Re: [PATCH v2] mm/page_owner: Don't access uninitialized memmaps when reading /proc/pagetypeinfo

[Vlastimil Babka]

On 10/11/19 4:06 PM, David Hildenbrand wrote:
> From: Qian Cai <cai@xxxxxx>
> 
> Uninitialized memmaps contain garbage and in the worst case trigger
> kernel BUGs, especially with CONFIG_PAGE_POISONING. They should not get
> touched.
> 
> For example, when not onlining a memory block that is spanned by a zone
> and reading /proc/pagetypeinfo with CONFIG_DEBUG_VM_PGFLAGS and
> CONFIG_PAGE_POISONING, we can trigger a kernel BUG:
> 
> :/# echo 1 > /sys/devices/system/memory/memory40/online
> :/# echo 1 > /sys/devices/system/memory/memory42/online
> :/# cat /proc/pagetypeinfo > test.file
>   [   42.489856] page:fffff2c585200000 is uninitialized and poisoned
>   [   42.489861] raw: ffffffffffffffff ffffffffffffffff ffffffffffffffff ffffffffffffffff
>   [   42.492235] raw: ffffffffffffffff ffffffffffffffff ffffffffffffffff ffffffffffffffff
>   [   42.493501] page dumped because: VM_BUG_ON_PAGE(PagePoisoned(p))
>   [   42.494533] There is not page extension available.
>   [   42.495358] ------------[ cut here ]------------
>   [   42.496163] kernel BUG at include/linux/mm.h:1107!
>   [   42.497069] invalid opcode: 0000 [#1] SMP NOPTI
> 
> Please not that this change does not affect ZONE_DEVICE, because
> pagetypeinfo_showmixedcount_print() is called from
> mm/vmstat.c:pagetypeinfo_showmixedcount() only for populated zones, and
> ZONE_DEVICE is never populated (zone->present_pages always 0).
> 
> Fixes: f1dd2cd13c4b ("mm, memory_hotplug: do not associate hotadded memory to zones until online") # visible after d0dc12e86b319
> Signed-off-by: Qian Cai <cai@xxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: "Peter Zijlstra (Intel)" <peterz@xxxxxxxxxxxxx>
> Cc: Miles Chen <miles.chen@xxxxxxxxxxxx>
> Cc: Mike Rapoport <rppt@xxxxxxxxxxxxxxxxxx>
> Cc: Qian Cai <cai@xxxxxx>
> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> [ move check to outer loop, add comment, rephrase description ]
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

> ---
> 
> Cai asked me to follow up on:
> 	[PATCH] mm/page_owner: fix a crash after memory offline
> 
> ---
>  mm/page_owner.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/page_owner.c b/mm/page_owner.c
> index dee931184788..7d149211f6be 100644
> --- a/mm/page_owner.c
> +++ b/mm/page_owner.c
> @@ -284,7 +284,8 @@ void pagetypeinfo_showmixedcount_print(struct seq_file *m,
>  	 * not matter as the mixed block count will still be correct
>  	 */
>  	for (; pfn < end_pfn; ) {
> -		if (!pfn_valid(pfn)) {
> +		page = pfn_to_online_page(pfn);
> +		if (!page) {
>  			pfn = ALIGN(pfn + 1, MAX_ORDER_NR_PAGES);
>  			continue;
>  		}
> @@ -292,13 +293,13 @@ void pagetypeinfo_showmixedcount_print(struct seq_file *m,
>  		block_end_pfn = ALIGN(pfn + 1, pageblock_nr_pages);
>  		block_end_pfn = min(block_end_pfn, end_pfn);
>  
> -		page = pfn_to_page(pfn);
>  		pageblock_mt = get_pageblock_migratetype(page);
>  
>  		for (; pfn < block_end_pfn; pfn++) {
>  			if (!pfn_valid_within(pfn))
>  				continue;
>  
> +			/* The pageblock is online, no need to recheck. */
>  			page = pfn_to_page(pfn);
>  
>  			if (page_zone(page) != zone)
>