On Wed, Dec 04, 2024 at 10:28:39AM +0100, David Hildenbrand wrote: > On 04.12.24 10:15, Oscar Salvador wrote: > > 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. > > I suspect because "GFP_USER" felt like the appropriate thing to do. Looking back at when the whole contiguous allocator patchset was posted, it seems that it kinda copied what memory-offline code was doing, which was migrating pages with GFP_HIGHUSER_MOVABLE (hotremove_migrate_alloc()). Then, the HIGHMEM modifier was dropped due to HIGHMEM restrictions on some systems, ending up with GFP_USER. -- Oscar Salvador SUSE Labs
