linux-kernel - Re: [PATCH] net: don't use kvzalloc for DMA memory

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Date:   Wed, 18 Apr 2018 13:38:43 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Cc:     Mikulas Patocka <mpatocka@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Joby Poriyath <joby.poriyath@...rix.com>,
        Ben Hutchings <bhutchings@...arflare.com>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net: don't use kvzalloc for DMA memory



On 04/18/2018 10:55 AM, Michael S. Tsirkin wrote:

> Imagine you want to pass some data to card.
> Natural thing is to just put it in a variable and start DMA.
> However DMA API disallows stack access nowdays,
> so it's natural to put this within struct device.
> 
> See e.g.
> 
> 	commit a725ee3e44e39dab1ec82cc745899a785d2a555e
> 	Author: Andy Lutomirski <luto@...nel.org>
> 	Date:   Mon Jul 18 15:34:49 2016 -0700
> 
> 	    virtio-net: Remove more stack DMA
>

Andy just moved the problem to another one, since at that time we already
had vmalloc() fallback for at least 2 years.

Note that my original patch had :

p = kzalloc(alloc_size, GFP_KERNEL | __GFP_NOWARN | __GFP_REPEAT);
if (!p)
      p = vzalloc(alloc_size);

So really, normal (less than PAGE_SIZE) allocations would have almost-zero-chance to end up to vmalloc(one_page)