From: David Hildenbrand (Arm) Date: Fri, 20 Mar 2026 22:13:33 +0000 (+0100) Subject: mm/memory_hotplug: fix possible race in scan_movable_pages() X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=89e69c7d187eca8a040d4db0e080f79b5b32163d;p=thirdparty%2Fkernel%2Flinux.git mm/memory_hotplug: fix possible race in scan_movable_pages() Patch series "mm: memory hot(un)plug and SPARSEMEM cleanups", v2. Some cleanups around memory hot(un)plug and SPARSEMEM. In essence, we can limit CONFIG_MEMORY_HOTPLUG to CONFIG_SPARSEMEM_VMEMMAP, remove some dead code, and move all the hotplug bits over to mm/sparse-vmemmap.c. Some further/related cleanups around other unnecessary code (memory hole handling and complicated usemap allocation). I have some further sparse.c cleanups lying around, and I'm planning on getting rid of bootmem_info.c entirely. This patch (of 15): If a hugetlb folio gets freed while we are in scan_movable_pages(), folio_nr_pages() could return 0, resulting in or'ing "0 - 1 = -1" to the PFN, resulting in PFN = -1. We're not holding any locks or references that would prevent that. for_each_valid_pfn() would then search for the next valid PFN, and could return a PFN that is outside of the range of the original requested range. do_migrate_page() would then try to migrate quite a big range, which is certainly undesirable. To fix it, simply test for valid folio_nr_pages() values. While at it, as PageHuge() really just does a page_folio() internally, we can just use folio_test_hugetlb() on the folio directly. scan_movable_pages() is expected to be fast, and we try to avoid taking locks or grabbing references. We cannot use folio_try_get() as that does not work for free hugetlb folios. We could grab the hugetlb_lock, but that just adds complexity. The race is unlikely to trigger in practice, so we won't be CCing stable. Link: https://lkml.kernel.org/r/20260320-sparsemem_cleanups-v2-0-096addc8800d@kernel.org Link: https://lkml.kernel.org/r/20260320-sparsemem_cleanups-v2-1-096addc8800d@kernel.org Fixes: 16540dae959d ("mm/hugetlb: mm/memory_hotplug: use a folio in scan_movable_pages()") Signed-off-by: David Hildenbrand (Arm) Reviewed-by: Lorenzo Stoakes (Oracle) Cc: Axel Rasmussen Cc: Liam Howlett Cc: Michal Hocko Cc: Mike Rapoport Cc: Oscar Salvador Cc: Sidhartha Kumar Cc: Suren Baghdasaryan Cc: Vlastimil Babka Cc: Wei Xu Cc: Yuanchu Xie Signed-off-by: Andrew Morton --- diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c index a602310bdf33d..c427967c78bbc 100644 --- a/mm/memory_hotplug.c +++ b/mm/memory_hotplug.c @@ -1746,6 +1746,7 @@ static int scan_movable_pages(unsigned long start, unsigned long end, unsigned long pfn; for_each_valid_pfn(pfn, start, end) { + unsigned long nr_pages; struct page *page; struct folio *folio; @@ -1762,9 +1763,9 @@ static int scan_movable_pages(unsigned long start, unsigned long end, if (PageOffline(page) && page_count(page)) return -EBUSY; - if (!PageHuge(page)) - continue; folio = page_folio(page); + if (!folio_test_hugetlb(folio)) + continue; /* * This test is racy as we hold no reference or lock. The * hugetlb page could have been free'ed and head is no longer @@ -1774,7 +1775,11 @@ static int scan_movable_pages(unsigned long start, unsigned long end, */ if (folio_test_hugetlb_migratable(folio)) goto found; - pfn |= folio_nr_pages(folio) - 1; + nr_pages = folio_nr_pages(folio); + if (unlikely(nr_pages < 1 || nr_pages > MAX_FOLIO_NR_PAGES || + !is_power_of_2(nr_pages))) + continue; + pfn |= nr_pages - 1; } return -ENOENT; found: