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SubjectRe: [RFC PATCH v5 06/10] ovl: implement overlayfs' ->write_inode operation
On Tue, 16 Nov 2021 at 03:20, Chengguang Xu <cgxu519@mykernel.net> wrote:
>
> ---- 在 星期四, 2021-10-07 21:34:19 Miklos Szeredi <miklos@szeredi.hu> 撰写 ----
> > 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()?
> >
> > I.e. call write_inode_now() as suggested by Jan.
> >
> > Also could just call mark_inode_dirty() on the overlay inode
> > regardless of the dirty flags on the upper inode since it shouldn't
> > matter and results in simpler logic.
> >
>
> Hi Miklos,
>
> Sorry for delayed response for this, I've been busy with another project.
>
> I agree with your suggesion above and further more how about just mark overlay inode dirty
> when it has upper inode? This approach will make marking dirtiness simple enough.

Are you suggesting that all non-lower overlay inodes should always be dirty?

The logic would be simple, no doubt, but there's the cost to walking
those overlay inodes which don't have a dirty upper inode, right? Can
you quantify this cost with a benchmark? Can be totally synthetic,
e.g. lookup a million upper files without modifying them, then call
syncfs.

Thanks,
Miklos

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