On Fri 20-10-17 11:13:29, Joonsoo Kim wrote: > On Thu, Oct 19, 2017 at 02:21:18PM +0200, Michal Hocko wrote: > > On Thu 19-10-17 10:20:41, Michal Hocko wrote: > > > On Thu 19-10-17 16:33:56, Joonsoo Kim wrote: > > > > On Thu, Oct 19, 2017 at 09:15:03AM +0200, Michal Hocko wrote: > > > > > On Thu 19-10-17 11:51:11, Joonsoo Kim wrote: > > > [...] > > > > > > Hello, > > > > > > > > > > > > This patch will break the CMA user. As you mentioned, CMA allocation > > > > > > itself isn't migrateable. So, after a single page is allocated through > > > > > > CMA allocation, has_unmovable_pages() will return true for this > > > > > > pageblock. Then, futher CMA allocation request to this pageblock will > > > > > > fail because it requires isolating the pageblock. > > > > > > > > > > Hmm, does this mean that the CMA allocation path depends on > > > > > has_unmovable_pages to return false here even though the memory is not > > > > > movable? This sounds really strange to me and kind of abuse of this > > > > > > > > Your understanding is correct. Perhaps, abuse or wrong function name. > > > > > > > > > function. Which path is that? Can we do the migrate type test theres? > > > > > > > > alloc_contig_range() -> start_isolate_page_range() -> > > > > set_migratetype_isolate() -> has_unmovable_pages() > > > > > > I see. It seems that the CMA and memory hotplug have a very different > > > view on what should happen during isolation. > > > > > > > We can add one argument, 'XXX' to set_migratetype_isolate() and change > > > > it to check migrate type rather than has_unmovable_pages() if 'XXX' is > > > > specified. > > > > > > Can we use the migratetype argument and do the special thing for > > > MIGRATE_CMA? Like the following diff? > > > > And with the full changelog. > > --- > > >From 8cbd811d741f5dd93d1b21bb3ef94482a4d0bd32 Mon Sep 17 00:00:00 2001 > > From: Michal Hocko <mhocko@xxxxxxxx> > > Date: Thu, 19 Oct 2017 14:14:02 +0200 > > Subject: [PATCH] mm: distinguish CMA and MOVABLE isolation in > > has_unmovable_pages > > > > Joonsoo has noticed that "mm: drop migrate type checks from > > has_unmovable_pages" would break CMA allocator because it relies on > > has_unmovable_pages returning false even for CMA pageblocks which in > > fact don't have to be movable: > > alloc_contig_range > > start_isolate_page_range > > set_migratetype_isolate > > has_unmovable_pages > > > > This is a result of the code sharing between CMA and memory hotplug > > while each one has a different idea of what has_unmovable_pages should > > return. This is unfortunate but fixing it properly would require a lot > > of code duplication. > > > > Fix the issue by introducing the requested migrate type argument > > and special case MIGRATE_CMA case where CMA page blocks are handled > > properly. This will work for memory hotplug because it requires > > MIGRATE_MOVABLE. > > Unfortunately, alloc_contig_range() can be called with > MIGRATE_MOVABLE so this patch cannot perfectly fix the problem. Yes, alloc_contig_range can be called with MIGRATE_MOVABLE but my understanding is that only CMA allocator really depends on this weird semantic and that does MIGRATE_CMA unconditionally. > I did a more thinking and found that it's strange to check if there is > unmovable page in the pageblock during the set_migratetype_isolate(). > set_migratetype_isolate() should be just for setting the migratetype > of the pageblock. Checking other things should be done by another > place, for example, before calling the start_isolate_page_range() in > __offline_pages(). How do we guarantee the atomicity? -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
