On Fri, Jan 06, 2023 at 10:22:44PM +0000, Aaron Thompson wrote:
> 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>
Applied, thanks!
> ---
> mm/memblock.c | 8 +++++++-
> tools/testing/memblock/internal.h | 4 ++++
> 2 files changed, 11 insertions(+), 1 deletion(-)
>
> diff --git a/mm/memblock.c b/mm/memblock.c
> index 511d4783dcf1..fc3d8fbd2060 100644
> --- a/mm/memblock.c
> +++ b/mm/memblock.c
> @@ -1640,7 +1640,13 @@ void __init memblock_free_late(phys_addr_t base, phys_addr_t size)
> 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();
> }
> }
> diff --git a/tools/testing/memblock/internal.h b/tools/testing/memblock/internal.h
> index fdb7f5db7308..85973e55489e 100644
> --- a/tools/testing/memblock/internal.h
> +++ b/tools/testing/memblock/internal.h
> @@ -15,6 +15,10 @@ bool mirrored_kernelcore = false;
>
> struct page {};
>
> +void __free_pages_core(struct page *page, unsigned int order)
> +{
> +}
> +
> void memblock_free_pages(struct page *page, unsigned long pfn,
> unsigned int order)
> {
> --
> 2.30.2
>
--
Sincerely yours,
Mike.
