On Tue, Jul 23, 2019 at 08:05:25AM +0200, Michal Hocko wrote: > [Cc linux-api - please always do CC this list when introducing a user > visible API] Sorry, will do. > On Mon 22-07-19 17:32:04, Joel Fernandes (Google) wrote: > > The page_idle tracking feature currently requires looking up the pagemap > > for a process followed by interacting with /sys/kernel/mm/page_idle. > > This is quite cumbersome and can be error-prone too. If between > > accessing the per-PID pagemap and the global page_idle bitmap, if > > something changes with the page then the information is not accurate. > > More over looking up PFN from pagemap in Android devices is not > > supported by unprivileged process and requires SYS_ADMIN and gives 0 for > > the PFN. > > > > This patch adds support to directly interact with page_idle tracking at > > the PID level by introducing a /proc/<pid>/page_idle file. This > > eliminates the need for userspace to calculate the mapping of the page. > > It follows the exact same semantics as the global > > /sys/kernel/mm/page_idle, however it is easier to use for some usecases > > where looking up PFN is not needed and also does not require SYS_ADMIN. > > It ended up simplifying userspace code, solving the security issue > > mentioned and works quite well. SELinux does not need to be turned off > > since no pagemap look up is needed. > > > > In Android, we are using this for the heap profiler (heapprofd) which > > profiles and pin points code paths which allocates and leaves memory > > idle for long periods of time. > > > > Documentation material: > > The idle page tracking API for virtual address indexing using virtual page > > frame numbers (VFN) is located at /proc/<pid>/page_idle. It is a bitmap > > that follows the same semantics as /sys/kernel/mm/page_idle/bitmap > > except that it uses virtual instead of physical frame numbers. > > > > This idle page tracking API can be simpler to use than physical address > > indexing, since the pagemap for a process does not need to be looked up > > to mark or read a page's idle bit. It is also more accurate than > > physical address indexing since in physical address indexing, address > > space changes can occur between reading the pagemap and reading the > > bitmap. In virtual address indexing, the process's mmap_sem is held for > > the duration of the access. > > I didn't get to read the actual code but the overall idea makes sense to > me. I can see this being useful for userspace memory management (along > with remote MADV_PAGEOUT, MADV_COLD). Thanks. > Normally I would object that a cumbersome nature of the existing > interface can be hidden in a userspace but I do agree that rowhammer has > made this one close to unusable for anything but a privileged process. Agreed, this is one of the primary motivations for the patch as you said. > I do not think you can make any argument about accuracy because > the information will never be accurate. Sure the race window is smaller > in principle but you can hardly say anything about how much or whether > at all. Sure, fair enough. That is why I wasn't beating the drum too much on the accuracy point. However, this surprisingly does work quite well. thanks, - Joel
