This is a note to let you know that I've just added the patch titled mm: Always release pages to the buddy allocator in memblock_free_late(). to the 5.4-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: mm-always-release-pages-to-the-buddy-allocator-in-memblock_free_late.patch and it can be found in the queue-5.4 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From 115d9d77bb0f9152c60b6e8646369fa7f6167593 Mon Sep 17 00:00:00 2001 From: Aaron Thompson <dev@xxxxxxxxxx> Date: Fri, 6 Jan 2023 22:22:44 +0000 Subject: mm: Always release pages to the buddy allocator in memblock_free_late(). From: Aaron Thompson <dev@xxxxxxxxxx> commit 115d9d77bb0f9152c60b6e8646369fa7f6167593 upstream. If CONFIG_DEFERRED_STRUCT_PAGE_INIT is enabled, memblock_free_pages() only releases pages to the buddy allocator if they are not in the deferred range. This is correct for free pages (as defined by for_each_free_mem_pfn_range_in_zone()) because free pages in the deferred range will be initialized and released as part of the deferred init process. memblock_free_pages() is called by memblock_free_late(), which is used to free reserved ranges after memblock_free_all() has run. All pages in reserved ranges have been initialized at that point, and accordingly, those pages are not touched by the deferred init process. This means that currently, if the pages that memblock_free_late() intends to release are in the deferred range, they will never be released to the buddy allocator. They will forever be reserved. In addition, memblock_free_pages() calls kmsan_memblock_free_pages(), which is also correct for free pages but is not correct for reserved pages. KMSAN metadata for reserved pages is initialized by kmsan_init_shadow(), which runs shortly before memblock_free_all(). For both of these reasons, memblock_free_pages() should only be called for free pages, and memblock_free_late() should call __free_pages_core() directly instead. One case where this issue can occur in the wild is EFI boot on x86_64. The x86 EFI code reserves all EFI boot services memory ranges via memblock_reserve() and frees them later via memblock_free_late() (efi_reserve_boot_services() and efi_free_boot_services(), respectively). If any of those ranges happens to fall within the deferred init range, the pages will not be released and that memory will be unavailable. For example, on an Amazon EC2 t3.micro VM (1 GB) booting via EFI: v6.2-rc2: # grep -E 'Node|spanned|present|managed' /proc/zoneinfo Node 0, zone DMA spanned 4095 present 3999 managed 3840 Node 0, zone DMA32 spanned 246652 present 245868 managed 178867 v6.2-rc2 + patch: # grep -E 'Node|spanned|present|managed' /proc/zoneinfo Node 0, zone DMA spanned 4095 present 3999 managed 3840 Node 0, zone DMA32 spanned 246652 present 245868 managed 222816 # +43,949 pages Fixes: 3a80a7fa7989 ("mm: meminit: initialise a subset of struct pages if CONFIG_DEFERRED_STRUCT_PAGE_INIT is set") Signed-off-by: Aaron Thompson <dev@xxxxxxxxxx> Link: https://lore.kernel.org/r/01010185892de53e-e379acfb-7044-4b24-b30a-e2657c1ba989-000000@xxxxxxxxxxxxxxxxxxxxxxx Signed-off-by: Mike Rapoport (IBM) <rppt@xxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- mm/memblock.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-) --- a/mm/memblock.c +++ b/mm/memblock.c @@ -1546,7 +1546,13 @@ void __init __memblock_free_late(phys_ad end = PFN_DOWN(base + size); for (; cursor < end; cursor++) { - memblock_free_pages(pfn_to_page(cursor), cursor, 0); + /* + * Reserved pages are always initialized by the end of + * memblock_free_all() (by memmap_init() and, if deferred + * initialization is enabled, memmap_init_reserved_pages()), so + * these pages can be released directly to the buddy allocator. + */ + __free_pages_core(pfn_to_page(cursor), 0); totalram_pages_inc(); } } Patches currently in stable-queue which might be from dev@xxxxxxxxxx are queue-5.4/mm-always-release-pages-to-the-buddy-allocator-in-memblock_free_late.patch