Re: [PATCH] remove use_sg_chaining

[Boaz Harrosh <bharrosh@panasas.com>]

On Tue, Jan 15 2008 at 19:52 +0200, James Bottomley <James.Bottomley@HansenPartnership.com> wrote:
> this patch depends on the sg branch of the block tree
> 
> James
> 
> ---
> From: James Bottomley <James.Bottomley@HansenPartnership.com>
> Date: Tue, 15 Jan 2008 11:11:46 -0600
> Subject: remove use_sg_chaining
> 
> With the sg table code, every SCSI driver is now either chain capable
> or broken, so there's no need to have a check in the host template.
> 
> Also tidy up the code by moving the scatterlist size defines into the
> SCSI includes and permit the last entry of the scatterlist pools not
> to be a power of two.
> ---

I have a theoretical problem that BUGed me from the beginning.

Could it happen that a memory critical IO, (that is needed to free
memory), be collected into an sg-chained large IO, and the allocation 
of the multiple sg-pool-allocations fail, thous dead locking on
out-of-memory? Is there a mechanism in place that will split large IO's 
into smaller chunks in the event of out-of-memory condition in prep_fn?

Is it possible to call blk_rq_map_sg() with less then what is present
at request to only map the starting portion?

Boaz