On Tue 15-12-20 00:20:39, Pavel Tatashin wrote: > > Ack to this. > > Thank you. > > > > > But I do not really understand this. All allocation contexts should have > > a proper gfp mask so why do we have to call current_gfp_context here? > > In fact moving the current_gfp_context in the allocator path should have > > made all this games unnecessary. Memcg reclaim path might need some > > careful check because gfp mask is used more creative there but the > > general reclaim paths should be ok. > > > > > diff --git a/mm/vmscan.c b/mm/vmscan.c > > > > Again, why do we need this when the gfp_mask > > > }; > > > > -- > > Hi Michal, > > Beside from __alloc_pages_nodemask(), the current_gfp_context() is > called from the following six functions: > > try_to_free_pages() > try_to_free_mem_cgroup_pages() > __node_reclaim() > __need_fs_reclaim() > alloc_contig_range() > pcpu_alloc() > > As I understand, the idea is that because the allocator now honors > gfp_context values for all paths, the call can be removed from some of > the above functions. I think you are correct. But, at least from a > quick glance, this is not obvious, and is not the case for all of the > above functions. > > For example: > > alloc_contig_range() > __alloc_contig_migrate_range > isolate_migratepages_range > isolate_migratepages_block > /* > * Only allow to migrate anonymous pages in GFP_NOFS context > * because those do not depend on fs locks. > */ > if (!(cc->gfp_mask & __GFP_FS) && page_mapping(page)) > goto isolate_fail; > > If we remove current_gfp_context() from alloc_contig_range(), the > cc->gfp_mask will not be updated with proper __GFP_FS flag. I do not think I was proposing to drop current_gfp_context from alloc_contig_range. ACR needs some work to be properly scoped gfp mask aware. This should be addressed but I do not think think the code works properly now so I wouldn't lose sleep over it in this series. At least __alloc_contig_migrate_range should follow the gfp mask given to alloc_contig_range. > I have studied some other paths, and they are also convoluted. > Therefore, I am worried about performing this optimization in this > series. Dropping current_gfp_context from the reclaim context should be done in a separate patch. I didn't mean to push for this here. All I meant was to simply not touch gfp/zone_idx in the reclaim path. The changelog should call out that the page allocator always provides proper gfp mask. -- Michal Hocko SUSE Labs
