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SubjectRe: [RFC PATCH v5 06/10] ovl: implement overlayfs' ->write_inode operation
 ---- 在 星期二, 2021-11-16 20:35:55 Miklos Szeredi <miklos@szeredi.hu> 撰写 ----
> 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?

That's true.

> 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.
>

No problem, I'll do some tests for the performance.

Thanks,
Chengguang

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