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 SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
