxx/ > > > > But seems to me that emergency_sync() and sync(2) are equally broken > > for this use case. > > > > I wonder if anyone cares enough about resilience of loop mounted fs to try > > and change the iterate_* functions to iterate supers/bdevs in reverse order... > > Now I see reason behind "sync; sync; sync; reboot" =) > > Order old -> new allows to not miss new items if list modifies. > Might be important for some users. > That's not the reason I suggested reverse order. The reason is that with loop mounted fs, the correct order of flushing is: 1. sync loop mounted fs inodes => writes to loop image file 2. sync loop mounted fs sb => fsyncs the loop image file 3. sync the loop image host fs sb With forward sb iteration order, #3 happens before #1, so the loop mounted fs changes are not really being made durable by a single sync(2) call. > bdev iteration seems already reversed: inode_sb_list_add adds to the head > I think bdev iteration order will not make a difference in this case. flushing /dev/loopX will not be needed and it happens too late anyway. Thanks, Amir.
