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charset="UTF-8" X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, Nov 18, 2020 at 11:55 AM Suren Baghdasaryan wrote: > > On Wed, Nov 18, 2020 at 11:51 AM Suren Baghdasaryan wrote: > > > > On Wed, Nov 18, 2020 at 11:32 AM Michal Hocko wrote: > > > > > > On Wed 18-11-20 11:22:21, Suren Baghdasaryan wrote: > > > > On Wed, Nov 18, 2020 at 11:10 AM Michal Hocko wrote: > > > > > > > > > > On Fri 13-11-20 18:16:32, Andrew Morton wrote: > > > > > [...] > > > > > > 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 was looking into how to work around the limitation of MAX_RW_COUNT and the conceptual issue there is the "struct iovec" which has its iov_len as size_t that lacks capacity for expressing ranges like "entire process memory". I would like to check your reaction to the following idea which can be implemented without painful surgeries to the import_iovec and its friends. process_madvise(pidfd, iovec = [ { range_start_addr, 0 }, { range_end_addr, 0 } ], vlen = 2, behavior=MADV_xxx, flags = PMADV_FLAG_RANGE) So, to represent a range we pass a new PMADV_FLAG_RANGE flag and construct a 2-element vector to express range start and range end using iovec.iov_base members. iov_len member of the iovec elements is ignored in this mode. I know it sounds hacky but I think it's the simplest way if we want the ability to express an arbitrarily large range. Another option is to do what Andrew described as "madvise((void *)0, (void *)-1, MADV_PAGEOUT)" which means this mode works only with the entire mm of the process. WDYT? > > > > > > > > > > I am not sure TBH. Is there any reasonable usecase where uncoordinated > > > > > memory tear down is OK and a target process which is able to see the > > > > > unmapped memory? > > > > > > > > I think uncoordinated memory tear down is a special case which makes > > > > sense only when the target process is being killed (and we can enforce > > > > that by allowing MADV_DONTNEED to be used only if the target process > > > > has pending SIGKILL). > > > > > > That would be safe but then I am wondering whether it makes sense to > > > implement as a madvise call. It is quite strange to expect somebody call > > > a syscall on a killed process. But this is more a detail. I am not a > > > great fan of a more generic MADV_DONTNEED on a remote process. This is > > > just too dangerous IMHO. > > > > Agree 100% > > I assumed here that by "a more generic MADV_DONTNEED on a remote > process" you meant "process_madvise(MADV_DONTNEED) applied to a > process that is not being killed". Re-reading your comment I realized > that you might have meant "process_madvice() with generic support to > large memory areas". I hope I understood you correctly. > > > > > > > > > > However, the ability to apply other flavors of > > > > process_madvise() to large memory areas spanning multiple VMAs can be > > > > useful in more cases. > > > > > > Yes I do agree with that. The error reporting would be more tricky but > > > I am not really sure that the exact reporting is really necessary for > > > advice like interface. > > > > Andrew's suggestion for this special mode to change return semantics > > to the usual "0 or error code" seems to me like the most reasonable > > way to deal with the return value limitation. > > > > > > > > > For example in Android we will use > > > > process_madvise(MADV_PAGEOUT) to "shrink" an inactive background > > > > process. > > > > > > That makes sense to me. > > > -- > > > Michal Hocko > > > SUSE Labs