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

[Amir Goldstein <amir73il@gmail.com>]

On Wed, Dec 1, 2021 at 8:31 AM Chengguang Xu <cgxu519@mykernel.net> wrote:
>
>
>  ---- 在 星期三, 2021-12-01 10:37:15 Chengguang Xu <cgxu519@mykernel.net> 撰写 ----
>  >
>  >  ---- 在 星期三, 2021-12-01 03:04:59 Amir Goldstein <amir73il@gmail.com> 撰写 ----
>  >  > >  > I was thinking about this a bit more and I don't think I buy this
>  >  > >  > explanation. What I rather think is happening is that real work for syncfs
>  >  > >  > (writeback_inodes_sb() and sync_inodes_sb() calls) gets offloaded to a flush
>  >  > >  > worker. E.g. writeback_inodes_sb() ends up calling
>  >  > >  > __writeback_inodes_sb_nr() which does:
>  >  > >  >
>  >  > >  > bdi_split_work_to_wbs()
>  >  > >  > wb_wait_for_completion()
>  >  > >  >
>  >  > >  > So you don't see the work done in the times accounted to your test
>  >  > >  > program. But in practice the flush worker is indeed burning 1.3s worth of
>  >  > >  > CPU to scan the 1 million inode list and do nothing.
>  >  > >  >
>  >  > >
>  >  > > That makes sense. However, in real container use case,  the upper dir is always empty,
>  >  > > so I don't think there is meaningful difference compare to accurately marking overlay
>  >  > > inode dirty.
>  >  > >
>  >  >
>  >  > It's true the that is a very common case, but...
>  >  >
>  >  > > I'm not very familiar with other use cases of overlayfs except container, should we consider
>  >  > > other use cases? Maybe we can also ignore the cpu burden because those use cases don't
>  >  > > have density deployment like container.
>  >  > >
>  >  >
>  >  > metacopy feature was developed for the use case of a container
>  >  > that chowns all the files in the lower image.
>  >  >
>  >  > In that case, which is now also quite common, all the overlay inodes are
>  >  > upper inodes.
>  >  >
>  >
>  > Regardless of metacopy or datacopy, that copy-up has already modified overlay inode
>  > so initialy marking dirty to all overlay inodes which have upper inode will not be a serious
>  > problem in this case too, right?
>  >
>  > I guess maybe you more concern about the re-mark dirtiness on above use case.
>  >
>  >
>  >
>  >  > What about only re-mark overlay inode dirty if upper inode is dirty or is
>  >  > writeably mmapped.
>  >  > For other cases, it is easy to know when overlay inode becomes dirty?
>  >  > Didn't you already try this?
>  >  >
>  >
>  > Yes, I've tried that approach in previous version but as Miklos pointed out in the
>  > feedback there are a few of racy conditions.
>  >

Right..

>
> 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)?

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?

Thanks,
Amir.