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

David Hildenbrand
2024-08-07T14:52:38Z

On 07.08.24 16:45, Zi Yan wrote:
> On 7 Aug 2024, at 5:57, David Hildenbrand wrote:
>
>> On 06.08.24 17:36, Zi Yan wrote:
>>> On 6 Aug 2024, at 6:24, David Hildenbrand wrote:
>>>
>>>> On 06.08.24 12:03, David Hildenbrand wrote:
>>>>> 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.
>>>>
>>>> Indeed, the following does the trick! Thanks Ryan, I could have sworn
>>>> I ran that selftest as well.
>>>>
>>>> TAP version 13
>>>> 1..12
>>>> ok 1 Split huge pages successful
>>>> ok 2 Split PTE-mapped huge pages successful
>>>> # Please enable pr_debug in split_huge_pages_in_file() for more info.
>>>> # Please check dmesg for more information
>>>> ok 3 File-backed THP split test done
>>>>
>>>> ...
>>>>
>>>>
>>>> @Andrew, can you squash the following?
>>>>
>>>>
>>>> From e5ea585de3e089ea89bf43d8447ff9fc9b371286 Mon Sep 17 00:00:00 2001
>>>> From: David Hildenbrand <david@redhat.com>
>>>> Date: Tue, 6 Aug 2024 12:08:17 +0200
>>>> Subject: [PATCH] fixup: mm/huge_memory: convert split_huge_pages_pid() from
>>>> follow_page() to folio_walk
>>>>
>>>> We have to teach can_split_folio() that we are not holding an additional
>>>> reference.
>>>>
>>>> Signed-off-by: David Hildenbrand <david@redhat.com>
>>>> ---
>>>> include/linux/huge_mm.h | 4 ++--
>>>> mm/huge_memory.c | 8 ++++----
>>>> mm/vmscan.c | 2 +-
>>>> 3 files changed, 7 insertions(+), 7 deletions(-)
>>>>
>>>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
>>>> index e25d9ebfdf89..ce44caa40eed 100644
>>>> --- a/include/linux/huge_mm.h
>>>> +++ b/include/linux/huge_mm.h
>>>> @@ -314,7 +314,7 @@ unsigned long thp_get_unmapped_area_vmflags(struct file *filp, unsigned long add
>>>> unsigned long len, unsigned long pgoff, unsigned long flags,
>>>> vm_flags_t vm_flags);
>>>> -bool can_split_folio(struct folio *folio, int *pextra_pins);
>>>> +bool can_split_folio(struct folio *folio, int caller_pins, int *pextra_pins);
>>>> int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>>>> unsigned int new_order);
>>>> static inline int split_huge_page(struct page *page)
>>>> @@ -470,7 +470,7 @@ thp_get_unmapped_area_vmflags(struct file *filp, unsigned long addr,
>>>> }
>>>> static inline bool
>>>> -can_split_folio(struct folio *folio, int *pextra_pins)
>>>> +can_split_folio(struct folio *folio, int caller_pins, int *pextra_pins)
>>>> {
>>>> return false;
>>>> }
>>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>>> index 697fcf89f975..c40b0dcc205b 100644
>>>> --- a/mm/huge_memory.c
>>>> +++ b/mm/huge_memory.c
>>>> @@ -3021,7 +3021,7 @@ static void __split_huge_page(struct page *page, struct list_head *list,
>>>> }
>>>> /* Racy check whether the huge page can be split */
>>>> -bool can_split_folio(struct folio *folio, int *pextra_pins)
>>>> +bool can_split_folio(struct folio *folio, int caller_pins, int *pextra_pins)
>>>> {
>>>> int extra_pins;
>>>> @@ -3033,7 +3033,7 @@ bool can_split_folio(struct folio *folio, int *pextra_pins)
>>>> extra_pins = folio_nr_pages(folio);
>>>> if (pextra_pins)
>>>> *pextra_pins = extra_pins;
>>>> - return folio_mapcount(folio) == folio_ref_count(folio) - extra_pins - 1;
>>>> + return folio_mapcount(folio) == folio_ref_count(folio) - extra_pins - caller_pins;
>>>> }
>>>> /*
>>>> @@ -3201,7 +3201,7 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>>>> * Racy check if we can split the page, before unmap_folio() will
>>>> * split PMDs
>>>> */
>>>> - if (!can_split_folio(folio, &extra_pins)) {
>>>> + if (!can_split_folio(folio, 1, &extra_pins)) {
>>>> ret = -EAGAIN;
>>>> goto out_unlock;
>>>> }
>>>> @@ -3537,7 +3537,7 @@ static int split_huge_pages_pid(int pid, unsigned long vaddr_start,
>>>> * can be split or not. So skip the check here.
>>>> */
>>>> if (!folio_test_private(folio) &&
>>>> - !can_split_folio(folio, NULL))
>>>> + !can_split_folio(folio, 0, NULL))
>>>> goto next;
>>>> if (!folio_trylock(folio))
>>>
>>> The diff below can skip a folio with private and extra pin(s) early instead
>>> of trying to lock and split it then failing at can_split_folio() inside
>>> split_huge_page_to_list_to_order().
>>>
>>> Maybe worth applying on top of yours?
>>>
>>>
>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>> index a218320a9233..ce992d54f1da 100644
>>> --- a/mm/huge_memory.c
>>> +++ b/mm/huge_memory.c
>>> @@ -3532,13 +3532,10 @@ static int split_huge_pages_pid(int pid, unsigned long vaddr_start,
>>> goto next;
>>>
>>> total++;
>>> - /*
>>> - * For folios with private, split_huge_page_to_list_to_order()
>>> - * will try to drop it before split and then check if the folio
>>> - * can be split or not. So skip the check here.
>>> - */
>>> - if (!folio_test_private(folio) &&
>>> - !can_split_folio(folio, 0, NULL))
>>> +
>>> + if (!can_split_folio(folio,
>>> + folio_test_private(folio) ? 1 : 0,
>>> + NULL))
>>
>> Hmm, it does look a bit odd. It's not something from the caller (caller_pins), but a
>> folio property. Likely should be handled differently.
>>
>> In vmscan code, we only call can_split_folio() on anon folios where
>> folio_test_private() does not apply.
>>
>> But indeed, in split_huge_page_to_list_to_order() we'd have to fail if
>> folio_test_private() still applies after
>>
>> Not sure if that is really better:
>
> Yeah, not worth the code churn to optimize for that debugfs code.
>
> As I looked at this patch and the fix long enough, feel free to add
> Reviewed-by: Zi Yan <ziy@nvidia.com>

Thanks! :)

--
Cheers,

David / dhildenb