Re: [PATCH v2 1/1] mm, memory_hotplug: Initialize struct pages for the full memory section

[Zaslonko Mikhail <zaslonko@linux.bm.com>]

On 13.12.2018 16:12, Wei Yang wrote:
> On Thu, Dec 13, 2018 at 01:37:16PM +0100, Zaslonko Mikhail wrote:
>> On 13.12.2018 04:46, Wei Yang wrote:
>>> On Wed, Dec 12, 2018 at 06:27:12PM +0100, Mikhail Zaslonko wrote:
>>>> 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 page access from
>>>> is_mem_section_removable() or test_pages_in_a_zone() function triggered by
>>>> memory_hotplug sysfs handlers:
>>>>
>>>> Here are the the panic examples:
>>>> CONFIG_DEBUG_VM=y
>>>> 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
>>>>
>>>> kernel parameter mem=3075M
>>>> --------------------------
>>>> page:000003d08300c000 is uninitialized and poisoned
>>>> page dumped because: VM_BUG_ON_PAGE(PagePoisoned(p))
>>>> Call Trace:
>>>> ([<000000000038596c>] is_mem_section_removable+0xb4/0x190)
>>>>  [<00000000008f12fa>] show_mem_removable+0x9a/0xd8
>>>>  [<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:
>>>>  [<000000000038596c>] is_mem_section_removable+0xb4/0x190
>>>> Kernel panic - not syncing: Fatal exception: panic_on_oops
>>>>
>>>> Fix the problem by initializing the last memory section of each zone
>>>> in memmap_init_zone() till the very end, even if it goes beyond the zone
>>>> end.
>>>>
>>>> Signed-off-by: Mikhail Zaslonko <zaslonko@linux.ibm.com>
>>>> Reviewed-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
>>>> Cc: <stable@vger.kernel.org>
>>>> ---
>>>> mm/page_alloc.c | 12 ++++++++++++
>>>> 1 file changed, 12 insertions(+)
>>>>
>>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>>> index 2ec9cc407216..e2afdb2dc2c5 100644
>>>> --- a/mm/page_alloc.c
>>>> +++ b/mm/page_alloc.c
>>>> @@ -5542,6 +5542,18 @@ void __meminit memmap_init_zone(unsigned long size, int nid, unsigned long zone,
>>>> 			cond_resched();
>>>> 		}
>>>> 	}
>>>> +#ifdef CONFIG_SPARSEMEM
>>>> +	/*
>>>> +	 * If the zone does not span the rest of the section then
>>>> +	 * we should at least initialize those pages. Otherwise we
>>>> +	 * could blow up on a poisoned page in some paths which depend
>>>> +	 * on full sections being initialized (e.g. memory hotplug).
>>>> +	 */
>>>> +	while (end_pfn % PAGES_PER_SECTION) {
>>>> +		__init_single_page(pfn_to_page(end_pfn), end_pfn, zone, nid);
>>>> +		end_pfn++;
>>>> +	}
>>>> +#endif
>>>> }
>>>
>>> What will happen if the end_pfn is PAGES_PER_SECTION aligned, but there
>>> is invalid pfn? For example, the page is skipped in the for loop of
>>> memmap_init_zone()?
>>>
>>
>> If the end_pfn is PAGES_PER_SECTION aligned, we do not do any extra 
>> initialization.
>> If the page is skipped in the loop, it will remain uninitialized for 
>> the reason (e.g. a memory hole). The behavior has not been changed here.
>>
> 
> I may not describe my question clearly.
> 
> If the page is skipped, then it is uninitialized. Then would this page
> trigger the call trace when read the removable sysfs?

Yes, it might still trigger PF_POISONED_CHECK if the first page 
of the pageblock is left uninitialized (poisoned).
But in order to cover these exceptional cases we would need to 
adjust memory_hotplug sysfs handler functions with similar 
checks (as in the for loop of memmap_init_zone()). And I guess 
that is what we were trying to avoid (adding special cases to 
memory_hotplug paths).

> 
>>>>
>>>> #ifdef CONFIG_ZONE_DEVICE
>>>> -- 
>>>> 2.16.4
>>>
>