Re: Scatter-gather list constraints

[FUJITA Tomonori]

On Thu, 26 Jun 2008 11:06:03 +0900
FUJITA Tomonori <fujita.tomonori@xxxxxxxxxxxxx> wrote:

> On Wed, 25 Jun 2008 10:23:00 -0400 (EDT)
> Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> 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.
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