linux-kernel - Re: [PATCH] scatterlist: Allocate a contiguous array instead of chaining

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Date:   Fri, 12 Jul 2019 14:56:15 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Sultan Alsawaf <sultan@...neltoast.com>
Cc:     Jason Gunthorpe <jgg@...pe.ca>, Palmer Dabbelt <palmer@...ive.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Gal Pressman <galpress@...zon.com>,
        Allison Randal <allison@...utok.net>,
        Christophe Leroy <christophe.leroy@....fr>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] scatterlist: Allocate a contiguous array instead of
 chaining

On Thu, Jul 11, 2019 at 11:36:56PM -0700, Sultan Alsawaf wrote:
> From: Sultan Alsawaf <sultan@...neltoast.com>
> 
> Typically, drivers allocate sg lists of sizes up to a few MiB in size.
> The current algorithm deals with large sg lists by splitting them into
> several smaller arrays and chaining them together. But if the sg list
> allocation is large, and we know the size ahead of time, sg chaining is
> both inefficient and unnecessary.
> 
> Rather than calling kmalloc hundreds of times in a loop for chaining
> tiny arrays, we can simply do it all at once with kvmalloc, which has
> the proper tradeoff on when to stop using kmalloc and instead use
> vmalloc.

vmalloc() may sleep, so it is impossible to be called in atomic context.

Thanks,
Ming