linux-kernel - Re: Scatter-gather list constraints

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Date:	Thu, 26 Jun 2008 19:39:16 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Boaz Harrosh <bharrosh@...asas.com>
Cc:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	stern@...land.harvard.edu, andi@...stfloor.org,
	linux-kernel@...r.kernel.org, antonio.lin@...ormicro.com,
	david.vrabel@....com,
	"Martin K. Petersen" <martin.petersen@...cle.com>
Subject: Re: Scatter-gather list constraints

On Thu, Jun 26 2008, Boaz Harrosh wrote:
> Jens Axboe wrote:
> > On Thu, Jun 26 2008, FUJITA Tomonori wrote:
> >> 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).
> > 
> > On the fs side, just setting a hw block size of 1k should fix the
> > problem, since that'd be your minimum transfer size AND alignment there
> > even for O_DIRECT IO.
> 
> Please forgive my ignorance, is there a way for devices to specify
> minimum block size to upper layer, say if we have a new sata with 1k
> sectors?

Sure, blk_queue_hardsect_size(). We support > 512b sector devices just
fine.

-- 
Jens Axboe

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