linux-kernel - Re: [PATCH] block: fix q->max_segment_size checking in blk_recalc_rq

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Message-Id: <20080724.145336.41899163.davem@davemloft.net>
Date:	Thu, 24 Jul 2008 14:53:36 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	mpatocka@...hat.com
Cc:	James.Bottomley@...senPartnership.com,
	fujita.tomonori@....ntt.co.jp, jens.axboe@...cle.com,
	linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org,
	linux-parisc@...r.kernel.org
Subject: Re: [PATCH] block: fix q->max_segment_size checking in
 blk_recalc_rq_segments about VMERGE

From: Mikulas Patocka <mpatocka@...hat.com>
Date: Thu, 24 Jul 2008 17:49:14 -0400 (EDT)

> * it is prone to bugs and hard to maintain, because the same value must be 
> calculated in blk-merge.c and in architectural iommu functions --- if the 
> value differs, you create too long request, corrupt kernel memory and 
> crash (happened on sparc64). Anyone changing blk-merge in the future will 
> risk breaking something on the architectures that use BIO_VMERGE_BOUNDARY 
> --- and because these architectures are so rare, the bug will go unnoticed 
> for long time --- like in the case of sparc64.

I completely agree with this point.

This VMERGE stuff is now a non-trivial maintainence burdon because
anyone who wants to hack on the block layer has to be mindful of
VMERGE but is very unlikely to have access to a system that it can
even be tested on.

And the answer isn't "James Bottomly will test your changes for you",
because that simply doesn't scale.

I still say we should definitely remove the VMERGE code.  It's not
worth the maintainence hassle just for some SG chaining test rig
on some obscure platform.

I really only hear one person who really wants this code around any
more.  Is that the Linux way? :-) Can't he patch it into his tree when
he needs it or write an alternative way to stress the SG chaining
code?  He has the source, right? :-)))
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