linux-kernel - Re: [RFC PATCH 0/3] target: try satisfying memory requests with higher-order allocations

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Date:	Wed, 05 Sep 2012 18:58:18 -0700
From:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	linux-kernel@...r.kernel.org, target-devel@...r.kernel.org,
	Christoph Hellwig <hch@....de>
Subject: Re: [RFC PATCH 0/3] target: try satisfying memory requests with
 higher-order allocations

On Wed, 2012-09-05 at 17:13 +0200, Paolo Bonzini wrote:
> Hi all,
> 
> while testing PSCSI I noticed that even requests for a smallish amount
> of data (approximately 700 KB) failed due to an excessive number of
> segments in the request.  In fact, using alloc_page resulted in a
> completely fragmented request, with no merging of consecutive pages
> at all.
> 
> This patch series fixes this problem by using higher-order allocations
> to build the data scatterlist.  The problem is that iscsi assumes that the
> scatterlist consists of single pages, which is not true anymore.  So
> patch 2 has to introduce some relatively complicated changes to
> iscsi_map_iovec and iscsi_unmap_iovec.
> 

So enabling multi-page per SGL support is a feature that has been
dormant within target core for a long time.  It's about time that we
start taking advantage of it again.  ;)

> While doing this, I noticed something strange in iscsit_do_crypto_hash_sg.
> Patch 1 adds a warning about it.
> 

Mmmmm, looks like a separate bug with DataDigest enabled.  

> The approach may be completely wrong and it needs more testing anyway.
> Please review!
> 

Adding my comments inline.

Thanks Paolo!

--nab

> Paolo
> 
> Paolo Bonzini (3):
>   tcm_iscsi: warn on incorrect precondition for iscsit_do_crypto_hash_sg
>   tcm_iscsi: support multiple sizes in the scatterlist
>   target: try satisfying memory requests with contiguous blocks
> 
>  drivers/target/iscsi/iscsi_target.c      |  106 +++++++++++++++++++++++++-----
>  drivers/target/iscsi/iscsi_target_core.h |    2 +-
>  drivers/target/target_core_transport.c   |   58 ++++++++++++++---
>  3 files changed, 138 insertions(+), 28 deletions(-)
> 
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