linux-ext4 - Re: Boot failure with ext2 and initrds

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Message-ID: <Pine.LNX.4.64.0611210508270.22957@blonde.wat.veritas.com>
Date:	Tue, 21 Nov 2006 05:39:06 +0000 (GMT)
From:	Hugh Dickins <hugh@...itas.com>
To:	Mingming Cao <cmm@...ibm.com>
cc:	Andrew Morton <akpm@...l.org>, Mel Gorman <mel@...net.ie>,
	"Martin J. Bligh" <mbligh@...igh.org>,
	linux-kernel@...r.kernel.org,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: Boot failure with ext2 and initrds

On Mon, 20 Nov 2006, Mingming Cao wrote:
> 
> So there is only one writer at the moment the hang was happening? 

I expect there were multiple writers when the task which hangs
first entered its ext2_prepare_write (it's a make -j20 build on
that ext2 filesystem); but by the time I come to look at the hang,
there's only that one writer active - everything else would be waiting
on that part of the build to complete (well, your question makes me
realize that I didn't look down the whole "ps" listing to see what
was waiting; but the hanging task is the only one I see running on
on any cpu, each time I break in).

> 
> hmm, is the filesystem relatively all being used or reserved, i.e, the
> free bits are all being reserved?  There is one extreme case that may
> cause starvation. If filesystem free blocks are all being reserved, when
> a  new writer need a free block, it has to go through the entire
> filesystems, try to reserve a space, which will repeatly calling
> rsv_window_add and rsv_window_remove, since. Finally give up and fall
> back to allocation without reservation. But this is all theory, not sure
> fits your case here.

I can understand that there may be a worst case like that: but I hope
it wouldn't take 20 hours to find a free block on a default 340MB ext2
filesystem!  And unless something else has gone wrong, this build would
not be filling the filesystem to that extent: it's probably around 80%
full at this stage, and shouldn't get fuller than 98% in the end.

Any suggestions for what I might check, next time it happens?

Hugh
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