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

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Message-ID: <a40fa6a3-f137-2adb-b432-8d60cbf3f525@redhat.com>
Date:   Tue, 3 Apr 2018 15:20:55 +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 18:39, Greg Kroah-Hartman wrote:
> On Thu, Mar 29, 2018 at 03:42:59PM +0200, Hans de Goede wrote:
>> 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.
> 
> If there are 50 users of such an api, why is the mm core not handling
> this for drivers automatically?  It seems harsh that they all have to do
> this type of allocation on their own.

True, but fixing that falls outside the scope of this patch-set,
which is purely a small bug-fix patchset fixing a bug which
is being hit be users in the field atm.

So it would be nice if we can get (version 2) of this patch-set
merged.

>> 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.
> 
> If they haven't noticed that their hardware isn't working, perhaps they
> don't need to set that flag?  :)

Or they have never tested with more then 4G of RAM, which is what
caused me to not find this problem in my own testing of the
vboxguest driver.

Regards,

Hans