On Fri, Nov 13, 2020 at 6:16 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > > On Fri, 13 Nov 2020 17:57:02 -0800 Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote: > > > On Fri, Nov 13, 2020 at 5:18 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > > > > > > On Fri, 13 Nov 2020 17:09:37 -0800 Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote: > > > > > > > > > > Seems to me that the ability to reap another process's memory is a > > > > > > > generally useful one, and that it should not be tied to delivering a > > > > > > > signal in this fashion. > > > > > > > > > > > > > > And we do have the new process_madvise(MADV_PAGEOUT). It may need a > > > > > > > few changes and tweaks, but can't that be used to solve this problem? > > > > > > > > > > > > Thank you for the feedback, Andrew. process_madvise(MADV_DONTNEED) was > > > > > > one of the options recently discussed in > > > > > > https://lore.kernel.org/linux-api/CAJuCfpGz1kPM3G1gZH+09Z7aoWKg05QSAMMisJ7H5MdmRrRhNQ@xxxxxxxxxxxxxx > > > > > > . The thread describes some of the issues with that approach but if we > > > > > > limit it to processes with pending SIGKILL only then I think that > > > > > > would be doable. > > > > > > > > > > Why would it be necessary to read /proc/pid/maps? I'd have thought > > > > > that a starting effort would be > > > > > > > > > > madvise((void *)0, (void *)-1, MADV_PAGEOUT) > > > > > > > > > > (after translation into process_madvise() speak). Which is equivalent > > > > > to the proposed process_madvise(MADV_DONTNEED_MM)? > > > > > > > > Yep, this is very similar to option #3 in > > > > https://lore.kernel.org/linux-api/CAJuCfpGz1kPM3G1gZH+09Z7aoWKg05QSAMMisJ7H5MdmRrRhNQ@xxxxxxxxxxxxxx > > > > and I actually have a tested prototype for that. > > > > > > Why is the `vector=NULL' needed? Can't `vector' point at a single iovec > > > which spans the whole address range? > > > > That would be the option #4 from the same discussion and the issues > > noted there are "process_madvise return value can't handle such a > > large number of bytes and there is MAX_RW_COUNT limit on max number of > > bytes one process_madvise call can handle". In my prototype I have a > > special handling for such "bulk operation" to work around the > > MAX_RW_COUNT limitation. > > Ah, OK, return value. Maybe process_madvise() shouldn't have done that > and should have simply returned 0 on success, like madvise(). > > I guess a special "nuke whole address space" command is OK. But, again > in the search for generality, the ability to nuke very large amounts of > address space (but not the entire address space) would be better. > > The process_madvise() return value issue could be addressed by adding a > process_madvise() mode which return 0 on success. > > And I guess the MAX_RW_COUNT issue is solvable by adding an > import_iovec() arg to say "don't check that". Along those lines. > > It's all sounding a bit painful (but not *too* painful). But to > reiterate, I do think that adding the ability for a process to shoot > down a large amount of another process's memory is a lot more generally > useful than tying it to SIGKILL, agree? I see. So you are suggesting a mode where process_madvise() can operate on large areas spanning multiple VMAs. This slightly differs from option 4 in the previous RFC which suggested a special mode that operates on the *entire* mm of the process. I agree, your suggestion is more generic. > > > > > > > > If that's the > > > > preferred method then I can post it quite quickly. > > > > > > I assume you've tested that prototype. How did its usefulness compare > > > with this SIGKILL-based approach? > > > > Just to make sure I understand correctly your question, you are asking > > about performance comparison of: > > > > // approach in this RFC > > pidfd_send_signal(SIGKILL, SYNC_REAP_MM) > > > > vs > > > > // option #4 in the previous RFC > > kill(SIGKILL); process_madvise(vector=NULL, MADV_DONTNEED); > > > > If so, I have results for the current RFC approach but the previous > > approach was testing on an older device, so don't have > > apples-to-apples comparison results at the moment. I can collect the > > data for fair comparison if desired, however I don't expect a > > noticeable performance difference since they both do pretty much the > > same thing (even on different devices my results are quite close). I > > think it's more a question of which API would be more appropriate. > > OK. I wouldn't expect performance to be very different (and things can > be sped up if so), but the API usefulness might be an issue. Using > process_madvise() (or similar) makes it a two-step operation, whereas > tying it to SIGKILL&&TASK_UNINTERRUPTIBLE provides a more precise tool. > Any thoughts on this? >
