On Fri, Aug 9, 2019 at 1:39 PM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote: > > On 8/9/19 11:05 AM, Mina Almasry wrote: > > On Fri, Aug 9, 2019 at 4:27 AM Michal Koutný <mkoutny@xxxxxxxx> wrote: > >>> Alternatives considered: > >>> [...] > >> (I did not try that but) have you considered: > >> 3) MAP_POPULATE while you're making the reservation, > > > > I have tried this, and the behaviour is not great. Basically if > > userspace mmaps more memory than its cgroup limit allows with > > MAP_POPULATE, the kernel will reserve the total amount requested by > > the userspace, it will fault in up to the cgroup limit, and then it > > will SIGBUS the task when it tries to access the rest of its > > 'reserved' memory. > > > > So for example: > > - if /proc/sys/vm/nr_hugepages == 10, and > > - your cgroup limit is 5 pages, and > > - you mmap(MAP_POPULATE) 7 pages. > > > > Then the kernel will reserve 7 pages, and will fault in 5 of those 7 > > pages, and will SIGBUS you when you try to access the remaining 2 > > pages. So the problem persists. Folks would still like to know they > > are crossing the limits on mmap time. > > If you got the failure at mmap time in the MAP_POPULATE case would this > be useful? > > Just thinking that would be a relatively simple change. Not quite, unfortunately. A subset of the folks that want to use hugetlb memory, don't want to use MAP_POPULATE (IIRC, something about mmaping a huge amount of hugetlb memory at their jobs' startup, and doing that with MAP_POPULATE adds so much to their startup time that it is prohibitively expensive - but that's just what I vaguely recall offhand. I can get you the details if you're interested). > -- > Mike Kravetz
