[PATCH 1/1] [RFC] drm/ttm: Don't init dma32_zone on 64-bit systems

Thomas Hellstrom thomas at shipmail.org
Wed Feb 20 08:14:45 UTC 2019 

On 2/20/19 9:07 AM, Christian König wrote:
> Am 20.02.19 um 07:41 schrieb Thomas Hellstrom:
>> On Tue, 2019-02-19 at 17:06 +0000, Kuehling, Felix wrote:
>>> On 2019-02-18 3:39 p.m., Thomas Hellstrom wrote:
>>>> On Mon, 2019-02-18 at 18:07 +0100, Christian König wrote:
>>>>> Am 18.02.19 um 10:47 schrieb Thomas Hellstrom:
>>>>>> On Mon, 2019-02-18 at 09:20 +0000, Koenig, Christian wrote:
>>>>>>> Another good question is also why the heck the acc_size
>>>>>>> counts
>>>>>>> towards
>>>>>>> the DMA32 zone?
>>>>>> DMA32 TTM pages are accounted in the DMA32 zone. Other pages
>>>>>> are
>>>>>> not.
>>>>> Yeah, I'm perfectly aware of this. But this is for the accounting
>>>>> size!
>>>>>
>>>>> We have an accounting for the stuff needed additional to the
>>>>> pages
>>>>> backing the BO (e.g. the page and DMA addr array).
>>>>>
>>>>> And from the bug description it sounds like we use the DMA32 zone
>>>>> for
>>>>> this accounting which of course is completely nonsense.
>>>> It's actually accounted in all available zones, since it would be
>>>> pretty hard to determine exactly where that memory should be
>>>> accounted.
>>>> In particular if it's vmalloced. It might be DMA32, it might not.
>>>> Given
>>>> the objective of stopping malicious user-space from exhausting the
>>>> DMA32 zone it was, at the time the code was written, a reasonable
>>>> approximation. With ever increasing memory sizes, there might be
>>>> better
>>>> solutions?
>>> As far as I can see, in TTM, ttm_mem_global_alloc is only used for
>>> the
>>> acc_size in ttm_bo_init_reserved. Other than that vmwgfx also seems
>>> to
>>> use it to account for a few things that are allocated with kmalloc.
>>>
>>> So would a better solution be to change ttm_mem_global_alloc to use
>>> only
>>> the kernel zone?
>>>
>> IMO we need to determine what functionality to keep and then the best
>> solution. The current code does its job, but is obviously too
>> restrictive. Both of the solutions you suggest open up for potential
>> DOS attacks (DMA32 and kernel zones are not mutually exclusive. They
>> overlap).
>
> Yeah and exactly because of this it doesn't make to much sense to 
> account the housekeeping stuff in both zones.

IMO it makes sense because (given the assumption that kmalloc/vmalloc 
doesn't care) , accounting only in DMA32 in low memory configurations 
will not guarantee that kernel is not exhausted. Accounting only in 
kernel in huge memory configurations will not guarantee that DMA32 is 
not exhausted.

>
> IIRC the kernel takes perfectly care by itself that kmalloced memory 
> doesn't eat up everything in the DMA32 zone.

If we can somehow verify this, or at least illustrate that it's likely, 
I'm perfectly fine with either patch from the vmwgfx POW.

Thanks,
Thomas



>
> Christian.
>
>>
>>
>> /Thomas
>>
>>
>>
>>
>>> Regards,
>>>     Felix
>>>
>>>
>>>> /Thomas
>>>>
>>>>> Christian.
>>>>>
>>>>>> For small persistent allocations using ttm_mem_global_alloc(),
>>>>>> they
>>>>>> are
>>>>>> accounted also in the DMA32 zone, which may cause over-
>>>>>> accounting
>>>>>> of
>>>>>> that zone, but that's pretty unlikely to be a big problem..
>>>>>>
>>>>>> /Thomas
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>> In other words why should the internal bookkeeping pages be
>>>>>>> allocated
>>>>>>> in
>>>>>>> the DMA32 zone?
>>>>>>>
>>>>>>> That doesn't sounds valid to me in any way,
>>>>>>> Christian.
>>>>>>>
>>>>>>> Am 18.02.19 um 09:02 schrieb Thomas Hellstrom:
>>>>>>>> Hmm,
>>>>>>>>
>>>>>>>> This zone was intended to stop TTM page allocations from
>>>>>>>> exhausting
>>>>>>>> the DMA32 zone. IIRC dma_alloc_coherent() uses DMA32 by
>>>>>>>> default,
>>>>>>>> which
>>>>>>>> means if we drop this check, other devices may stop
>>>>>>>> functioning
>>>>>>>> unexpectedly?
>>>>>>>>
>>>>>>>> However, in the end I'd expect the kernel page allocation
>>>>>>>> system
>>>>>>>> to
>>>>>>>> make sure there are some pages left in the DMA32 zone,
>>>>>>>> otherwise
>>>>>>>> random non-IO page allocations would also potentially
>>>>>>>> exhaust
>>>>>>>> the
>>>>>>>> DMA32 zone without anybody caring, which means removing
>>>>>>>> this
>>>>>>>> zone
>>>>>>>> wouldn't be any worse than whatever other subsystems may be
>>>>>>>> doing
>>>>>>>> already...
>>>>>>>>
>>>>>>>> /Thomas
>>>>>>>>
>>>>>>>>
>>>>>>>> On 2/16/19 12:02 AM, Kuehling, Felix wrote:
>>>>>>>>> This is an RFC. I'm not sure this is the right solution,
>>>>>>>>> but
>>>>>>>>> it
>>>>>>>>> highlights the problem I'm trying to solve.
>>>>>>>>>
>>>>>>>>> The dma32_zone limits the acc_size of all allocated BOs
>>>>>>>>> to
>>>>>>>>> 2GB.
>>>>>>>>> On a
>>>>>>>>> 64-bit system with hundreds of GB of system memory and
>>>>>>>>> GPU
>>>>>>>>> memory,
>>>>>>>>> this can become a bottle neck. We're seeing TTM memory
>>>>>>>>> allocation
>>>>>>>>> failures not because we're truly out of memory, but
>>>>>>>>> because
>>>>>>>>> we're
>>>>>>>>> out of space in the dma32_zone for the acc_size needed
>>>>>>>>> for
>>>>>>>>> our BO
>>>>>>>>> book-keeping.
>>>>>>>>>
>>>>>>>>> Signed-off-by: Felix Kuehling <Felix.Kuehling at amd.com>
>>>>>>>>> CC: thellstrom at vmware.com
>>>>>>>>> CC: christian.koenig at amd.com
>>>>>>>>> ---
>>>>>>>>>      drivers/gpu/drm/ttm/ttm_memory.c | 4 ++--
>>>>>>>>>      1 file changed, 2 insertions(+), 2 deletions(-)
>>>>>>>>>
>>>>>>>>> diff --git a/drivers/gpu/drm/ttm/ttm_memory.c
>>>>>>>>> b/drivers/gpu/drm/ttm/ttm_memory.c
>>>>>>>>> index f1567c3..bb05365 100644
>>>>>>>>> --- a/drivers/gpu/drm/ttm/ttm_memory.c
>>>>>>>>> +++ b/drivers/gpu/drm/ttm/ttm_memory.c
>>>>>>>>> @@ -363,7 +363,7 @@ static int
>>>>>>>>> ttm_mem_init_highmem_zone(struct
>>>>>>>>> ttm_mem_global *glob,
>>>>>>>>>          glob->zones[glob->num_zones++] = zone;
>>>>>>>>>          return 0;
>>>>>>>>>      }
>>>>>>>>> -#else
>>>>>>>>> +#elifndef CONFIG_64BIT
>>>>>>>>>      static int ttm_mem_init_dma32_zone(struct
>>>>>>>>> ttm_mem_global
>>>>>>>>> *glob,
>>>>>>>>>                         const struct sysinfo *si)
>>>>>>>>>      {
>>>>>>>>> @@ -441,7 +441,7 @@ int ttm_mem_global_init(struct
>>>>>>>>> ttm_mem_global
>>>>>>>>> *glob)
>>>>>>>>>          ret = ttm_mem_init_highmem_zone(glob, &si);
>>>>>>>>>          if (unlikely(ret != 0))
>>>>>>>>>              goto out_no_zone;
>>>>>>>>> -#else
>>>>>>>>> +#elifndef CONFIG_64BIT
>>>>>>>>>          ret = ttm_mem_init_dma32_zone(glob, &si);
>>>>>>>>>          if (unlikely(ret != 0))
>>>>>>>>>              goto out_no_zone;
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