Re: [PATCH 2/2] mm, memory_hotplug: test_pages_in_a_zone do not pass the end of zone

[Oscar Salvador]

On Mon, Jan 28, 2019 at 03:45:06PM +0100, Michal Hocko wrote:
> From: Mikhail Zaslonko <zaslonko@xxxxxxxxxxxxx>
> 
> If memory end is not aligned with the sparse memory section boundary, the
> mapping of such a section is only partly initialized. This may lead to
> VM_BUG_ON due to uninitialized struct pages access from test_pages_in_a_zone()
> function triggered by memory_hotplug sysfs handlers.
> 
> Here are the the panic examples:
>  CONFIG_DEBUG_VM_PGFLAGS=y
>  kernel parameter mem=2050M
>  --------------------------
>  page:000003d082008000 is uninitialized and poisoned
>  page dumped because: VM_BUG_ON_PAGE(PagePoisoned(p))
>  Call Trace:
>  ([<0000000000385b26>] test_pages_in_a_zone+0xde/0x160)
>   [<00000000008f15c4>] show_valid_zones+0x5c/0x190
>   [<00000000008cf9c4>] dev_attr_show+0x34/0x70
>   [<0000000000463ad0>] sysfs_kf_seq_show+0xc8/0x148
>   [<00000000003e4194>] seq_read+0x204/0x480
>   [<00000000003b53ea>] __vfs_read+0x32/0x178
>   [<00000000003b55b2>] vfs_read+0x82/0x138
>   [<00000000003b5be2>] ksys_read+0x5a/0xb0
>   [<0000000000b86ba0>] system_call+0xdc/0x2d8
>  Last Breaking-Event-Address:
>   [<0000000000385b26>] test_pages_in_a_zone+0xde/0x160
>  Kernel panic - not syncing: Fatal exception: panic_on_oops
> 
> Fix this by checking whether the pfn to check is within the zone.
> 
> [mhocko@xxxxxxxx: separated this change from
> http://lkml.kernel.org/r/20181105150401.97287-2-zaslonko@xxxxxxxxxxxxx]
> Signed-off-by: Mikhail Zaslonko <zaslonko@xxxxxxxxxxxxx>
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>

Looks good to me:

Reviewed-by: Oscar Salvador <osalvador@xxxxxxx>

> ---
>  mm/memory_hotplug.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 07872789d778..7711d0e327b6 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1274,6 +1274,9 @@ int test_pages_in_a_zone(unsigned long start_pfn, unsigned long end_pfn,
>  				i++;
>  			if (i == MAX_ORDER_NR_PAGES || pfn + i >= end_pfn)
>  				continue;
> +			/* Check if we got outside of the zone */
> +			if (zone && !zone_spans_pfn(zone, pfn + i))
> +				return 0;
>  			page = pfn_to_page(pfn + i);

Since we are already checking if the zone spans that pfn, is it safe to get
rid of the below check? Or maybe not because we might have intersected zones?

>  			if (zone && page_zone(page) != zone)
>  				return 0;

-- 
Oscar Salvador
SUSE L3