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

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

On 11.12.2018 10:49, Wei Yang wrote:
> On Mon, Dec 10, 2018 at 02:24:51PM +0100, Michal Hocko wrote:
>> On Mon 10-12-18 14:07:12, 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.
>>
>> It would be great to mention how you can end up in the situation like
>> this(a user provided memmap or a strange HW). 
>>
>>> 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.
>>
>> Why do we need to restrict this to the highest zone? In other words, why
>> cannot we do what I was suggesting earlier [1]. What does prevent other
>> zones to have an incomplete section boundary?
>>
>> [1] http://lkml.kernel.org/r/20181105183533.GQ4361@dhcp22.suse.cz
>>
> 
> I tried to go through the original list and make myself familiar with
> the bug.
> 
> Confused why initialize the *last* end_pfn could fix this, since
> is_mem_section_removable() will iterate on each page of a section. This
> means we need to initialize all the pages left in the section.
That's exactly what the fix does. We initialize all the pages left in 
the section.

> 
> One way to fix this in my mind is to record the last pfn in mem_section.
Do you mean last initialized pfn? I guess we have agreed upon that the 
entire section should be initialized.  

> This could be done in memory_preset(), since after that we may assume
> the section is full. Not sure whether you would like it.
>