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

[Chengguang Xu]

 ---- 在 星期二, 2021-11-16 20:35:55 Miklos Szeredi <miklos@xxxxxxxxxx> 撰写 ----
 > On Tue, 16 Nov 2021 at 03:20, Chengguang Xu <cgxu519@xxxxxxxxxxxx> wrote:
 > >
 > >  ---- 在 星期四, 2021-10-07 21:34:19 Miklos Szeredi <miklos@xxxxxxxxxx> 撰写 ----
 > >  > On Thu, 7 Oct 2021 at 15:10, Chengguang Xu <cgxu519@xxxxxxxxxxxx> 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