On Mon 23-10-17 20:32:25, Balbir Singh wrote: > On Mon, Oct 23, 2017 at 7:49 PM, Michal Hocko <mhocko@xxxxxxxx> wrote: > > On Sat 21-10-17 08:55:59, Balbir Singh wrote: > >> On Sat, Oct 21, 2017 at 12:08 AM, Michal Hocko <mhocko@xxxxxxxx> wrote: > >> > On Wed 18-10-17 17:31:23, Balbir Singh wrote: > >> >> With HMM, we can have either public or private zone > >> >> device pages. With private zone device pages, they should > >> >> show up as swapped entities. For public zone device pages > >> >> the smaps output can be confusing and incomplete. > >> >> > >> >> This patch adds a new attribute to just smaps to show > >> >> device memory usage. > >> > > >> > As this will become user API which we will have to maintain for ever I > >> > would really like to hear about who is going to use this information and > >> > what for. > >> > >> This is something I observed when running some tests with HMM/CDM. > >> The issue I had was that there was no visibility of what happened to the > >> pages after the following sequence > >> > >> 1. malloc/mmap pages > >> 2. migrate_vma() to ZONE_DEVICE (hmm/cdm space) > >> 3. look at smaps > >> > >> If we look at smaps after 1 and the pages are faulted in we can see the > >> pages for the region, but at point 3, there is absolutely no visibility of > >> what happened to the pages. I thought smaps is a good way to provide > >> the visibility as most developers use that interface. It's more to fix the > >> inconsistency I saw w.r.t visibility and accounting. > > > > Yes I can see how this can be confusing. But, well, I have grown overly > > cautious regarding user APIs over time. So I would rather not add > > something new until we have a real user with a usecase in mind. We can > > always add this later but once we have exposed the accounting we are > > bound to maintain it for ever. > > I see your point. But we are beginning to build on top of this. I'll just add > it as a private patch in my patchset. But soon, we'll need to address HMM/CDM > pages of different sizes as well. My problem right now is to ensure correctness > of the design and expectations and a large part of it is tracking > where the pages are. I would wait for more experiences to carve a userspace API into stone. Adding one for debugging purposes is almost _always_ a bad idea that back fires sooner or later. -- 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>
