Re: [Kiobuf-io-devel] RFC: Kernel mechanism: Compound event wait

["Jeff V. Merkey" <jmerkey@vger.timpanogas.org>]

On Tue, Feb 06, 2001 at 04:50:19PM -0800, Linus Torvalds wrote:
> 
> 
> On Wed, 7 Feb 2001, Stephen C. Tweedie wrote:
> > 
> > That gets us from 512-byte blocks to 4k, but no more (ll_rw_block
> > enforces a single blocksize on all requests but that relaxing that
> > requirement is no big deal).  Buffer_heads can't deal with data which
> > spans more than a page right now.
> 
> Stephen, you're so full of shit lately that it's unbelievable. You're
> batting a clear 0.000 so far.
> 
> "struct buffer_head" can deal with pretty much any size: the only thing it
> cares about is bh->b_size.
> 
> It so happens that if you have highmem support, then "create_bounce()"
> will work on a per-page thing, but that just means that you'd better have
> done your bouncing into low memory before you call generic_make_request().
> 
> Have you ever spent even just 5 minutes actually _looking_ at the block
> device layer, before you decided that you think it needs to be completely
> re-done some other way? It appears that you never bothered to.
> 
> Sure, I would not be surprised if some device driver ends up being
> surpised if you start passing it different request sizes than it is used
> to. But that's a driver and testing issue, nothing more.
> 
> (Which is not to say that "driver and testing" issues aren't important as
> hell: it's one of the more scary things in fact, and it can take a long
> time to get right if you start doing somehting that historically has never
> been done and thus has historically never gotten any testing. So I'm not
> saying that it should work out-of-the-box. But I _am_ saying that there's
> no point in trying to re-design upper layers that already do ALL of this
> with no problems at all).
> 
> 		Linus
> 

I remember Linus asking to try this variable buffer head chaining 
thing 512-1024-512 kind of stuff several months back, and mixing them to 
see what would happen -- result.  About half the drivers break with it.  
The interface allows you to do it, I've tried it, (works on Andre's 
drivers, but a lot of SCSI drivers break) but a lot of drivers seem to 
have assumptions about these things all being the same size in a 
buffer head chain. 

:-)

Jeff


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