Re: [PATCH 1/2] mm, memory_hotplug: is_mem_section_removable do not pass the end of a zone

[Oscar Salvador <osalvador@suse.de>]

On Mon, Jan 28, 2019 at 03:45:05PM +0100, Michal Hocko wrote:
> From: Michal Hocko <mhocko@suse.com>
> 
> Mikhail has reported the following VM_BUG_ON triggered when reading
> sysfs removable state of a memory block:
>  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
> 
> The reason is that the memory block spans the zone boundary and we are
> stumbling over an unitialized struct page. Fix this by enforcing zone
> range in is_mem_section_removable so that we never run away from a
> zone.

Does that mean that the remaining pages(escaping from the current zone) are not tied to
any other zone? Why? Are these pages "holes" or how that came to be?

> 
> Reported-by: Mikhail Zaslonko <zaslonko@linux.ibm.com>
> Debugged-by: Mikhail Zaslonko <zaslonko@linux.ibm.com>
> Signed-off-by: Michal Hocko <mhocko@suse.com>
> ---
>  mm/memory_hotplug.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index b9a667d36c55..07872789d778 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1233,7 +1233,8 @@ static bool is_pageblock_removable_nolock(struct page *page)
>  bool is_mem_section_removable(unsigned long start_pfn, unsigned long nr_pages)
>  {
>  	struct page *page = pfn_to_page(start_pfn);
> -	struct page *end_page = page + nr_pages;
> +	unsigned long end_pfn = min(start_pfn + nr_pages, zone_end_pfn(page_zone(page)));
> +	struct page *end_page = pfn_to_page(end_pfn);
>  
>  	/* Check the starting page of each pageblock within the range */
>  	for (; page < end_page; page = next_active_pageblock(page)) {
> -- 
> 2.20.1
> 

-- 
Oscar Salvador
SUSE L3