Re: [PATCH v2 2/2] memblock: do not start bottom-up allocations with kernel_end

[Mike Rapoport]

On Thu, Dec 17, 2020 at 12:12:14PM -0800, Roman Gushchin wrote:
> With kaslr the kernel image is placed at a random place, so starting
> the bottom-up allocation with the kernel_end can result in an
> allocation failure and a warning like this one:
> 
> [    0.002920] hugetlb_cma: reserve 2048 MiB, up to 2048 MiB per node
> [    0.002921] ------------[ cut here ]------------
> [    0.002922] memblock: bottom-up allocation failed, memory hotremove may be affected
> [    0.002937] WARNING: CPU: 0 PID: 0 at mm/memblock.c:332 memblock_find_in_range_node+0x178/0x25a
> [    0.002937] Modules linked in:
> [    0.002939] CPU: 0 PID: 0 Comm: swapper Not tainted 5.10.0+ #1169
> [    0.002940] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-1.fc33 04/01/2014
> [    0.002942] RIP: 0010:memblock_find_in_range_node+0x178/0x25a
> [    0.002944] Code: e9 6d ff ff ff 48 85 c0 0f 85 da 00 00 00 80 3d 9b 35 df 00 00 75 15 48 c7 c7 c0 75 59 88 c6 05 8b 35 df 00 01 e8 25 8a fa ff <0f> 0b 48 c7 44 24 20 ff ff ff ff 44 89 e6 44 89 ea 48 c7 c1 70 5c
> [    0.002945] RSP: 0000:ffffffff88803d18 EFLAGS: 00010086 ORIG_RAX: 0000000000000000
> [    0.002947] RAX: 0000000000000000 RBX: 0000000240000000 RCX: 00000000ffffdfff
> [    0.002948] RDX: 00000000ffffdfff RSI: 00000000ffffffea RDI: 0000000000000046
> [    0.002948] RBP: 0000000100000000 R08: ffffffff88922788 R09: 0000000000009ffb
> [    0.002949] R10: 00000000ffffe000 R11: 3fffffffffffffff R12: 0000000000000000
> [    0.002950] R13: 0000000000000000 R14: 0000000080000000 R15: 00000001fb42c000
> [    0.002952] FS:  0000000000000000(0000) GS:ffffffff88f71000(0000) knlGS:0000000000000000
> [    0.002953] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [    0.002954] CR2: ffffa080fb401000 CR3: 00000001fa80a000 CR4: 00000000000406b0
> [    0.002956] Call Trace:
> [    0.002961]  ? memblock_alloc_range_nid+0x8d/0x11e
> [    0.002963]  ? cma_declare_contiguous_nid+0x2c4/0x38c
> [    0.002964]  ? hugetlb_cma_reserve+0xdc/0x128
> [    0.002968]  ? flush_tlb_one_kernel+0xc/0x20
> [    0.002969]  ? native_set_fixmap+0x82/0xd0
> [    0.002971]  ? flat_get_apic_id+0x5/0x10
> [    0.002973]  ? register_lapic_address+0x8e/0x97
> [    0.002975]  ? setup_arch+0x8a5/0xc3f
> [    0.002978]  ? start_kernel+0x66/0x547
> [    0.002980]  ? load_ucode_bsp+0x4c/0xcd
> [    0.002982]  ? secondary_startup_64_no_verify+0xb0/0xbb
> [    0.002986] random: get_random_bytes called from __warn+0xab/0x110 with crng_init=0
> [    0.002988] ---[ end trace f151227d0b39be70 ]---
> 
> At the same time, the kernel image is protected with memblock_reserve(),
> so we can just start searching at PAGE_SIZE. In this case the
> bottom-up allocation has the same chances to success as a top-down
> allocation, so there is no reason to fallback in the case of a
> failure. All together it simplifies the logic.
> 
> Signed-off-by: Roman Gushchin <guro@xxxxxx>

Reviewed-by: Mike Rapoport <rppt@xxxxxxxxxxxxx>

> ---
>  mm/memblock.c | 49 ++++++-------------------------------------------
>  1 file changed, 6 insertions(+), 43 deletions(-)
> 
> diff --git a/mm/memblock.c b/mm/memblock.c
> index b68ee86788af..10bd7d1ef0f4 100644
> --- a/mm/memblock.c
> +++ b/mm/memblock.c
> @@ -275,14 +275,6 @@ __memblock_find_range_top_down(phys_addr_t start, phys_addr_t end,
>   *
>   * Find @size free area aligned to @align in the specified range and node.
>   *
> - * When allocation direction is bottom-up, the @start should be greater
> - * than the end of the kernel image. Otherwise, it will be trimmed. The
> - * reason is that we want the bottom-up allocation just near the kernel
> - * image so it is highly likely that the allocated memory and the kernel
> - * will reside in the same node.
> - *
> - * If bottom-up allocation failed, will try to allocate memory top-down.
> - *
>   * Return:
>   * Found address on success, 0 on failure.
>   */
> @@ -291,8 +283,6 @@ static phys_addr_t __init_memblock memblock_find_in_range_node(phys_addr_t size,
>  					phys_addr_t end, int nid,
>  					enum memblock_flags flags)
>  {
> -	phys_addr_t kernel_end, ret;
> -
>  	/* pump up @end */
>  	if (end == MEMBLOCK_ALLOC_ACCESSIBLE ||
>  	    end == MEMBLOCK_ALLOC_KASAN)
> @@ -301,40 +291,13 @@ static phys_addr_t __init_memblock memblock_find_in_range_node(phys_addr_t size,
>  	/* avoid allocating the first page */
>  	start = max_t(phys_addr_t, start, PAGE_SIZE);
>  	end = max(start, end);
> -	kernel_end = __pa_symbol(_end);
> -
> -	/*
> -	 * try bottom-up allocation only when bottom-up mode
> -	 * is set and @end is above the kernel image.
> -	 */
> -	if (memblock_bottom_up() && end > kernel_end) {
> -		phys_addr_t bottom_up_start;
> -
> -		/* make sure we will allocate above the kernel */
> -		bottom_up_start = max(start, kernel_end);
>  
> -		/* ok, try bottom-up allocation first */
> -		ret = __memblock_find_range_bottom_up(bottom_up_start, end,
> -						      size, align, nid, flags);
> -		if (ret)
> -			return ret;
> -
> -		/*
> -		 * we always limit bottom-up allocation above the kernel,
> -		 * but top-down allocation doesn't have the limit, so
> -		 * retrying top-down allocation may succeed when bottom-up
> -		 * allocation failed.
> -		 *
> -		 * bottom-up allocation is expected to be fail very rarely,
> -		 * so we use WARN_ONCE() here to see the stack trace if
> -		 * fail happens.
> -		 */
> -		WARN_ONCE(IS_ENABLED(CONFIG_MEMORY_HOTREMOVE),
> -			  "memblock: bottom-up allocation failed, memory hotremove may be affected\n");
> -	}
> -
> -	return __memblock_find_range_top_down(start, end, size, align, nid,
> -					      flags);
> +	if (memblock_bottom_up())
> +		return __memblock_find_range_bottom_up(start, end, size, align,
> +						       nid, flags);
> +	else
> +		return __memblock_find_range_top_down(start, end, size, align,
> +						      nid, flags);
>  }
>  
>  /**
> -- 
> 2.26.2
> 

-- 
Sincerely yours,
Mike.