Re: [PATCH v1 07/11] mm/huge_memory: convert split_huge_pages_pid() from follow_page() to folio_walk

[David Hildenbrand <david@redhat.com>]

On 06.08.24 11:56, David Hildenbrand wrote:
> On 06.08.24 11:46, Ryan Roberts wrote:
>> On 02/08/2024 16:55, David Hildenbrand wrote:
>>> Let's remove yet another follow_page() user. Note that we have to do the
>>> split without holding the PTL, after folio_walk_end(). We don't care
>>> about losing the secretmem check in follow_page().
>>
>> Hi David,
>>
>> Our (arm64) CI is showing a regression in split_huge_page_test from mm selftests from next-20240805 onwards. Navigating around a couple of other lurking bugs, I was able to bisect to this change (which smells about right).
>>
>> Newly failing test:
>>
>> # # ------------------------------
>> # # running ./split_huge_page_test
>> # # ------------------------------
>> # # TAP version 13
>> # # 1..12
>> # # Bail out! Still AnonHugePages not split
>> # # # Planned tests != run tests (12 != 0)
>> # # # Totals: pass:0 fail:0 xfail:0 xpass:0 skip:0 error:0
>> # # [FAIL]
>> # not ok 52 split_huge_page_test # exit=1
>>
>> It's trying to split some pmd-mapped THPs then checking and finding that they are not split. The split is requested via /sys/kernel/debug/split_huge_pages, which I believe ends up in this function you are modifying here. Although I'll admit that looking at the change, there is nothing obviously wrong! Any ideas?
> 
> Nothing jumps at me as well. Let me fire up the debugger :)

Ah, very likely the can_split_folio() check expects a raised refcount 
already.

-- 
Cheers,

David / dhildenb