On Fri, Apr 21, 2023 at 03:56:57PM +0100, Mel Gorman wrote: > On Tue, Apr 18, 2023 at 03:13:12PM -0400, Johannes Weiner wrote: > > Fallbacks are already unlikely due to watermarks being enforced > > against MIGRATE_FREE blocks. Eliminate them altogether. This allows > > compaction to look exclusively at movable blocks, reducing the number > > of pageblocks it needs to scan on an ongoing basis. > > > > Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx> > > Conceptually this could be fun if a GFP_NOFS allocation cannot migrate > enough memory to free one pageblock and there are no pageblocks > available of the correct migratetype. Fallbacks might be unlikely but > never being able to fallback is a livelock risk, no? The reserves below the watermarks are maintained in neutral MIGRATE_FREE blocks. So just like today, critical/limited allocation contexts are ensured forward progress as long as there are reserves. An argument could be made that because smaller orders type-claim the entire neutral block on allocation now, the reserves can deplete faster than they do today given comparable watermarks. I haven't run into this issue during testing that would have me raise the reserves by a factor of NR_MIGRATETYPES. But something to keep an eye out for.
