linux-kernel - Re: [PATCH block/for-linus] writeback: fix syncing of I_DIRTY

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Message-ID: <20150824214535.GM28944@mtj.duckdns.org>
Date:	Mon, 24 Aug 2015 17:45:35 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Jan Kara <jack@...e.cz>
Cc:	Dave Chinner <david@...morbit.com>, Eryu Guan <eguan@...hat.com>,
	Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org,
	xfs@....sgi.com, axboe@...com, Jan Kara <jack@...e.com>,
	linux-fsdevel@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH block/for-linus] writeback: fix syncing of I_DIRTY_TIME
 inodes

Hello,

On Mon, Aug 24, 2015 at 11:09:27PM +0200, Jan Kara wrote:
> It is inefficient, yes. But note that 'writeback' and 'dirty' states are
> completely independent. Page can be in any of the !dirty & !writeback,

That isn't true for pages being dirtied through set_page_dirty().
It's guaranteed that a dirty inode remains on one of the b_* lists
till there's no dirty page and writeback is complete.

> dirty & !writeback, !dirty & writeback, dirty & writeback states. So mixing
> tracking of writeback and dirty state of an inode just makes the code even
> messier.

I'm curious where and why they would deviate.  Can you give me some
examples?  AFAICS, anything which uses the usual set_page_dirty() path
shouldn't do that.

> > > a list to track inodes with pages under writeback but they clashed with
> > > your patch series and they didn't get rebased yet AFAIR.
> > 
> > Wouldn't it make more sense to simply put them on one of the existing
> > b_* lists?
> 
> Logically it just doesn't make sense because as I wrote above dirty and
> writeback states are completely independent. Also you'd have to detect &
> skip inodes that don't really have any dirty pages to write and all the
> detection of "is there any data to write" would get more complicated. A
> separate list for inodes under writeback as Josef did is IMO the cleanest
> solution.

Given that the usual code path tracks dirty and writeback together, I
don't think it's nonsensical; however, I'm more curious how common
writeback w/o dirtying case is.

Thanks.

-- 
tejun
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