From 10eeadf3045c35fc83649ac586973eb28255add9 Mon Sep 17 00:00:00 2001 From: Oscar Salvador Date: Mon, 13 May 2019 17:17:29 -0700 Subject: [PATCH] mm,memory_hotplug: unlock 1GB-hugetlb on x86_64 On x86_64, 1GB-hugetlb pages could never be offlined due to the fact that hugepage_migration_supported() returned false for PUD_SHIFT. So whenever we wanted to offline a memblock containing a gigantic hugetlb page, we never got beyond has_unmovable_pages() check. This changed with [1], where now we also return true for PUD_SHIFT. After that patch, the check in has_unmovable_pages() and scan_movable_pages() returned true, but we still had a final barrier in do_migrate_range(): if (compound_order(head) > PFN_SECTION_SHIFT) { ret = -EBUSY; break; } This is not really nice, and we do not really need it. It is perfectly possible to migrate a gigantic page as long as another node has a spare gigantic page for us. In alloc_huge_page_nodemask(), we calculate the __real__ number of free pages, and if any, we try to dequeue one from another node. This all works fine when we do have another node with a spare gigantic page, but if that is not the case, alloc_huge_page_nodemask() ends up calling alloc_migrate_huge_page() which bails out if the wanted page is gigantic. That is mainly because finding a 1GB (or even 16GB on powerpc) contiguous memory is quite unlikely when the system has been running for a while. In that situation, we will keep looping forever because scan_movable_pages() will give us the same page and we will fail again because there is no node where we can dequeue a gigantic page from. This is not nice, and it has been raised that we might want to treat -ENOMEM as a fatal error in do_migrate_range(), but this has to be checked further. Anyway, I would tend say that this is the administrator's job, to make sure that the system can keep up with the memory to be offlined, so that would mean that if we want to use gigantic pages, make sure that the other nodes have at least enough gigantic pages to keep up in case we need to offline memory. Just for the sake of completeness, this is one of the tests done: # echo 1 > /sys/devices/system/node/node1/hugepages/hugepages-1048576kB/nr_hugepages # echo 1 > /sys/devices/system/node/node2/hugepages/hugepages-1048576kB/nr_hugepages # cat /sys/devices/system/node/node1/hugepages/hugepages-1048576kB/nr_hugepages 1 # cat /sys/devices/system/node/node1/hugepages/hugepages-1048576kB/free_hugepages 1 # cat /sys/devices/system/node/node2/hugepages/hugepages-1048576kB/nr_hugepages 1 # cat /sys/devices/system/node/node2/hugepages/hugepages-1048576kB/free_hugepages 1 (hugetlb1gb is a program that maps 1GB region using MAP_HUGE_1GB) # numactl -m 1 ./hugetlb1gb # cat /sys/devices/system/node/node1/hugepages/hugepages-1048576kB/free_hugepages 0 # cat /sys/devices/system/node/node2/hugepages/hugepages-1048576kB/free_hugepages 1 # offline node1 memory # cat /sys/devices/system/node/node2/hugepages/hugepages-1048576kB/free_hugepages 0 [1] https://lore.kernel.org/patchwork/patch/998796/ Link: http://lkml.kernel.org/r/20190320152658.10855-2-osalvador@suse.de Signed-off-by: Oscar Salvador Acked-by: Michal Hocko Cc: David Hildenbrand Cc: Mike Kravetz Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/memory_hotplug.c | 4 ---- 1 file changed, 4 deletions(-) diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c index b236069ff0d8..df41b467e020 100644 --- a/mm/memory_hotplug.c +++ b/mm/memory_hotplug.c @@ -1382,10 +1382,6 @@ do_migrate_range(unsigned long start_pfn, unsigned long end_pfn) if (PageHuge(page)) { struct page *head = compound_head(page); - if (compound_order(head) > PFN_SECTION_SHIFT) { - ret = -EBUSY; - break; - } pfn = page_to_pfn(head) + (1<