linux-kernel - Re: [PATCH 5/9] writeback: introduce the pageout work

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Date:	Mon, 12 Mar 2012 16:21:49 +0200
From:	Artem Bityutskiy <dedekind1@...il.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Fengguang Wu <fengguang.wu@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg Thelen <gthelen@...gle.com>,
	Ying Han <yinghan@...gle.com>,
	"hannes@...xchg.org" <hannes@...xchg.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Minchan Kim <minchan.kim@...il.com>,
	Linux Memory Management List <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Adrian Hunter <adrian.hunter@...el.com>
Subject: Re: [PATCH 5/9] writeback: introduce the pageout work

On Mon, 2012-03-12 at 15:02 +0100, Jan Kara wrote:
> > The second part of the overall deletion job will be when we commit - the
> > updated version of the FS index will be written to the flash media.
>   Oh, I see. This is what I was missing. And I presume you always make sure
> to have enough space for new FS index so it cannot deadlock when trying to
> push out dirty pages.

Yes, this is one of the hardest part and this is what the budgeting
subsystem does. Every VFS call (even unlink()) first invokes something
like 'ubifs_budget_space()' with arguments describing the space needs,
and the budgeting subsystem will account for the space, including the
possibility of the index growth. And the budgeting subsystem actually
forces write-back when it sees that there is not enough free space for
the operation. Because all the calculations are pessimistic, write-back
helps: the data nodes are compressed, and so on. The budgeting subsystem
may also force commit, which will clarify many unclarities and make the
calculations more precise. If nothing helps - ENOSPC is reported. For
deletions we also have a bit of reserve space to prevent -ENOSPC when
you actually want to delete a file on full file-system.

But the shorted answer: yes, we reserve 2 times the current index size
of the space for the index growths.

Long time ago I tried to describe this and the surrounding issues here:
http://www.linux-mtd.infradead.org/doc/ubifs.html#L_spaceacc

-- 
Best Regards,
Artem Bityutskiy

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