GEM memory DOS (WAS Re: [PATCH 3/3] drm/ttm: under memory pressure minimize the size of memory pool)

Jesse Barnes jbarnes at
Thu Aug 14 15:29:15 PDT 2014

On Wed, 13 Aug 2014 17:13:56 +0200
Thomas Hellstrom <thellstrom at> wrote:

> On 08/13/2014 03:01 PM, Daniel Vetter wrote:
> > On Wed, Aug 13, 2014 at 02:35:52PM +0200, Thomas Hellstrom wrote:
> >> On 08/13/2014 12:42 PM, Daniel Vetter wrote:
> >>> On Wed, Aug 13, 2014 at 11:06:25AM +0200, Thomas Hellstrom wrote:
> >>>> On 08/13/2014 05:52 AM, Jérôme Glisse wrote:
> >>>>> From: Jérôme Glisse <jglisse at>
> >>>>>
> >>>>> When experiencing memory pressure we want to minimize pool size so that
> >>>>> memory we just shrinked is not added back again just as the next thing.
> >>>>>
> >>>>> This will divide by 2 the maximum pool size for each device each time
> >>>>> the pool have to shrink. The limit is bumped again is next allocation
> >>>>> happen after one second since the last shrink. The one second delay is
> >>>>> obviously an arbitrary choice.
> >>>> Jérôme,
> >>>>
> >>>> I don't like this patch. It adds extra complexity and its usefulness is
> >>>> highly questionable.
> >>>> There are a number of caches in the system, and if all of them added
> >>>> some sort of voluntary shrink heuristics like this, we'd end up with
> >>>> impossible-to-debug unpredictable performance issues.
> >>>>
> >>>> We should let the memory subsystem decide when to reclaim pages from
> >>>> caches and what caches to reclaim them from.
> >>> Yeah, artificially limiting your cache from growing when your shrinker
> >>> gets called will just break the equal-memory pressure the core mm uses to
> >>> rebalance between all caches when workload changes. In i915 we let
> >>> everything grow without artificial bounds and only rely upon the shrinker
> >>> callbacks to ensure we don't consume more than our fair share of available
> >>> memory overall.
> >>> -Daniel
> >> Now when you bring i915 memory usage up, Daniel,
> >> I can't refrain from bringing up the old user-space unreclaimable kernel
> >> memory issue, for which gem open is a good example ;) Each time
> >> user-space opens a gem handle, some un-reclaimable kernel memory is
> >> allocated, for which there is no accounting, so theoretically I think a
> >> user can bring a system to unusability this way.
> >>
> >> Typically there are various limits on unreclaimable objects like this,
> >> like open file descriptors, and IIRC the kernel even has an internal
> >> limit on the number of struct files you initialize, based on the
> >> available system memory, so dma-buf / prime should already have some
> >> sort of protection.
> > Oh yeah, we have zero cgroups limits or similar stuff for gem allocations,
> > so there's not really a way to isolate gpu memory usage in a sane way for
> > specific processes. But there's also zero limits on actual gpu usage
> > itself (timeslices or whatever) so I guess no one asked for this yet.
> In its simplest form (like in TTM if correctly implemented by drivers)
> this type of accounting stops non-privileged malicious GPU-users from
> exhausting all system physical memory causing grief for other kernel
> systems but not from causing grief for other GPU users. I think that's
> the minimum level that's intended also for example also for the struct
> file accounting.
> > My comment really was about balancing mm users under the assumption that
> > they're all unlimited.
> Yeah, sorry for stealing the thread. I usually bring this up now and
> again but nowadays with an exponential backoff.

Yeah I agree we're missing some good limits stuff in i915 and DRM in
general probably.  Chris started looking at this awhile back, but I
haven't seen anything recently.  Tying into the ulimits/rlimits might
make sense, and at the very least we need to account for things
properly so we can add new limits where needed.

Jesse Barnes, Intel Open Source Technology Center

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