Re: [PATCH v3 4/6] mm: Introduce a pageflag for partially mapped folios

[Usama Arif <usamaarif642@gmail.com>]

On 14/08/2024 12:20, Barry Song wrote:
> On Wed, Aug 14, 2024 at 11:11 PM Usama Arif <usamaarif642@gmail.com> wrote:
>>
>>
>>
>> On 14/08/2024 11:44, Barry Song wrote:
>>> On Wed, Aug 14, 2024 at 12:03 AM Usama Arif <usamaarif642@gmail.com> 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@gmail.com>
>>>> ---
>>>>  include/linux/huge_mm.h    |  4 ++--
>>>>  include/linux/page-flags.h |  3 +++
>>>>  mm/huge_memory.c           | 21 +++++++++++++--------
>>>>  mm/hugetlb.c               |  1 +
>>>>  mm/internal.h              |  4 +++-
>>>>  mm/memcontrol.c            |  3 ++-
>>>>  mm/migrate.c               |  3 ++-
>>>>  mm/page_alloc.c            |  5 +++--
>>>>  mm/rmap.c                  |  3 ++-
>>>>  mm/vmscan.c                |  3 ++-
>>>>  10 files changed, 33 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..cecc1bad7910 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, /* was identified to be partially mapped */
>>>>  };
>>>>
>>>>  #define PAGEFLAGS_MASK         ((1UL << NR_PAGEFLAGS) - 1)
>>>> @@ -861,8 +862,10 @@ static inline void ClearPageCompound(struct page *page)
>>>>         ClearPageHead(page);
>>>>  }
>>>>  FOLIO_FLAG(large_rmappable, FOLIO_SECOND_PAGE)
>>>> +FOLIO_FLAG(partially_mapped, FOLIO_SECOND_PAGE)
>>>>  #else
>>>>  FOLIO_FLAG_FALSE(large_rmappable)
>>>> +FOLIO_FLAG_FALSE(partially_mapped)
>>>>  #endif
>>>>
>>>>  #define PG_head_mask ((1UL << PG_head))
>>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>>> index 6df0e9f4f56c..c024ab0f745c 100644
>>>> --- a/mm/huge_memory.c
>>>> +++ b/mm/huge_memory.c
>>>> @@ -3397,6 +3397,7 @@ 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) {
>>>> @@ -3453,11 +3454,12 @@ 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);
>>>>         }
>>>>         spin_unlock_irqrestore(&ds_queue->split_queue_lock, flags);
>>>>  }
>>>>
>>>> -void deferred_split_folio(struct folio *folio)
>>>> +void deferred_split_folio(struct folio *folio, bool partially_mapped)
>>>>  {
>>>>         struct deferred_split *ds_queue = get_deferred_split_queue(folio);
>>>>  #ifdef CONFIG_MEMCG
>>>> @@ -3485,14 +3487,17 @@ void deferred_split_folio(struct folio *folio)
>>>>         if (folio_test_swapcache(folio))
>>>>                 return;
>>>>
>>>> -       if (!list_empty(&folio->_deferred_list))
>>>> -               return;
>>>> -
>>>>         spin_lock_irqsave(&ds_queue->split_queue_lock, flags);
>>>> +       if (partially_mapped)
>>>> +               folio_set_partially_mapped(folio);
>>>> +       else
>>>> +               folio_clear_partially_mapped(folio);
>>>
>>> Hi Usama,
>>>
>>> Do we need this? When can a partially_mapped folio on deferred_list become
>>> non-partially-mapped and need a clear? I understand transferring from
>>> entirely_map
>>> to partially_mapped is a one way process? partially_mapped folios can't go back
>>> to entirely_mapped?
>>>
>> Hi Barry,
>>
>> deferred_split_folio function is called in 3 places after this series, at fault, collapse and partial mapping. partial mapping can only happen after fault/collapse, and we have FOLIO_FLAG_FALSE(partially_mapped), i.e. flag initialized to false, so technically its not needed. A partially_mapped folio on deferred list wont become non-partially mapped.
>>
>> I just did it as a precaution if someone ever changes the kernel and calls deferred_split_folio with partially_mapped set to false after it had been true. The function arguments of deferred_split_folio make it seem that passing partially_mapped=false as an argument would clear it, which is why I cleared it as well. I could change the patch to something like below if it makes things better? i.e. add a comment at the top of the function:
>>
> 
> to me, it seems a BUG to call with false after a folio has been
> partially_mapped. So I'd rather
> VM_WARN_ON_FOLIO(folio_test_partially_mapped(folio), folio);
> 
> The below should also fix the MTHP_STAT_SPLIT_DEFERRED
> counter this patch is breaking?
> 
> @@ -3515,16 +3522,18 @@ void deferred_split_folio(struct folio *folio,
> bool partially_mapped)
>                 return;
> 
>         spin_lock_irqsave(&ds_queue->split_queue_lock, flags);
> -       if (partially_mapped)
> -               folio_set_partially_mapped(folio);
> -       else
> -               folio_clear_partially_mapped(folio);
> -       if (list_empty(&folio->_deferred_list)) {
> -               if (partially_mapped) {
> +       if (partially_mapped) {
> +               if (!folio_test_set_partially_mapped(folio)) {
> +                       mod_mthp_stat(folio_order(folio),
> +                               MTHP_STAT_NR_SPLIT_DEFERRED, 1);
>                         if (folio_test_pmd_mappable(folio))
>                                 count_vm_event(THP_DEFERRED_SPLIT_PAGE);
>                         count_mthp_stat(folio_order(folio),
> MTHP_STAT_SPLIT_DEFERRED);
>                 }
> +       }
> +       VM_WARN_ON_FOLIO(folio_test_partially_mapped(folio), folio);
> +
> +       if (list_empty(&folio->_deferred_list)) {
>                 list_add_tail(&folio->_deferred_list, &ds_queue->split_queue);
>                 ds_queue->split_queue_len++;
>  #ifdef CONFIG_MEMCG
> 
> 
So I had sent the below without the VM_WARN_ON_FOLIO as a reply to the other email, below is with VM_WARN.


-void deferred_split_folio(struct folio *folio)
+/* partially_mapped=false won't clear PG_partially_mapped folio flag */
+void deferred_split_folio(struct folio *folio, bool partially_mapped)
 {
        struct deferred_split *ds_queue = get_deferred_split_queue(folio);
 #ifdef CONFIG_MEMCG
@@ -3485,14 +3488,17 @@ void deferred_split_folio(struct folio *folio)
        if (folio_test_swapcache(folio))
                return;
 
-       if (!list_empty(&folio->_deferred_list))
-               return;
-
        spin_lock_irqsave(&ds_queue->split_queue_lock, flags);
-       if (list_empty(&folio->_deferred_list)) {
+       if (partially_mapped) {
+               folio_set_partially_mapped(folio);
                if (folio_test_pmd_mappable(folio))
                        count_vm_event(THP_DEFERRED_SPLIT_PAGE);
                count_mthp_stat(folio_order(folio), MTHP_STAT_SPLIT_DEFERRED);
+       } else {
+               /* partially mapped folios cannont become partially unmapped */
+               VM_WARN_ON_FOLIO(folio_test_partially_mapped(folio), folio);
+       }
+       if (list_empty(&folio->_deferred_list)) {
                list_add_tail(&folio->_deferred_list, &ds_queue->split_queue);
                ds_queue->split_queue_len++;
 #ifdef CONFIG_MEMCG

 Thanks