Re: [11/36] Use page_cache_xxx in fs/buffer.c

[Jens Axboe]

On Fri, Aug 31 2007, Christoph Lameter wrote:
> On Fri, 31 Aug 2007, Jens Axboe wrote:
> 
> > > Could you be more specific?
> > 
> > Size of a single segment, for instance. Or if the bio crosses a dma
> > boundary. If your block is 64kb and the maximum segment size is 32kb,
> > then you would need to clone the bio and split it into two.
> 
> A DMA boundary cannot be crossed AFAIK. The compound pages are aligned to 
> the power of two boundaries and the page allocator will not create pages 
> that cross the zone boundaries.

With a 64k page and a dma boundary of 0x7fff, that's two segments.

> It looks like the code will correctly signal a failure if you try to write 
> a 64k block on a device with a maximum segment size of 32k. Isnt this 
> okay? One would not want to use a larger block size than supported by the 
> underlying hardware?

That's just the size in sectors limitation again. And that also needs to
be handled, the fact that it currently errors out is reassuring but
definitely a long term solution. You don't want to knowingly setup such
a system where the fs block size is larger than what the hardware would
want, but it should work. You could be moving hardware around, for
recovery or otherwise.

> > Things like that. This isn't a problem with single page requests, as we
> > based the lower possible boundaries on that.
> 
> submit_bh() is used to submit a single buffer and I think that was our 
> main concern here.

And how large can that be?

-- 
Jens Axboe

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