Re: [PATCH] mm, hotplug: fix page online with DEBUG_PAGEALLOC compiled but not enabled

[David Hildenbrand]

On 24.02.20 10:46, Vlastimil Babka wrote:
> Commit cd02cf1aceea ("mm/hotplug: fix an imbalance with DEBUG_PAGEALLOC") fixed
> memory hotplug with debug_pagealloc enabled, where onlining a page goes through
> page freeing, which removes the direct mapping. Some arches don't like when the
> page is not mapped in the first place, so generic_online_page() maps it first.
> This is somewhat wasteful, but better than special casing page freeing fast
> paths.
> 
> The commit however missed that DEBUG_PAGEALLOC configured doesn't mean it's
> actually enabled. One has to test debug_pagealloc_enabled() since 031bc5743f15
> ("mm/debug-pagealloc: make debug-pagealloc boottime configurable"), or alternatively
> debug_pagealloc_enabled_static() since 8e57f8acbbd1 ("mm, debug_pagealloc:
> don't rely on static keys too early"), but this is not done.
> 
> As a result, a s390 kernel with DEBUG_PAGEALLOC configured but not enabled
> will crash:
> 
> Unable to handle kernel pointer dereference in virtual kernel address space
> Failing address: 0000000000000000 TEID: 0000000000000483
> Fault in home space mode while using kernel ASCE.
> AS:0000001ece13400b R2:000003fff7fd000b R3:000003fff7fcc007 S:000003fff7fd7000 P:000000000000013d
> Oops: 0004 ilc:2 [#1] SMP
> CPU: 1 PID: 26015 Comm: chmem Kdump: loaded Tainted: GX 5.3.18-5-default #1 SLE15-SP2 (unreleased)
> Krnl PSW : 0704e00180000000 0000001ecd281b9e (__kernel_map_pages+0x166/0x188)
> R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:0 AS:3 CC:2 PM:0 RI:0 EA:3
> Krnl GPRS: 0000000000000000 0000000000000800 0000400b00000000 0000000000000100
> 0000000000000001 0000000000000000 0000000000000002 0000000000000100
> 0000001ece139230 0000001ecdd98d40 0000400b00000100 0000000000000000
> 000003ffa17e4000 001fffe0114f7d08 0000001ecd4d93ea 001fffe0114f7b20
> Krnl Code: 0000001ecd281b8e: ec17ffff00d8 ahik %r1,%r7,-1
> 0000001ecd281b94: ec111dbc0355 risbg %r1,%r1,29,188,3
>> 0000001ecd281b9e: 94fb5006 ni 6(%r5),251
> 0000001ecd281ba2: 41505008 la %r5,8(%r5)
> 0000001ecd281ba6: ec51fffc6064 cgrj %r5,%r1,6,1ecd281b9e
> 0000001ecd281bac: 1a07 ar %r0,%r7
> 0000001ecd281bae: ec03ff584076 crj %r0,%r3,4,1ecd281a5e
> Call Trace:
> [<0000001ecd281b9e>] __kernel_map_pages+0x166/0x188
> [<0000001ecd4d9516>] online_pages_range+0xf6/0x128
> [<0000001ecd2a8186>] walk_system_ram_range+0x7e/0xd8
> [<0000001ecda28aae>] online_pages+0x2fe/0x3f0
> [<0000001ecd7d02a6>] memory_subsys_online+0x8e/0xc0
> [<0000001ecd7add42>] device_online+0x5a/0xc8
> [<0000001ecd7d0430>] state_store+0x88/0x118
> [<0000001ecd5b9f62>] kernfs_fop_write+0xc2/0x200
> [<0000001ecd5064b6>] vfs_write+0x176/0x1e0
> [<0000001ecd50676a>] ksys_write+0xa2/0x100
> [<0000001ecda315d4>] system_call+0xd8/0x2c8
> 
> Fix this by checking debug_pagealloc_enabled_static() before calling
> kernel_map_pages(). Backports for kernel before 5.5 should use
> debug_pagealloc_enabled() instead. Also add comments.
> 
> Fixes: cd02cf1aceea ("mm/hotplug: fix an imbalance with DEBUG_PAGEALLOC")
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> Reported-by: Gerald Schaefer <gerald.schaefer@xxxxxxxxxx>
> Cc: Qian Cai <cai@xxxxxx>
> Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>
> ---
>  include/linux/mm.h  | 4 ++++
>  mm/memory_hotplug.c | 8 +++++++-
>  2 files changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git include/linux/mm.h include/linux/mm.h
> index 52269e56c514..c54fb96cb1e6 100644
> --- include/linux/mm.h
> +++ include/linux/mm.h
> @@ -2715,6 +2715,10 @@ static inline bool debug_pagealloc_enabled_static(void)
>  #if defined(CONFIG_DEBUG_PAGEALLOC) || defined(CONFIG_ARCH_HAS_SET_DIRECT_MAP)
>  extern void __kernel_map_pages(struct page *page, int numpages, int enable);
>  
> +/*
> + * When called in DEBUG_PAGEALLOC context, the call should most likely be
> + * guarded by debug_pagealloc_enabled() or debug_pagealloc_enabled_static()
> + */
>  static inline void
>  kernel_map_pages(struct page *page, int numpages, int enable)
>  {
> diff --git mm/memory_hotplug.c mm/memory_hotplug.c
> index 0a54ffac8c68..19389cdc16a5 100644
> --- mm/memory_hotplug.c
> +++ mm/memory_hotplug.c
> @@ -574,7 +574,13 @@ EXPORT_SYMBOL_GPL(restore_online_page_callback);
>  
>  void generic_online_page(struct page *page, unsigned int order)
>  {
> -	kernel_map_pages(page, 1 << order, 1);
> +	/*
> +	 * Freeing the page with debug_pagealloc enabled will try to unmap it,
> +	 * so we should map it first. This is better than introducing a special
> +	 * case in page freeing fast path.
> +	 */
> +	if (debug_pagealloc_enabled_static())
> +		kernel_map_pages(page, 1 << order, 1);
>  	__free_pages_core(page, order);
>  	totalram_pages_add(1UL << order);
>  #ifdef CONFIG_HIGHMEM
> 

Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>

-- 
Thanks,

David / dhildenb