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

From: Chengguang Xu
Date: Tue Nov 30 2021 - 21:38:00 EST



---- 在 星期三, 2021-12-01 03:04:59 Amir Goldstein <amir73il@xxxxxxxxx> 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.



Thanks,
Chengguang