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SubjectRe: [PATCH] memory_hotplug: fix the panic when memory end is not on the section boundary
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On 10.09.2018 15:46, Pasha Tatashin wrote:
>
> On 9/10/18 9:17 AM, Michal Hocko wrote:
>> [Cc Pavel]
>>
>> On Mon 10-09-18 14:35:27, Mikhail Zaslonko wrote:
>>> If memory end is not aligned with the linux memory section boundary, such
>>> a section is only partly initialized. This may lead to VM_BUG_ON due to
>>> uninitialized struct pages access from is_mem_section_removable() or
>>> test_pages_in_a_zone() function.
>>>
>>> Here is one of the panic examples:
>>> CONFIG_DEBUG_VM_PGFLAGS=y
>>> kernel parameter mem=3075M
>> OK, so the last memory section is not full and we have a partial memory
>> block right?
>>
>>> page dumped because: VM_BUG_ON_PAGE(PagePoisoned(p))
>> OK, this means that the struct page is not fully initialized. Do you
>> have a specific place which has triggered this assert?
>>
>>> ------------[ cut here ]------------
>>> Call Trace:
>>> ([<000000000039b8a4>] is_mem_section_removable+0xcc/0x1c0)
>>> [<00000000009558ba>] show_mem_removable+0xda/0xe0
>>> [<00000000009325fc>] dev_attr_show+0x3c/0x80
>>> [<000000000047e7ea>] sysfs_kf_seq_show+0xda/0x160
>>> [<00000000003fc4e0>] seq_read+0x208/0x4c8
>>> [<00000000003cb80e>] __vfs_read+0x46/0x180
>>> [<00000000003cb9ce>] vfs_read+0x86/0x148
>>> [<00000000003cc06a>] ksys_read+0x62/0xc0
>>> [<0000000000c001c0>] system_call+0xdc/0x2d8
>>>
>>> This fix checks if the page lies within the zone boundaries before
>>> accessing the struct page data. The check is added to both functions.
>>> Actually similar check has already been present in
>>> is_pageblock_removable_nolock() function but only after the struct page
>>> is accessed.
>>>
>> Well, I am afraid this is not the proper solution. We are relying on the
>> full pageblock worth of initialized struct pages at many other place. We
>> used to do that in the past because we have initialized the full
>> section but this has been changed recently. Pavel, do you have any ideas
>> how to deal with this partial mem sections now?
> We have:
>
> remove_memory()
> BUG_ON(check_hotplug_memory_range(start, size))
>
> That supposed to safely check for this condition: if [start, start +
> size) not block size aligned (and we know block size is section
> aligned), hot remove is not allowed. The problem is this check is late,
> and only happens when invalid range has already passed through previous
> checks.
>
> We could add check_hotplug_memory_range() to is_mem_section_removable():
>
> is_mem_section_removable(start_pfn, nr_pages)
> if (check_hotplug_memory_range(PFN_PHYS(start_pfn), PFN_PHYS(nr_pages)))
> return false;
>
> I think it should work.

I don't think so since is_mem_section_removable() is called for for the
entire
section. Thus [start_pfn, start_pfn + nr_pages) is always memory block
aligned.

>
> Pavel
>
>>> Signed-off-by: Mikhail Zaslonko <zaslonko@linux.ibm.com>
>>> Reviewed-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
>>> Cc: <stable@vger.kernel.org>
>>> ---
>>> mm/memory_hotplug.c | 20 +++++++++++---------
>>> 1 file changed, 11 insertions(+), 9 deletions(-)
>>>
>>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>>> index 9eea6e809a4e..8e20e8fcc3b0 100644
>>> --- a/mm/memory_hotplug.c
>>> +++ b/mm/memory_hotplug.c
>>> @@ -1229,9 +1229,8 @@ static struct page *next_active_pageblock(struct page *page)
>>> return page + pageblock_nr_pages;
>>> }
>>>
>>> -static bool is_pageblock_removable_nolock(struct page *page)
>>> +static bool is_pageblock_removable_nolock(struct page *page, struct zone **zone)
>>> {
>>> - struct zone *zone;
>>> unsigned long pfn;
>>>
>>> /*
>>> @@ -1241,15 +1240,14 @@ static bool is_pageblock_removable_nolock(struct page *page)
>>> * We have to take care about the node as well. If the node is offline
>>> * its NODE_DATA will be NULL - see page_zone.
>>> */
>>> - if (!node_online(page_to_nid(page)))
>>> - return false;
>>> -
>>> - zone = page_zone(page);
>>> pfn = page_to_pfn(page);
>>> - if (!zone_spans_pfn(zone, pfn))
>>> + if (*zone && !zone_spans_pfn(*zone, pfn))
>>> return false;
>>> + if (!node_online(page_to_nid(page)))
>>> + return false;
>>> + *zone = page_zone(page);
>>>
>>> - return !has_unmovable_pages(zone, page, 0, MIGRATE_MOVABLE, true);
>>> + return !has_unmovable_pages(*zone, page, 0, MIGRATE_MOVABLE, true);
>>> }
>>>
>>> /* Checks if this range of memory is likely to be hot-removable. */
>>> @@ -1257,10 +1255,11 @@ 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;
>>> + struct zone *zone = NULL;
>>>
>>> /* Check the starting page of each pageblock within the range */
>>> for (; page < end_page; page = next_active_pageblock(page)) {
>>> - if (!is_pageblock_removable_nolock(page))
>>> + if (!is_pageblock_removable_nolock(page, &zone))
>>> return false;
>>> cond_resched();
>>> }
>>> @@ -1296,6 +1295,9 @@ 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))
>>> + return 0;
>>> page = pfn_to_page(pfn + i);
>>> if (zone && page_zone(page) != zone)
>>> return 0;
>>> --
>>> 2.16.4

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