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

Zi Yan
2024-08-07T14:45:59Z

[-- Attachment #1: Type: text/plain, Size: 7338 bytes --]

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>

Best Regards,
Yan, Zi

[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 854 bytes --]