linux-kernel - Re: [ext3][kernels >= 2.6.20.7 at least] KDE going comatose when FS is under heavy write load (massive starvation)

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Date:	Fri, 04 May 2007 10:57:12 +0400
From:	Alex Tomas <alex@...sterfs.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Andreas Dilger <adilger@...sterfs.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Marat Buharov <marat.buharov@...il.com>,
	Mike Galbraith <efault@....de>,
	LKML <linux-kernel@...r.kernel.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: [ext3][kernels >= 2.6.20.7 at least] KDE going comatose when
 FS is under heavy write load (massive starvation)

Andrew Morton wrote:
> On Fri, 04 May 2007 10:18:12 +0400 Alex Tomas <alex@...sterfs.com> wrote:
> 
>> Andrew Morton wrote:
>>> Yes, there can be issues with needing to allocate journal space within the
>>> context of a commit.  But
>> no-no, this isn't required. we only need to mark pages/blocks within
>> transaction, otherwise race is possible when we allocate blocks in transaction,
>> then transacton starts to commit, then we mark pages/blocks to be flushed
>> before commit.
> 
> I don't understand.  Can you please describe the race in more detail?

if I understood your idea right, then in data=ordered mode, commit thread writes
all dirty mapped blocks before real commit.

say, we have two threads: t1 is a thread doing flushing and t2 is a commit thread

t1					t2
find dirty inode I
find some dirty unallocated blocks
journal_start()
allocate blocks
attach them to I
journal_stop()

					going to commit
					find inode I dirty
					do NOT find these blocks because they're
					  allocated only, but pages/bhs aren't mapped
					  to them
					start commit


map pages/bhs to just allocate blocks


so, either we mark pages/bhs someway within journal_start()--journal_stop() or
commit thread should do lookup for all dirty pages. the latter doesn't sound nice, IMHO.

thanks, Alex



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