Re: [PATCH 6/6] mm/hmm: documents how device memory is accounted in rss and memcg

From: Michal Hocko
Date: Fri Jul 14 2017 - 09:27:17 EST

On Thu 13-07-17 17:15:32, Jérôme Glisse wrote:
> For now we account device memory exactly like a regular page in
> respect to rss counters and memory cgroup. We do this so that any
> existing application that starts using device memory without knowing
> about it will keep running unimpacted. This also simplify migration
> code.
> We will likely revisit this choice once we gain more experience with
> how device memory is use and how it impacts overall memory resource
> management. For now we believe this is a good enough choice.
> Note that device memory can not be pin. Nor by device driver, nor
> by GUP thus device memory can always be free and unaccounted when
> a process exit.

I have to look at the implementation but this gives a good idea of what
is going on and why.

> Signed-off-by: Jérôme Glisse <jglisse@xxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

> ---
> Documentation/vm/hmm.txt | 40 ++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 40 insertions(+)
> diff --git a/Documentation/vm/hmm.txt b/Documentation/vm/hmm.txt
> index 192dcdb38bd1..4d3aac9f4a5d 100644
> --- a/Documentation/vm/hmm.txt
> +++ b/Documentation/vm/hmm.txt
> @@ -15,6 +15,15 @@ section present the new migration helper that allow to leverage the device DMA
> engine.
> +1) Problems of using device specific memory allocator:
> +2) System bus, device memory characteristics
> +3) Share address space and migration
> +4) Address space mirroring implementation and API
> +5) Represent and manage device memory from core kernel point of view
> +6) Migrate to and from device memory
> +7) Memory cgroup (memcg) and rss accounting
> +
> +
> -------------------------------------------------------------------------------
> 1) Problems of using device specific memory allocator:
> @@ -342,3 +351,34 @@ that happens then the finalize_and_map() can catch any pages that was not
> migrated. Note those page were still copied to new page and thus we wasted
> bandwidth but this is considered as a rare event and a price that we are
> willing to pay to keep all the code simpler.
> +
> +
> +-------------------------------------------------------------------------------
> +
> +7) Memory cgroup (memcg) and rss accounting
> +
> +For now device memory is accounted as any regular page in rss counters (either
> +anonymous if device page is use for anonymous, file if device page is use for
> +file back page or shmem if device page is use for share memory). This is a
> +deliberate choice to keep existing application that might start using device
> +memory without knowing about it to keep runing unimpacted.
> +
> +Drawbacks is that OOM killer might kill an application using a lot of device
> +memory and not a lot of regular system memory and thus not freeing much system
> +memory. We want to gather more real world experience on how application and
> +system react under memory pressure in the presence of device memory before
> +deciding to account device memory differently.
> +
> +
> +Same decision was made for memory cgroup. Device memory page are accounted
> +against same memory cgroup a regular page would be accounted to. This does
> +simplify migration to and from device memory. This also means that migration
> +back from device memory to regular memory can not fail because it would
> +go above memory cgroup limit. We might revisit this choice latter on once we
> +get more experience in how device memory is use and its impact on memory
> +resource control.
> +
> +
> +Note that device memory can never be pin nor by device driver nor through GUP
> +and thus such memory is always free upon process exit. Or when last reference
> +is drop in case of share memory or file back memory.
> --
> 2.13.0

Michal Hocko