linux-kernel - Re: [PATCH 2/3] virt: vbox: Use __get_free_pages instead of kmalloc for DMA32 memory

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Date:   Thu, 29 Mar 2018 15:42:59 +0200
From:   Hans de Goede <hdegoede@...hat.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Michael Thayer <michael.thayer@...cle.com>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH 2/3] virt: vbox: Use __get_free_pages instead of kmalloc
 for DMA32 memory

Hi,

On 29-03-18 13:58, Greg Kroah-Hartman wrote:
> On Thu, Mar 29, 2018 at 01:21:15PM +0200, Hans de Goede wrote:
>> It is not possible to get DMA32 zone memory through kmalloc,
> 
> Why can't we just fix that issue here instead?

AFAIK that would go against the design of the whole slab
allocator, it creates buckets (kmem_cache objects) for
differently sized allocs out of the normal memzone,
you cannot select what memzone you want with kmalloc,
if you need to specify a memzone you need to use either
dma specific malloc functions such as the dmapool.h
functions, or directly call alloc_pages / get_free_pages.

A quick grep on the drivers dir shows 50 files using
GFP_DMA32 and almost all are passing it to alloc_page
or __get_page.

I do actually see 2 drivers which are passing it to
kmalloc, which as said will not work I will mail the
maintainers of those about this.

Regards,

Hans