On Wed, Oct 14, 2020 at 09:57:20AM -0700, Suren Baghdasaryan wrote: > On Wed, Oct 14, 2020 at 5:09 AM Michal Hocko <mhocko@xxxxxxxx> wrote: > > > > [Sorry for a late reply] > > > > On Mon 14-09-20 17:45:44, Suren Baghdasaryan wrote: > > > + linux-kernel@xxxxxxxxxxxxxxx > > > > > > On Mon, Sep 14, 2020 at 5:43 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote: > > > > > > > > Last year I sent an RFC about using oom-reaper while killing a > > > > process: https://patchwork.kernel.org/cover/10894999. During LSFMM2019 > > > > discussion https://lwn.net/Articles/787217 a couple of alternative > > > > options were discussed with the most promising one (outlined in the > > > > last paragraph of https://lwn.net/Articles/787217) suggesting to use a > > > > remote version of madvise(MADV_DONTNEED) operation to force memory > > > > reclaim of a killed process. With process_madvise() making its way > > > > through reviews (https://patchwork.kernel.org/patch/11747133/), I > > > > would like to revive this discussion and get feedback on several > > > > possible options, their pros and cons. > > > > Thanks for reviving this! > > Thanks for your feedback! > > > > > > > The need is similar to why oom-reaper was introduced - when a process > > > > is being killed to free memory we want to make sure memory is freed > > > > even if the victim is in uninterruptible sleep or is busy and reaction > > > > to SIGKILL is delayed by an unpredictable amount of time. I > > > > experimented with enabling process_madvise(MADV_DONTNEED) operation > > > > and using it to force memory reclaim of the target process after > > > > sending SIGKILL. Unfortunately this approach requires the caller to > > > > read proc/pid/maps to extract the list of VMAs to pass as an input to > > > > process_madvise(). > > > > Well I would argue that this is not really necessary. You can simply > > call process_madvise with the full address range and let the kernel > > operated only on ranges which are safe to tear down asynchronously. > > Sure that would require some changes to the existing code to not fail > > on those ranges if they contain incompatible vmas but that should be > > possible. If we are worried about backward compatibility then a > > dedicated flag could override. > > > > IIUC this is very similar to the last option I proposed. I think this > is doable if we treat it as a special case. process_madvise() return > value not being able to handle a large range would still be a problem. > Maybe we can return MAX_INT in those cases? Or, maybe we could just return 0 if the operation succeeds without any error. > > > [...] > > > > > > While the objective is to guarantee forward progress even when the > > > > victim cannot terminate, we still want this mechanism to be efficient > > > > because we perform these operations to relieve memory pressure before > > > > it affects user experience. > > > > > > > > Alternative options I would like your feedback are: > > > > 1. Introduce a dedicated process_madvise(MADV_DONTNEED_MM) > > > > specifically for this case to indicate that the whole mm can be freed. > > > > This shouldn't be any different from madvise on the full address range, > > right? > > > > Yep, just a matter of choosing the most appropriate API. I agree full range or just NULL passing to indicate entire address space would be better than introducing a new advise in that we could avoid MADV_PAGEOUT_MM, MADV_COLD_MM. > > > > > 2. A new syscall to efficiently obtain a vector of VMAs (start, > > > > length, flags) of the process instead of reading /proc/pid/maps. The > > > > size of the vector is still limited by UIO_MAXIOV (1024), so several > > > > calls might be needed to query larger number of VMAs, however it will > > > > still be an order of magnitude more efficient than reading > > > > /proc/pid/maps file in 4K or smaller chunks. > > > > While this might be interesting for other usecases - userspace memory > > management in general - I do not think it is directly related to this > > particular feature. > > > > True but such a syscall would be useful for other use cases, like > MADV_COLD/MADV_PAGEOUT that Minchan was working on. Maybe we can kill > more than one bird here? Minchan, any thought? Generally, it could be helpful but I don't see it as desperate at this moment.
