---- 在 星期四, 2021-12-02 06:47:25 Amir Goldstein <amir73il@xxxxxxxxx> 撰写 ---- > On Wed, Dec 1, 2021 at 6:24 PM Chengguang Xu <cgxu519@xxxxxxxxxxxx> wrote: > > > > ---- 在 星期三, 2021-12-01 21:46:10 Jan Kara <jack@xxxxxxx> 撰写 ---- > > > On Wed 01-12-21 09:19:17, Amir Goldstein wrote: > > > > On Wed, Dec 1, 2021 at 8:31 AM Chengguang Xu <cgxu519@xxxxxxxxxxxx> wrote: > > > > > So the final solution to handle all the concerns looks like accurately > > > > > mark overlay inode diry on modification and re-mark dirty only for > > > > > mmaped file in ->write_inode(). > > > > > > > > > > Hi Miklos, Jan > > > > > > > > > > Will you agree with new proposal above? > > > > > > > > > > > > > Maybe you can still pull off a simpler version by remarking dirty only > > > > writably mmapped upper AND inode_is_open_for_write(upper)? > > > > > > Well, if inode is writeably mapped, it must be also open for write, doesn't > > > it? The VMA of the mapping will hold file open. So remarking overlay inode > > > dirty during writeback while inode_is_open_for_write(upper) looks like > > > reasonably easy and presumably there won't be that many inodes open for > > > writing for this to become big overhead? > > I think it should be ok and a good tradeoff of complexity vs. performance. IMO, mark dirtiness on write is relatively simple, so I think we can mark the overlayfs inode dirty during real write behavior and only remark writable mmap unconditionally in ->write_inode(). > > > > > > > > If I am not mistaken, if you always mark overlay inode dirty on ovl_flush() > > > > of FMODE_WRITE file, there is nothing that can make upper inode dirty > > > > after last close (if upper is not mmaped), so one more inode sync should > > > > be enough. No? > > > > > > But we still need to catch other dirtying events like timestamp updates, > > > truncate(2) etc. to mark overlay inode dirty. Not sure how reliably that > > > can be done... > > > > > Oh yeh, we have those as well :) > All those cases should be covered by ovl_copyattr() that updates the > ovl inode ctime/mtime, so always dirty in ovl_copyattr() should be good. Currently ovl_copyattr() does not cover all the cases, so I think we still need to carefully check all the places of calling mnt_want_write(). Thanks, Chengguang > I *think* the only case of ovl_copyattr() that should not dirty is in > ovl_inode_init(), so need some special helper there. > > > > > To be honest I even don't fully understand what's the ->flush() logic in overlayfs. > > Why should we open new underlying file when calling ->flush()? > > Is it still correct in the case of opening lower layer first then copy-uped case? > > > > The semantics of flush() are far from being uniform across filesystems. > most local filesystems do nothing on close. > most network fs only flush dirty data when a writer closes a file > but not when a reader closes a file. > It is hard to imagine that applications rely on flush-on-close of > rdonly fd behavior and I agree that flushing only if original fd was upper > makes more sense, so I am not sure if it is really essential for > overlayfs to open an upper rdonly fd just to do whatever the upper fs > would have done on close of rdonly fd, but maybe there is no good > reason to change this behavior either. > > Thanks, > Amir. >
