Re: [PATCH] mm/swap: add function get_total_swap_pages to expose total_swap_pages

[Michal Hocko <mhocko@kernel.org>]

On Tue 30-01-18 11:32:49, Christian König wrote:
> Am 30.01.2018 um 11:18 schrieb Michal Hocko:
> > On Tue 30-01-18 10:00:07, Christian König wrote:
> > > Am 30.01.2018 um 08:55 schrieb Michal Hocko:
> > > > On Tue 30-01-18 02:56:51, He, Roger wrote:
> > > > > Hi Michal:
> > > > > 
> > > > > We need a API to tell TTM module the system totally has how many swap
> > > > > cache.  Then TTM module can use it to restrict how many the swap cache
> > > > > it can use to prevent triggering OOM.  For Now we set the threshold of
> > > > > swap size TTM used as 1/2 * total size and leave the rest for others
> > > > > use.
> > > > Why do you so much memory? Are you going to use TB of memory on large
> > > > systems? What about memory hotplug when the memory is added/released?
> > > For graphics and compute applications on GPUs it isn't unusual to use large
> > > amounts of system memory.
> > > 
> > > Our standard policy in TTM is to allow 50% of system memory to be pinned for
> > > use with GPUs (the hardware can't do page faults).
> > > 
> > > When that limit is exceeded (or the shrinker callbacks tell us to make room)
> > > we wait for any GPU work to finish and copy buffer content into a shmem
> > > file.
> > > 
> > > This copy into a shmem file can easily trigger the OOM killer if there isn't
> > > any swap space left and that is something we want to avoid.
> > > 
> > > So what we want to do is to apply this 50% rule to swap space as well and
> > > deny allocation of buffer objects when it is exceeded.
> > How does that help when the rest of the system might eat swap?
> 
> Well it doesn't, but that is not the problem here.
> 
> When an application keeps calling malloc() it sooner or later is confronted
> with an OOM killer.
> 
> But when it keeps for example allocating OpenGL textures the expectation is
> that this sooner or later starts to fail because we run out of memory and
> not trigger the OOM killer.

There is nothing like running out of memory and not triggering the OOM
killer. You can make a _particular_ allocation to bail out without the
oom killer. Just use __GFP_NORETRY. But that doesn't make much
difference when you have already depleted your memory and live with the
bare remainings. Any desperate soul trying to get its memory will simply
trigger the OOM.

> So what we do is to allow the application to use all of video memory + a
> certain amount of system memory + swap space as last resort fallback (e.g.
> when you Alt+Tab from your full screen game back to your browser).
> 
> The problem we try to solve is that we haven't limited the use of swap space
> somehow.

I do think you should completely ignore the size of the swap space. IMHO
you should forbid further allocations when your current buffer storage
cannot be reclaimed. So you need some form of feedback mechanism that
would tell you: "Your buffers have grown too much". If you cannot do
that then simply assume that you cannot swap at all rather than rely on
having some portion of it for yourself. There are many other users of
memory outside of your subsystem. Any scaling based on the 50% of resource
belonging to me is simply broken.
-- 
Michal Hocko
SUSE Labs