Re: [00/17] Large Blocksize Support V3

From: Jens Axboe
Date: Thu Apr 26 2007 - 13:46:59 EST

On Fri, Apr 27 2007, Nick Piggin wrote:
> Why do we limit drivers to 128 sg entries?

No particular reason, except than to avoid 2^bigger order allocations.
2MiB requests would require 3 contig pages to setup the sg list, which
is (probably) a little troublesome especially since it's sometimes
atomically allocated.

Larger pages are by no means a prerequisite to getting larger requests,
assuming your hardware can handle the bigger sglist. There are other
ways of doing that, I've contemplated doing chained sglists and adding
sg_for_each_segment() macros for iterating these things. Drivers that
want larger sglists would then be required to update their sg mapping
loop to use the provided macros. It wouldn't be too hard.

Jens Axboe

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