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

[Jan Kara <jack@suse.cz>]

On Thu 18-11-21 14:32:36, Chengguang Xu wrote:
> 
>  ---- 在 星期三, 2021-11-17 14:11:29 Chengguang Xu <cgxu519@mykernel.net> 撰写 ----
>  >  ---- 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.
>  > 
> 
> Hi Miklos,
> 
> I did some rough tests and the results like below.  In practice,  I don't
> think that 1.3s extra time of syncfs will cause significant problem.
> What do you think?

Well, burning 1.3s worth of CPU time for doing nothing seems like quite a
bit to me. I understand this is with 1000000 inodes but although that is
quite a few it is not unheard of. If there would be several containers
calling sync_fs(2) on the machine they could easily hog the machine... That
is why I was originally against keeping overlay inodes always dirty and
wanted their dirtiness to at least roughly track the real need to do
writeback.

								Honza

> Test bed: kvm vm 
> 2.50GHz cpu 32core
> 64GB mem
> vm kernel  5.15.0-rc1+ (with ovl syncfs patch V6)
> 
> one millon files spread to 2 level of dir hierarchy.
> test step:
> 1) create testfiles in ovl upper dir
> 2) mount overlayfs
> 3) excute ls -lR to lookup all file in overlay merge dir
> 4) excute slabtop to make sure overlay inode number
> 5) call syncfs to the file in merge dir
> 
> Tested five times and the reusults are in 1.310s ~ 1.326s
> 
> root@VM-144-4-centos test]# time ./syncfs ovl-merge/create-file.sh 
> syncfs success
> 
> real    0m1.310s
> user    0m0.000s
> sys     0m0.001s
> [root@VM-144-4-centos test]# time ./syncfs ovl-merge/create-file.sh 
> syncfs success
> 
> real    0m1.326s
> user    0m0.001s
> sys     0m0.000s
> [root@VM-144-4-centos test]# time ./syncfs ovl-merge/create-file.sh 
> syncfs success
> 
> real    0m1.321s
> user    0m0.000s
> sys     0m0.001s
> [root@VM-144-4-centos test]# time ./syncfs ovl-merge/create-file.sh 
> syncfs success
> 
> real    0m1.316s
> user    0m0.000s
> sys     0m0.001s
> [root@VM-144-4-centos test]# time ./syncfs ovl-merge/create-file.sh 
> syncfs success
> 
> real    0m1.314s
> user    0m0.001s
> sys     0m0.001s
> 
> 
> Directly run syncfs to the file in ovl-upper dir.
> Tested five times and the reusults are in 0.001s ~ 0.003s
> 
> [root@VM-144-4-centos test]# time ./syncfs a
> syncfs success
> 
> real    0m0.002s
> user    0m0.001s
> sys     0m0.000s
> [root@VM-144-4-centos test]# time ./syncfs ovl-upper/create-file.sh 
> syncfs success
> 
> real    0m0.003s
> user    0m0.001s
> sys     0m0.000s
> [root@VM-144-4-centos test]# time ./syncfs ovl-upper/create-file.sh 
> syncfs success
> 
> real    0m0.001s
> user    0m0.000s
> sys     0m0.001s
> [root@VM-144-4-centos test]# time ./syncfs ovl-upper/create-file.sh 
> syncfs success
> 
> real    0m0.001s
> user    0m0.000s
> sys     0m0.001s
> [root@VM-144-4-centos test]# time ./syncfs ovl-upper/create-file.sh 
> syncfs success
> 
> real    0m0.001s
> user    0m0.000s
> sys     0m0.001s
> [root@VM-144-4-centos test]# time ./syncfs ovl-upper/create-file.sh 
> syncfs success
> 
> real    0m0.001s
> user    0m0.000s
> sys     0m0.001
> 
> 
> 
> 
> 
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR