linux-kernel - Re: [PATCH] mm/dmapool.c: avoid duplicate memset within dma_pool

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Message-Id: <20220716180110.eb9402180137d0ce84e3971c@linux-foundation.org>
Date:   Sat, 16 Jul 2022 18:01:10 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Liu Song <liusong@...ux.alibaba.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/dmapool.c: avoid duplicate memset within
 dma_pool_alloc

On Thu, 14 Jul 2022 11:25:00 +0800 Liu Song <liusong@...ux.alibaba.com> wrote:

> From: Liu Song <liusong@...ux.alibaba.com>
> 
> In "dma_direct_alloc", the allocated memory is explicitly set to 0.
> If use direct alloc, we need to avoid possible duplicate memset in
> dma_pool_alloc.

I'm having trouble seeing how this change is safe and correct and
maintainable.  Please describe the code flow more completely?

> --- a/mm/dmapool.c
> +++ b/mm/dmapool.c
> @@ -21,6 +21,7 @@
>  
>  #include <linux/device.h>
>  #include <linux/dma-mapping.h>
> +#include <linux/dma-map-ops.h>
>  #include <linux/dmapool.h>
>  #include <linux/kernel.h>
>  #include <linux/list.h>
> @@ -372,7 +373,7 @@ void *dma_pool_alloc(struct dma_pool *pool, gfp_t mem_flags,
>  #endif
>  	spin_unlock_irqrestore(&pool->lock, flags);
>  
> -	if (want_init_on_alloc(mem_flags))
> +	if (want_init_on_alloc(mem_flags) && get_dma_ops(pool->dev))
>  		memset(retval, 0, pool->size);

That DMAPOOL_DEBUG memset a couple of lines earlier could/should be
testing the same condition - there's no point in poisoning an area
which we're about to zero out.