On Thu, Jan 09, 2020 at 03:32:33PM +0300, Kirill A. Shutemov wrote:
>On Thu, Jan 02, 2020 at 11:04:21AM +0800, Wei Yang wrote:
>> Currently we behave differently between PMD-mapped THP and PTE-mapped
>> THP on memory_failure.
>>
>> User detected difference:
>>
>> For PTE-mapped THP, the whole 2M range will trigger MCE after
>> memory_failure(), while only 4K range for PMD-mapped THP will.
>>
>> Direct reason:
>>
>> All the 512 PTE entry will be marked as hwpoison entry for a PTE-mapped
>> THP while only one PTE will be marked for a PMD-mapped THP.
>>
>> Root reason:
>>
>> The root cause is PTE-mapped page doesn't need to split pmd which skip
>> the SPLIT_FREEZE process.
>
>I don't follow how SPLIT_FREEZE is related to pisoning. Cold you
>laraborate?
>
Sure. Let me try to explain this a little.
split_huge_page_to_list
unmap_page
try_to_unmap_one
...
__split_huge_pmd_locked
__split_huge_page
remap_page
There are two dimensions:
* PMD mapped THP and PTE mapped THP
* HWPOISON-ed page and non-HWPOISON-ed page
So there are total 4 cases.
1. First let's take a look at the normal case, when HWPOISON is not set.
If the page is PMD-mapped, SPLIT_FREEZE is passed down in flags. And finally
passed to __split_huge_pmd_locked. In this function, when freeze is true, PTE
will be set to migration entry. And because __split_huge_pmd_locked save
migration entry to PTE, try_to_unmap_one will not do real unmap. Then
remap_page restore those migration entry back.
If the page is PTE-mapped, __split_huge_pmd_locked will be skipped since this
is already done. This means try_to_unmap_one will do the real unmap. Because
SPLIT_FREEZE is passed, PTE will be set to migration entry, which is the same
behavior as PMD-mapped page. Then remap_page restore those migration entry
back.
This shows PMD-mapped and PTE-mapped page share the same result on split.
While difference is who sets PTE as migration entry
* __split_huge_pmd_locked does this job for PMD-mapped page
* try_to_unmap_one does this job for PTE-mapped page
2. Now let's take a look at the HWPOISON case.
There are two critical differences
* __split_huge_pmd_locked is skipped for PTE-mapped page
* HWPOISON effects the behavior of try_to_unmap_one
Then for PMD-mapped page, HWPOISON has no effect on split. But for PTE-mapped
page, all PTE will be set to hwpoison entry.
Then in memory_failure, the page split will have two different PTE result.
Not sure I explain it clearly.
--
Wei Yang
Help you, Help me
