On Mon, Oct 23, 2017 at 8:36 PM, Michal Hocko <mhocko@xxxxxxxx> wrote: > 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. Fair enough, I am going to keep these in my private branch for now Balbir Singh. -- 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>
