linux-kernel - Re: [PATCH] dm: max_segments=1 if merge_bvec

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Message-ID: <20100319084844.7ea9db41@notabene.brown>
Date:	Fri, 19 Mar 2010 08:48:44 +1100
From:	Neil Brown <neilb@...e.de>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Lars Ellenberg <lars.ellenberg@...bit.com>,
	Mikulas Patocka <mpatocka@...hat.com>,
	Alasdair G Kergon <agk@...hat.com>,
	device-mapper development <dm-devel@...hat.com>,
	jens.axboe@...cle.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dm: max_segments=1 if merge_bvec_fn is not supported

On Thu, 18 Mar 2010 11:48:58 -0700
Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Mon, 8 Mar 2010 14:14:49 +0100
> Lars Ellenberg <lars.ellenberg@...bit.com> wrote:
> 
> > On Mon, Mar 08, 2010 at 03:35:37AM -0500, Mikulas Patocka wrote:
> > > Hi
> > > 
> > > That patch with limits->max_segments = 1; is wrong. It fixes this bug 
> > > sometimes and sometimes not.
> > > 
> > > The problem is, if someone attempts to create a bio with two vector 
> > > entries, the first maps the last sector contained in some page and the 
> > > second maps the first sector of the next physical page: it has one 
> > > segment, it has size <= PAGE_SIZE, but it still may cross raid stripe and 
> > > the raid driver will reject it.
> > 
> > Now that you put it that way ;)
> > You are right.
> > 
> > My asumption that "single segment" was  
> > equalvalent in practice with "single bvec"
> > does not hold true in that case.
> > 
> > Then, what about adding seg_boundary_mask restrictions as well?
> > 	max_sectors = PAGE_SIZE >> 9;
> > 	max_segments = 1;
> > 	seg_boundary_mask = PAGE_SIZE -1;
> > or some such.
> > 
> > > > > This is not the first time this has been patched, btw.
> > > > > See https://bugzilla.redhat.com/show_bug.cgi?id=440093
> > > > > and the patch by Mikulas:
> > > > > https://bugzilla.redhat.com/attachment.cgi?id=342638&action=diff
> > > 
> > > Look at this patch, it is the proper way how to fix it: create a 
> > > merge_bvec_fn that reject more than one biovec entry.
> > 
> > If adding seg_boundary_mask is still not sufficient,
> > lets merge that patch instead?
> > Why has it been dropped, respectively never been merged?
> > It became obsolete for dm-linear by 7bc3447b,
> > but in general the bug is still there, or am I missing something?
> > 
> 
> This all seemed to die.  Does Neil's mysterypatch fix all these issues?
> 
> Neil, was that patch tagged for -stable backporting?

The patch at the top of my 'for-linus' branch (which Linus doesn't seem to
have pulled yet) fixes this for md and is tagged for -stable backporting.
I just sets max_segments and seg_boundary_mask.  There is no point setting
max_sectors as well.  I found that setting merge_bvec_fn, while a perfectly
correct approach, was more cumbersome.

My patch doesn't fix this for dm.  I assume the dm developers will do that.

NeilBrown
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