Re: [PATCH v4 4/6] mm: Introduce a pageflag for partially mapped folios

[Usama Arif]

On 19/08/2024 22:55, Barry Song wrote:
> On Tue, Aug 20, 2024 at 9:34 AM Barry Song <baohua@xxxxxxxxxx> wrote:
>>
>> On Tue, Aug 20, 2024 at 8:16 AM Usama Arif <usamaarif642@xxxxxxxxx> wrote:
>>>
>>>
>>>
>>> On 19/08/2024 20:00, Barry Song wrote:
>>>> On Tue, Aug 20, 2024 at 2:17 AM Usama Arif <usamaarif642@xxxxxxxxx> wrote:
>>>>>
>>>>>
>>>>>
>>>>> On 19/08/2024 09:29, Barry Song wrote:
>>>>>> Hi Usama,
>>>>>>
>>>>>> I feel it is much better now! thanks!
>>>>>>
>>>>>> On Mon, Aug 19, 2024 at 2:31 PM Usama Arif <usamaarif642@xxxxxxxxx> wrote:
>>>>>>>
>>>>>>> Currently folio->_deferred_list is used to keep track of
>>>>>>> partially_mapped folios that are going to be split under memory
>>>>>>> pressure. In the next patch, all THPs that are faulted in and collapsed
>>>>>>> by khugepaged are also going to be tracked using _deferred_list.
>>>>>>>
>>>>>>> This patch introduces a pageflag to be able to distinguish between
>>>>>>> partially mapped folios and others in the deferred_list at split time in
>>>>>>> deferred_split_scan. Its needed as __folio_remove_rmap decrements
>>>>>>> _mapcount, _large_mapcount and _entire_mapcount, hence it won't be
>>>>>>> possible to distinguish between partially mapped folios and others in
>>>>>>> deferred_split_scan.
>>>>>>>
>>>>>>> Eventhough it introduces an extra flag to track if the folio is
>>>>>>> partially mapped, there is no functional change intended with this
>>>>>>> patch and the flag is not useful in this patch itself, it will
>>>>>>> become useful in the next patch when _deferred_list has non partially
>>>>>>> mapped folios.
>>>>>>>
>>>>>>> Signed-off-by: Usama Arif <usamaarif642@xxxxxxxxx>
>>>>>>> ---
>>>>>>>  include/linux/huge_mm.h    |  4 ++--
>>>>>>>  include/linux/page-flags.h | 11 +++++++++++
>>>>>>>  mm/huge_memory.c           | 23 ++++++++++++++++-------
>>>>>>>  mm/internal.h              |  4 +++-
>>>>>>>  mm/memcontrol.c            |  3 ++-
>>>>>>>  mm/migrate.c               |  3 ++-
>>>>>>>  mm/page_alloc.c            |  5 +++--
>>>>>>>  mm/rmap.c                  |  5 +++--
>>>>>>>  mm/vmscan.c                |  3 ++-
>>>>>>>  9 files changed, 44 insertions(+), 17 deletions(-)
>>>>>>>
>>>>>>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
>>>>>>> index 4c32058cacfe..969f11f360d2 100644
>>>>>>> --- a/include/linux/huge_mm.h
>>>>>>> +++ b/include/linux/huge_mm.h
>>>>>>> @@ -321,7 +321,7 @@ static inline int split_huge_page(struct page *page)
>>>>>>>  {
>>>>>>>         return split_huge_page_to_list_to_order(page, NULL, 0);
>>>>>>>  }
>>>>>>> -void deferred_split_folio(struct folio *folio);
>>>>>>> +void deferred_split_folio(struct folio *folio, bool partially_mapped);
>>>>>>>
>>>>>>>  void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
>>>>>>>                 unsigned long address, bool freeze, struct folio *folio);
>>>>>>> @@ -495,7 +495,7 @@ static inline int split_huge_page(struct page *page)
>>>>>>>  {
>>>>>>>         return 0;
>>>>>>>  }
>>>>>>> -static inline void deferred_split_folio(struct folio *folio) {}
>>>>>>> +static inline void deferred_split_folio(struct folio *folio, bool partially_mapped) {}
>>>>>>>  #define split_huge_pmd(__vma, __pmd, __address)        \
>>>>>>>         do { } while (0)
>>>>>>>
>>>>>>> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
>>>>>>> index a0a29bd092f8..c3bb0e0da581 100644
>>>>>>> --- a/include/linux/page-flags.h
>>>>>>> +++ b/include/linux/page-flags.h
>>>>>>> @@ -182,6 +182,7 @@ enum pageflags {
>>>>>>>         /* At least one page in this folio has the hwpoison flag set */
>>>>>>>         PG_has_hwpoisoned = PG_active,
>>>>>>>         PG_large_rmappable = PG_workingset, /* anon or file-backed */
>>>>>>> +       PG_partially_mapped = PG_reclaim, /* was identified to be partially mapped */
>>>>>>>  };
>>>>>>>
>>>>>>>  #define PAGEFLAGS_MASK         ((1UL << NR_PAGEFLAGS) - 1)
>>>>>>> @@ -861,8 +862,18 @@ static inline void ClearPageCompound(struct page *page)
>>>>>>>         ClearPageHead(page);
>>>>>>>  }
>>>>>>>  FOLIO_FLAG(large_rmappable, FOLIO_SECOND_PAGE)
>>>>>>> +FOLIO_TEST_FLAG(partially_mapped, FOLIO_SECOND_PAGE)
>>>>>>> +/*
>>>>>>> + * PG_partially_mapped is protected by deferred_split split_queue_lock,
>>>>>>> + * so its safe to use non-atomic set/clear.
>>>>>>> + */
>>>>>>> +__FOLIO_SET_FLAG(partially_mapped, FOLIO_SECOND_PAGE)
>>>>>>> +__FOLIO_CLEAR_FLAG(partially_mapped, FOLIO_SECOND_PAGE)
>>>>>>>  #else
>>>>>>>  FOLIO_FLAG_FALSE(large_rmappable)
>>>>>>> +FOLIO_TEST_FLAG_FALSE(partially_mapped)
>>>>>>> +__FOLIO_SET_FLAG_NOOP(partially_mapped)
>>>>>>> +__FOLIO_CLEAR_FLAG_NOOP(partially_mapped)
>>>>>>>  #endif
>>>>>>>
>>>>>>>  #define PG_head_mask ((1UL << PG_head))
>>>>>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>>>>>> index 2d77b5d2291e..70ee49dfeaad 100644
>>>>>>> --- a/mm/huge_memory.c
>>>>>>> +++ b/mm/huge_memory.c
>>>>>>> @@ -3398,6 +3398,7 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>>>>>>>                          * page_deferred_list.
>>>>>>>                          */
>>>>>>>                         list_del_init(&folio->_deferred_list);
>>>>>>> +                       __folio_clear_partially_mapped(folio);
>>>>>>>                 }
>>>>>>>                 spin_unlock(&ds_queue->split_queue_lock);
>>>>>>>                 if (mapping) {
>>>>>>> @@ -3454,11 +3455,13 @@ void __folio_undo_large_rmappable(struct folio *folio)
>>>>>>>         if (!list_empty(&folio->_deferred_list)) {
>>>>>>>                 ds_queue->split_queue_len--;
>>>>>>>                 list_del_init(&folio->_deferred_list);
>>>>>>> +               __folio_clear_partially_mapped(folio);
>>>>>>
>>>>>> is it possible to make things clearer by
>>>>>>
>>>>>>  if (folio_clear_partially_mapped)
>>>>>>     __folio_clear_partially_mapped(folio);
>>>>>>
>>>>>> While writing without conditions isn't necessarily wrong, adding a condition
>>>>>> will improve the readability of the code and enhance the clarity of my mTHP
>>>>>> counters series. also help decrease smp cache sync if we can avoid
>>>>>> unnecessary writing?
>>>>>>
>>>>>
>>>>> Do you mean if(folio_test_partially_mapped(folio))?
>>>>>
>>>>> I don't like this idea. I think it makes the readability worse? If I was looking at if (test) -> clear for the first time, I would become confused why its being tested if its going to be clear at the end anyways?
>>>>
>>>> In the pmd-order case, the majority of folios are not partially mapped.
>>>> Unconditional writes will trigger cache synchronization across all
>>>> CPUs (related to the MESI protocol), making them more costly. By
>>>> using conditional writes, such as "if(test) write," we can avoid
>>>> most unnecessary writes, which is much more efficient. Additionally,
>>>> we only need to manage nr_split_deferred when the condition
>>>> is met. We are carefully evaluating all scenarios to determine
>>>> if modifications to the partially_mapped flag are necessary.
>>>>
>>>
>>>
>>> Hmm okay, as you said its needed for nr_split_deferred anyways. Something like below is ok to fold in?
>>>
>>> commit 4ae9e2067346effd902b342296987b97dee29018 (HEAD)
>>> Author: Usama Arif <usamaarif642@xxxxxxxxx>
>>> Date:   Mon Aug 19 21:07:16 2024 +0100
>>>
>>>     mm: Introduce a pageflag for partially mapped folios fix
>>>
>>>     Test partially_mapped flag before clearing it. This should
>>>     avoid unnecessary writes and will be needed in the nr_split_deferred
>>>     series.
>>>
>>>     Signed-off-by: Usama Arif <usamaarif642@xxxxxxxxx>
>>>
>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>> index 5d67d3b3c1b2..ccde60aaaa0f 100644
>>> --- a/mm/huge_memory.c
>>> +++ b/mm/huge_memory.c
>>> @@ -3479,7 +3479,8 @@ void __folio_undo_large_rmappable(struct folio *folio)
>>>         if (!list_empty(&folio->_deferred_list)) {
>>>                 ds_queue->split_queue_len--;
>>>                 list_del_init(&folio->_deferred_list);
>>> -               __folio_clear_partially_mapped(folio);
>>> +               if (folio_test_partially_mapped(folio))
>>> +                       __folio_clear_partially_mapped(folio);
>>>         }
>>>         spin_unlock_irqrestore(&ds_queue->split_queue_lock, flags);
>>>  }
>>> @@ -3610,7 +3611,8 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
>>>                 } else {
>>>                         /* We lost race with folio_put() */
>>>                         list_del_init(&folio->_deferred_list);
>>> -                       __folio_clear_partially_mapped(folio);
>>> +                       if (folio_test_partially_mapped(folio))
>>> +                               __folio_clear_partially_mapped(folio);
>>>                         ds_queue->split_queue_len--;
>>>                 }
>>>                 if (!--sc->nr_to_scan)
>>>
>>
>> Do we also need if (folio_test_partially_mapped(folio)) in
>> split_huge_page_to_list_to_order()?
>>
>> I recall that in Yu Zhao's TAO, there’s a chance of splitting (shattering)
>> non-partially-mapped folios. To be future-proof, we might want to handle
>> both cases equally.
> 
> we recall we also have a real case which can split entirely_mapped
> folio:
> 
> mm: huge_memory: enable debugfs to split huge pages to any order
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=fc4d182316bd5309b4066fd9ef21529ea397a7d4
> 
>>
>> By the way, we might not need to clear the flag for a new folio. This differs
>> from the init_list, which is necessary. If a new folio has the partially_mapped
>> flag, it indicates that we failed to clear it when freeing the folio to
>> the buddy system, which is a bug we need to fix in the free path.
>>
>> Thanks
>> Barry

I believe the below fixlet should address all concerns: