Re: [PATCH] memblock: fix page count when preserving reserve_mem regions

[Mike Rapoport <rppt@kernel.org>]

Hi,

On Wed, Sep 02, 2026 at 01:52:35AM +0900, Sang-Heon Jeon wrote:
> When map->start or map->size is not page aligned,
> reserved_mem_preserve() does not preserve the last pages of the region

I believe it's the last page, not pages.

> from map->start to map->start + map->size.
> 
> After kexec the new kernel reserves only the preserved pages, so the
> last pages are released to the buddy allocator. But
> reserve_mem_find_by_name() still returns the whole region including the
> last pages, so they can be allocated while the subsystem is still using
> them.
> 
> Preserve every page the region spans so the new kernel reserves all of
> it.

If there are two adjacent not page aligned reserve_mem regions, we'll
preserve them twice, or worse unintentionally unpreserve one of them on
error handling of another.

I think it's better to restrict the reserve_mem alignment to PAGE_SIZE.
 
> Fixes: 8375b76517cb ("kho: replace kho_preserve_phys() with kho_preserve_pages()")
> Signed-off-by: Sang-Heon Jeon <ekffu200098@gmail.com>
> ---
> 
> QEMU-based test results
> 
> - Booted with "memblock=debug kho=on reserve_mem=5000:64:oops"
> 
>   # kexec -s -l /bzImage --initrd=/initramfs.gz --reuse-cmdline
>   # kexec -e
>   # dmesg | grep kho_preserved_memory_reserve
> 
> 1) AS-IS (before fix)
> [    0.188536] __memblock_reserve: [0x000000013ffee000-0x000000013ffeefff] nid=-1 flags=0 kho_preserved_memory_reserve+0x3d/0x70
> 
> 2) TO-BE (after fix)
> [    0.076935] __memblock_reserve: [0x000000013ffee000-0x000000013ffeffff] nid=-1 flags=0 kho_preserved_memory_reserve+0x3d/0x70
> 
>  mm/memblock.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/memblock.c b/mm/memblock.c
> index 777c69f05400..a17b1cccaf3d 100644
> --- a/mm/memblock.c
> +++ b/mm/memblock.c
> @@ -2534,7 +2534,8 @@ static int __init reserved_mem_preserve(void)
>  	for (unsigned int i = 0; i < reserved_mem_count; i++, nr_preserved++) {
>  		struct reserve_mem_table *map = &reserved_mem_table[i];
>  		struct page *page = phys_to_page(map->start);
> -		unsigned int nr_pages = map->size >> PAGE_SHIFT;
> +		unsigned int nr_pages = PFN_UP(map->start + map->size) -
> +					PFN_DOWN(map->start);
>  
>  		err = kho_preserve_pages(page, nr_pages);
>  		if (err)
> @@ -2547,7 +2548,8 @@ static int __init reserved_mem_preserve(void)
>  	for (unsigned int i = 0; i < nr_preserved; i++) {
>  		struct reserve_mem_table *map = &reserved_mem_table[i];
>  		struct page *page = phys_to_page(map->start);
> -		unsigned int nr_pages = map->size >> PAGE_SHIFT;
> +		unsigned int nr_pages = PFN_UP(map->start + map->size) -
> +					PFN_DOWN(map->start);
>  
>  		kho_unpreserve_pages(page, nr_pages);
>  	}
> 
> base-commit: dde94c26af59f5020adafafa08ece21ccd32590b
> -- 
> 2.43.0
> 

-- 
Sincerely yours,
Mike.