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

Zaslonko Mikhail
2018-12-10T16:14:41Z

Hello,

On 10.12.2018 16:10, Wei Yang wrote:
> On Mon, Dec 10, 2018 at 02:07:12PM +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:
>>
>> 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 the problem by initializing the last memory section of the highest 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 | 15 +++++++++++++++
>> 1 file changed, 15 insertions(+)
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 2ec9cc407216..41ef5508e5f1 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -5542,6 +5542,21 @@ void __meminit memmap_init_zone(unsigned long size, int nid, unsigned long zone,
>> cond_resched();
>> }
>> }
>> +#ifdef CONFIG_SPARSEMEM
>> + /*
>> + * If there is no zone spanning 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).
>> + */
>> + if (end_pfn == max_pfn) {
>> + while (end_pfn % PAGES_PER_SECTION) {
>> + __init_single_page(pfn_to_page(end_pfn), end_pfn, zone,
>> + nid);
>> + end_pfn++;
>> + }
>> + }
>> +#endif
>
> If my understanding is correct, end_pfn is not a valid range.
>
> memmap_init_zone() initialize the range [start_pfn, start_pfn + size). I
> am afraid this will break the syntax.
>
> And max_pfn is also not a valid one. For example, on x86,
I used pfn_max here to check for the highest zone. What would be a better way?

> update_end_of_memory_vars() will update max_pfn, which is calculated by:
>
> end_pfn = PFN_UP(start + size);
>
> BTW, as you mentioned this apply to hotplug case. And then why this couldn't
> happen during boot up? What differ these two cases?

Well, the pages left uninitialized during bootup (initial problem), but the panic itself takes
place when we try to process memory_hotplug sysfs attributes (thus triggering sysfs handlers).
You can find more details in the original thread:
https://marc.info/?t=153658306400001&r=1&w=2

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

Thanks,
Mikhail Zaslonko