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

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Message-ID: <20150824210927.GA8823@quack.suse.cz>
Date:	Mon, 24 Aug 2015 23:09:27 +0200
From:	Jan Kara <jack@...e.cz>
To:	Tejun Heo <tj@...nel.org>
Cc:	Jan Kara <jack@...e.cz>, 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

On Mon 24-08-15 15:32:42, Tejun Heo wrote:
> Hello, Jan.
> 
> On Mon, Aug 24, 2015 at 09:08:47PM +0200, Jan Kara wrote:
> > Inode may contain writeback pages (but not dirty pages) without being on
> > any of the dirty lists. That is correct. Josef Bacik had patches to create
> 
> Hmmm... Can you please expand on how / why that happens?  It's kinda
> weird to require writeback to walk all inodes regardless of their
> dirty states.

It is inefficient, yes. But note that 'writeback' and 'dirty' states are
completely independent. Page can be in any of the !dirty & !writeback,
dirty & !writeback, !dirty & writeback, dirty & writeback states. So mixing
tracking of writeback and dirty state of an inode just makes the code even
messier.

> > 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.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR
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