Re: [PATCH v3 1/1] mm: Always release pages to the buddy allocator in memblock_free_late().

[Mike Rapoport <rppt@kernel.org>]

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@aaront.org>

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.