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SubjectRe: [RFC PATCH v5 06/10] ovl: implement overlayfs' ->write_inode operation
On Thu 07-10-21 20:51:47, Miklos Szeredi wrote:
> On Thu, 7 Oct 2021 at 16:53, Chengguang Xu <cgxu519@mykernel.net> wrote:
> >
> > ---- 在 星期四, 2021-10-07 22:46:46 Jan Kara <jack@suse.cz> 撰写 ----
> > > On Thu 07-10-21 15:34:19, Miklos Szeredi wrote:
> > > > On Thu, 7 Oct 2021 at 15:10, Chengguang Xu <cgxu519@mykernel.net> wrote:
> > > > > > However that wasn't what I was asking about. AFAICS ->write_inode()
> > > > > > won't start write back for dirty pages. Maybe I'm missing something,
> > > > > > but there it looks as if nothing will actually trigger writeback for
> > > > > > dirty pages in upper inode.
> > > > > >
> > > > >
> > > > > Actually, page writeback on upper inode will be triggered by overlayfs ->writepages and
> > > > > overlayfs' ->writepages will be called by vfs writeback function (i.e writeback_sb_inodes).
> > > >
> > > > Right.
> > > >
> > > > But wouldn't it be simpler to do this from ->write_inode()?
> > >
> > > You could but then you'd have to make sure you have I_DIRTY_SYNC always set
> > > when I_DIRTY_PAGES is set on the upper inode so that your ->write_inode()
> > > callback gets called. Overall I agree the logic would be probably simpler.
> > >
> >
>
> And it's not just for simplicity. The I_SYNC logic in
> writeback_single_inode() is actually necessary to prevent races
> between instances on a specific inode. I.e. if inode writeback is
> started by background wb then syncfs needs to synchronize with that
> otherwise it will miss the inode, or worse, mess things up by calling
> ->write_inode() multiple times in parallel. So going throught
> writeback_single_inode() is actually a must AFAICS.

Yes, you are correct.

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR

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