linux-kernel - Re: [PATCH v2 1/1] block: blk-merge: don't merge the pages with non-contiguous descriptors

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Date:	Thu, 17 Jan 2013 11:40:37 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
Cc:	martin.petersen@...cle.com, linux-scsi@...r.kernel.org,
	linux-arm-msm@...r.kernel.org, linux-mmc@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Subhash Jadavani <subhashj@...eaurora.org>,
	Tejun Heo <tj@...nel.org>, asias@...hat.com,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 1/1] block: blk-merge: don't merge the pages with
	non-contiguous descriptors

On Thu, Jan 17, 2013 at 11:19:21AM +0000, James Bottomley wrote:
> On Thu, 2013-01-17 at 11:04 +0000, Russell King - ARM Linux wrote:
> > On Thu, Jan 17, 2013 at 11:01:47AM +0000, James Bottomley wrote:
> > > On Thu, 2013-01-17 at 10:47 +0000, Russell King - ARM Linux wrote:
> > > > Also, couldn't the addition of the scatterlist offset to the page also
> > > > be buggy too?
> > > 
> > > No, fortunately, offset must be within the first page from the point of
> > > view of block generated sg lists.  As long as nothing within arm
> > > violates this, it should be a safe assumption ... although the code
> > > seems to assume otherwise.
> > 
> > Are you absolutely sure about that?  I believe I have seen cases where
> > that has been violated in the past, though it was many years ago.
> 
> >From the point of view of the block layer, absolutely: the scatterlist
> is generated from an array of bio_vecs.  Each bio_vec is a page, offset
> and length element and obeys the rule that offset must be within the
> page and offset + length cannot stray over the page.

Well, I found it when working on the mmc stuff initially, long before
it got complex.  The scatterlists were unmodified from the block layer,
and I'm positive I saw occasions where the offset in the scatter lists
were larger than PAGE_SIZE.

> >From the point of view of other arm stuff, I don't know.

I'm not talking about anything ARM specific here.
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