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

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Message-Id: <20080715.201042.193704915.davem@davemloft.net>
Date:	Tue, 15 Jul 2008 20:10:42 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	fujita.tomonori@....ntt.co.jp
Cc:	andi@...stfloor.org, mpatocka@...hat.com,
	sparclinux@...r.kernel.org, linux-kernel@...r.kernel.org,
	jens.axboe@...cle.com
Subject: Re: [SUGGESTION]: drop virtual merge accounting in I/O requests

From: FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
Date: Tue, 15 Jul 2008 02:45:03 +0900

> dma_4u_map_sg() has:
> 
> if (segstart != s) {
> 	/* We cannot merge if:
> 	 * - allocated dma_addr isn't contiguous to previous allocation
> 	 */
> 	if ((dma_addr != dma_next) ||
> 	    (outs->dma_length + s->length > max_seg_size) ||
> 	    (is_span_boundary(out_entry, base_shift,
> 		/* Can't merge: create a new segment */
> 		segstart = s;
> 		outcount++;
> 		outs = sg_next(outs);
> 
> So if the IOMMU allocated dma_addr isn't contiguous to previous
> allocation, it might not merge segments that the block layer expected
> the IOMMU to merge.
> 
> We need kinda two phase merging code such as the old SPARC64 IOMMU
> code and PARISC IOMMUs though I like the new simple SPARC64 IOMMU
> code.

I see.

I wonder if all that complexity is really worth it.  Also, all of this
IOMMU allocation and mapping code runs under a spinlock with hw IRQs
disabled.

More and more I'm seeing that it's likely better to remove the VMERGE
code.  I can't see what it really buys us anymore, and to make it work
requires quite a large amount of complexity in the IOMMU layer.
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