linux-kernel - Re: [SUGGESTION]: drop virtual merge accounting in I/O requests

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Message-Id: <20080711152054C.fujita.tomonori@lab.ntt.co.jp>
Date:	Fri, 11 Jul 2008 15:20:56 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	mpatocka@...hat.com
Cc:	sparclinux@...r.kernel.org, linux-kernel@...r.kernel.org,
	jens.axboe@...cle.com
Subject: Re: [SUGGESTION]: drop virtual merge accounting in I/O requests

On Thu, 10 Jul 2008 17:56:08 -0400 (EDT)
Mikulas Patocka <mpatocka@...hat.com> wrote:

> When I thought about it more, I realized that this accounting of virtual 
> segments in I/O layer can't work correctly at all. If an architecture 
> defines symbols BIOVEC_VIRT_MERGEABLE and BIOVEC_VIRT_OVERSIZE, it 
> declares that it's IOMMU must merge any two regions satisfying these 
> conditions. But in an IOMMU, it is impossible to guarantee, because:

Yeah, IOMMUs can't guarantee that. The majority of architectures set
BIO_VMERGE_BOUNDARY to 0 so they don't hit this, I think.


> * the bus address is allocated basiclly randomly, so we can hit 
> dev->dma_parms->segment_boundary_mask any time. This will prevent virtual 
> merging from happenning. I/O layer doesn't know the bus address at the 
> time it merges requests, so it can't predict when this happens.
> 
> * the IOMMU isn't guaranteed to find a continuous space in it's bus 
> address space. If it skips over already mapped regions, it can't perform 
> virtual merging.
> 
> * when creating the mapping, we can hit per-device limit 
> "dev->dma_parms->max_segment_size" --- but the I/O layer checks only 
> against global limit BIOVEC_VIRT_OVERSIZE. (this last issue is fixable, 
> the previous two are not).

I think that the block layer can handle this properly via
q->max_segment_size. We have the same value at two different
places. Yeah, it's not good...


BTW, inia100_template sets sg_tablesize to SG_ALL. If the controller
has at most 32 SG entries per request, we need to fix that.
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