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

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Date:	Thu, 18 Mar 2010 11:48:58 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Lars Ellenberg <lars.ellenberg@...bit.com>
Cc:	Mikulas Patocka <mpatocka@...hat.com>, Neil Brown <neilb@...e.de>,
	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 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?

Thanks.
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