On Wed, Dec 04, 2024 at 10:03:28AM +0100, Vlastimil Babka wrote: > On 12/4/24 09:59, Oscar Salvador wrote: > > On Tue, Dec 03, 2024 at 08:19:02PM +0100, David Hildenbrand wrote: > >> It was always set using "GFP_USER | __GFP_MOVABLE | __GFP_RETRY_MAYFAIL", > >> and I removed the same flag combination in #2 from memory offline code, and > >> we do have the exact same thing in do_migrate_range() in > >> mm/memory_hotplug.c. > >> > >> We should investigate if__GFP_HARDWALL is the right thing to use here, and > >> if we can get rid of that by switching to GFP_KERNEL in all these places. > > > > Why would not we want __GFP_HARDWALL set? > > Without it, we could potentially migrate the page to a node which is not > > part of the cpuset of the task that originally allocated it, thus violating the > > policy? Is not that a problem? > > The task doing the alloc_contig_range() will likely not be the same task as > the one that originally allocated the page, so its policy would be > different, no? So even with __GFP_HARDWALL we might be already migrating > outside the original tasks's constraint? Am I missing something? Yes, that is right, I thought we derive the policy from the old page somehow when migrating it, but reading the code does not seem to be the case. Looking at prepare_alloc_pages(), if !ac->nodemask, which would be the case here, we would get the policy from the current task (alloc_contig_range()) when cpusets are enabled. So yes, I am a bit puzzled why __GFP_HARDWALL was chosen in the first place. -- Oscar Salvador SUSE Labs
