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

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Date:	Tue, 15 Jul 2008 10:37:58 -0400 (EDT)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
cc:	jens.axboe@...cle.com, linux-kernel@...r.kernel.org,
	linux-scsi@...r.kernel.org, davem@...emloft.net,
	linux-parisc@...r.kernel.org
Subject: Re: [PATCH] block: fix q->max_segment_size checking in blk_recalc_rq_segments
 about VMERGE

>>> This bug could happen on alpha, parisc, and sparc, which use VMERGE.
>>
>> Parisc doesn't use virtual merge accounting (there is variable for it but
>> it's always 0).
>
> Hmm, really? Looks like PARISC IOMMUs (ccio-dma.c and sba_iomm.c) set
> parisc_vmerge_boundary (CC'ed PARISC mailing list).

That's right, I looked only at arch and include.

>> On sparc64 it is broken anyway with or without your patch.
>
> Yeah, we need to modify SPARC64 IOMMU code (I'm not sure that it's
> worth). Right now, the best fix is setting BIO_VMERGE_BOUNDARY to 0.

Even if we fix it now, the question is: how long it will stay fixed? Until 
someone makes another change to struct device that restricts boundaries on 
some wacky hardware.

Mikulas

>> And alpha alone doesn't justify substantial code bloat in generic block
>> layer. So I propose this patch to drop it at all.
>
> Jens, what do you think about removing VMERGE code?
>
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