Re: [RFC PATCH v5 06/10] ovl: implement overlayfs' ->write_inode operation

[Chengguang Xu]

 ---- 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.
 >