linux-kernel - Re: [RFC PATCH v5 06/10] ovl: implement overlayfs' ->write

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Message-ID: <17d36d37022.1227b6f102736.1047689367927335302@mykernel.net>
Date:   Fri, 19 Nov 2021 14:12:46 +0800
From:   Chengguang Xu <cgxu519@...ernel.net>
To:     "Jan Kara" <jack@...e.cz>
Cc:     "Miklos Szeredi" <miklos@...redi.hu>,
        "Amir Goldstein" <amir73il@...il.com>,
        "linux-fsdevel" <linux-fsdevel@...r.kernel.org>,
        "overlayfs" <linux-unionfs@...r.kernel.org>,
        "linux-kernel" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v5 06/10] ovl: implement overlayfs' ->write_inode
 operation

 ---- 在 星期五, 2021-11-19 00:43:49 Jan Kara <jack@...e.cz> 撰写 ----
 > On Thu 18-11-21 20:02:09, Chengguang Xu wrote:
 > >  ---- 在 星期四, 2021-11-18 19:23:15 Jan Kara <jack@...e.cz> 撰写 ----
 > >  > On Thu 18-11-21 14:32:36, Chengguang Xu wrote:
 > >  > > 
 > >  > >  ---- 在 星期三, 2021-11-17 14:11:29 Chengguang Xu <cgxu519@...ernel.net> 撰写 ----
 > >  > >  >  ---- 在 星期二, 2021-11-16 20:35:55 Miklos Szeredi <miklos@...redi.hu> 撰写 ----
 > >  > >  >  > On Tue, 16 Nov 2021 at 03:20, Chengguang Xu <cgxu519@...ernel.net> wrote:
 > >  > >  >  > >
 > >  > >  >  > >  ---- 在 星期四, 2021-10-07 21:34:19 Miklos Szeredi <miklos@...redi.hu> 撰写 ----
 > >  > >  >  > >  > On Thu, 7 Oct 2021 at 15:10, Chengguang Xu <cgxu519@...ernel.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.
 > >  > 
 > > 
 > > Hi Jan,
 > > 
 > > Actually, the time on user and sys are almost same with directly excute syncfs on underlying fs.
 > > IMO, it only extends syncfs(2) waiting time for perticular container but not burning cpu.
 > > What am I missing?
 > 
 > Ah, right, I've missed that only realtime changed, not systime. I'm sorry
 > for confusion. But why did the realtime increase so much? Are we waiting
 > for some IO?
 > 

There are many places to call cond_resched() in writeback process,
so sycnfs process was scheduled several times.

Thanks,
Chengguang