linux-kernel - Re: [PATCH 1/1] mm: Always release pages to the buddy allocator in memblock_free

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Date:   Wed, 4 Jan 2023 21:34:57 +0200
From:   Mike Rapoport <rppt@...nel.org>
To:     Aaron Thompson <dev@...ont.org>
Cc:     linux-mm@...ck.org, "H. Peter Anvin" <hpa@...or.com>,
        Alexander Potapenko <glider@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Shevchenko <andy@...radead.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Darren Hart <dvhart@...radead.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Ingo Molnar <mingo@...hat.com>, Marco Elver <elver@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        kasan-dev@...glegroups.com, linux-efi@...r.kernel.org,
        linux-kernel@...r.kernel.org, platform-driver-x86@...r.kernel.org,
        x86@...nel.org
Subject: Re: [PATCH 1/1] mm: Always release pages to the buddy allocator in
 memblock_free_late().

Hi,

On Wed, Jan 04, 2023 at 07:43:36AM +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. memblock_free_all() initializes all pages in reserved ranges, and

To be precise, memblock_free_all() frees pages, or releases them to the
pages allocator, rather than initializes.

> 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.

Overall looks fine to me and I couldn't spot potential issues.

I'd appreciate if you add a paragraph about the actual issue with EFI boot
you described in the cover letter to the commit message.

> Fixes: 3a80a7fa7989 ("mm: meminit: initialise a subset of struct pages if CONFIG_DEFERRED_STRUCT_PAGE_INIT is set")
> Signed-off-by: Aaron Thompson <dev@...ont.org>
> ---
>  mm/memblock.c                     | 2 +-
>  tools/testing/memblock/internal.h | 4 ++++
>  2 files changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/memblock.c b/mm/memblock.c
> index 511d4783dcf1..56a5b6086c50 100644
> --- a/mm/memblock.c
> +++ b/mm/memblock.c
> @@ -1640,7 +1640,7 @@ 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);
> +		__free_pages_core(pfn_to_page(cursor), 0);

Please add a comment that explains why it is safe to call __free_pages_core() here.
Something like

	/*
	 * Reserved pages are always initialized by the end of
	 * memblock_free_all() either during memmap_init() or, with deferred
	 * initialization if struct page in reserve_bootmem_region()
	 */

>  		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.