On Wed, May 16, 2018 at 11:35:55AM +0200, Vlastimil Babka wrote: > On 05/08/2018 03:00 AM, Joonsoo Kim wrote: > >> classzone predates my involvement with Linux but I would be less concerneed > >> about what the original intent was and instead ensure that classzone index > >> is consistent, sane and potentially renamed while preserving the intent of > >> "reserve pages in lower zones when an allocation request can use higher > >> zones". While historically the critical intent was to preserve Normal and > >> to a lesser extent DMA on 32-bit systems, there still should be some care > >> of DMA32 so we should not lose that. > > > > Agreed! > > > >> With the patch, the allocator looks like it would be fine as just > >> reservations change. I think it's unlikely that CMA usage will result > >> in lowmem starvation. Compaction becomes a bit weird as classzone index > >> has no special meaning versis highmem and I think it'll be very easy to > >> forget. > > I don't understand this point, what do you mean about highmem here? I mean it has no special meaning as compaction is not primarily concerned with lowmem protections as it compacts within a zone. It preserves watermarks but it does not have the same degree of criticality as the page allocator and reclaim is concerned with. > I've > checked and compaction seems to use classzone_idx 1) to pass it to > watermark checks as part of compaction suitability checks, i.e. the > usual lowmem protection, and 2) to limit compaction of higher zones in > kcompactd if the direct compactor can't use them anyway - seems this > part has currently the same zone imbalance problem as reclaim. > Originally the watermark check for compaction was primarily about not depleting a single zone but the checks were duplicated anyway. It's not actually super critical for it to preserve lowmem zones as any memory usage by compaction is transient. > > Agreed! > > I will update this patch to reflect your comment. If someone have an idea > > on renaming this variable, please let me know. > > Pehaps max_zone_idx? Seems a bit more clear than "high_zoneidx". And I > have no idea what was actually meant by "class". > I don't have a better suggestion. -- Mel Gorman SUSE Labs
