linux-kernel - Re: Scatter-gather list constraints

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Message-Id: <20080626155834I.fujita.tomonori@lab.ntt.co.jp>
Date:	Thu, 26 Jun 2008 15:58:24 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	jens.axboe@...cle.com
Cc:	fujita.tomonori@....ntt.co.jp, stern@...land.harvard.edu,
	andi@...stfloor.org, linux-kernel@...r.kernel.org,
	antonio.lin@...ormicro.com, david.vrabel@....com
Subject: Re: Scatter-gather list constraints

On Thu, 26 Jun 2008 08:35:59 +0200
Jens Axboe <jens.axboe@...cle.com> wrote:

> On Thu, Jun 26 2008, FUJITA Tomonori wrote:
> > On Thu, 26 Jun 2008 11:06:03 +0900
> > FUJITA Tomonori <fujita.tomonori@....ntt.co.jp> wrote:
> > 
> > > On Wed, 25 Jun 2008 10:23:00 -0400 (EDT)
> > > Alan Stern <stern@...land.harvard.edu> wrote:
> > > 
> > > > On Wed, 25 Jun 2008, FUJITA Tomonori wrote:
> > > > 
> > > > > > For example, suppose an I/O request starts out with two S-G elements
> > > > > > of 1536 bytes and 2048 bytes respectively, and the DMA requirement is
> > > > > > that all elements except the last must have length divisible by 1024.  
> > > > > > Then the request could be broken up into three requests of 1024, 512,
> > > > > > and 2048 bytes.
> > > > > 
> > > > > I can't say that it's easy to implement a clean mechanism to break up
> > > > > a request into multiple requests until I see a patch.
> > > > 
> > > > And I can't write a patch without learning a lot more about how the
> > > > block core works.
> > > > 
> > > > > What I said is that you think that this is about extending something
> > > > > in the block layer but it's about adding a new concept to the block
> > > > > layer.
> > > > 
> > > > Is it?  What does the block layer do when it receives an I/O request
> > > > that don't satisfy the other constraints (max_sectors or
> > > > dma_alignment_mask, for example)?
> > > 
> > > As I explained, you need something new.
> > > 
> > > I don't think that max_sectors works as you expect.
> > 
> > The block layer looks at max_sectors when merging two things (or add
> > one to another). So the test fails, it doesn't merge them.
> > 
> > 
> > > dma_alignment_mask is not used in the FS path. And I think that
> > > dma_alignment_mask doens't solve your problems.
> > 
> > If dma_alignment_mask test fails, the block layer allocates temporary
> > buffers and does memory copies.
> 
> I don't think adding anything in the general IO path makes a lot of
> sense, this is a really screwy case. I don't mind adding work-arounds to
> the block layer to cater for hardware weirdness, but this is getting a
> little silly. We could provide a helper function for 'bouncing' this
> request and thus reuse the block bounce buffer for this, but I'm not
> even sure how to simply express this generically. As it is likely of no
> use outside of this specific case, putting it in the driver (or usb
> layer, if you expect more of these similar cases) is the best option.

Yeah, agreed, as I wrote in the first mail:

http://marc.info/?l=linux-kernel&m=121430416329618&w=2

I guess that a generic mechanism reserving some buffers in the block
layer might work for them. I also need such a mechnism to convert sg
and st to use the block layer (yeah, it's overdue but still on my todo
list).
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