On Mon 03-06-19 13:27:17, Johannes Weiner wrote: > On Mon, Jun 03, 2019 at 09:16:07AM +0200, Michal Hocko wrote: > > On Fri 31-05-19 23:34:07, Minchan Kim wrote: > > > On Fri, May 31, 2019 at 04:03:32PM +0200, Michal Hocko wrote: > > > > On Fri 31-05-19 22:39:04, Minchan Kim wrote: > > > > > On Fri, May 31, 2019 at 10:47:52AM +0200, Michal Hocko wrote: > > > > > > On Fri 31-05-19 15:43:08, Minchan Kim wrote: > > > > > > > When a process expects no accesses to a certain memory range, it could > > > > > > > give a hint to kernel that the pages can be reclaimed when memory pressure > > > > > > > happens but data should be preserved for future use. This could reduce > > > > > > > workingset eviction so it ends up increasing performance. > > > > > > > > > > > > > > This patch introduces the new MADV_COLD hint to madvise(2) syscall. > > > > > > > MADV_COLD can be used by a process to mark a memory range as not expected > > > > > > > to be used in the near future. The hint can help kernel in deciding which > > > > > > > pages to evict early during memory pressure. > > > > > > > > > > > > > > Internally, it works via deactivating pages from active list to inactive's > > > > > > > head if the page is private because inactive list could be full of > > > > > > > used-once pages which are first candidate for the reclaiming and that's a > > > > > > > reason why MADV_FREE move pages to head of inactive LRU list. Therefore, > > > > > > > if the memory pressure happens, they will be reclaimed earlier than other > > > > > > > active pages unless there is no access until the time. > > > > > > > > > > > > [I am intentionally not looking at the implementation because below > > > > > > points should be clear from the changelog - sorry about nagging ;)] > > > > > > > > > > > > What kind of pages can be deactivated? Anonymous/File backed. > > > > > > Private/shared? If shared, are there any restrictions? > > > > > > > > > > Both file and private pages could be deactived from each active LRU > > > > > to each inactive LRU if the page has one map_count. In other words, > > > > > > > > > > if (page_mapcount(page) <= 1) > > > > > deactivate_page(page); > > > > > > > > Why do we restrict to pages that are single mapped? > > > > > > Because page table in one of process shared the page would have access bit > > > so finally we couldn't reclaim the page. The more process it is shared, > > > the more fail to reclaim. > > > > So what? In other words why should it be restricted solely based on the > > map count. I can see a reason to restrict based on the access > > permissions because we do not want to simplify all sorts of side channel > > attacks but memory reclaim is capable of reclaiming shared pages and so > > far I haven't heard any sound argument why madvise should skip those. > > Again if there are any reasons, then document them in the changelog. > > I think it makes sense. It could be explained, but it also follows > established madvise semantics, and I'm not sure it's necessarily > Minchan's job to re-iterate those. > > Sharing isn't exactly transparent to userspace. The kernel does COW, > ksm etc. When you madvise, you can really only speak for your own > reference to that memory - "*I* am not using this." > > This is in line with other madvise calls: MADV_DONTNEED clears the > local page table entries and drops the corresponding references, so > shared pages won't get freed. MADV_FREE clears the pte dirty bit and > also has explicit mapcount checks before clearing PG_dirty, so again > shared pages don't get freed. Right, being consistent with other madvise syscalls is certainly a way to go. And I am not pushing one way or another, I just want this to be documented with a reasoning behind. Consistency is certainly an argument to use. On the other hand these non-destructive madvise operations are quite different and the shared policy might differ as a result as well. We are aging objects rather than destroying them after all. Being able to age a pagecache with a sufficient privileges sounds like a useful usecase to me. In other words you are able to cause the same effect indirectly without the madvise operation so it kinda makes sense to allow it in a more sophisticated way. That being said, madvise is just a _hint_ and the kernel will be always free to ignore it so the future implementation might change so we can start simple and consistent with existing MADV_$FOO operations now and extend later on. But let's document the intention in the changelog and make the decision clear. I am sorry to be so anal about this but I have seen so many ad-hoc policies that were undocumented and it was so hard to guess when revisiting later on and make some sense of it. -- Michal Hocko SUSE Labs
