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

Usama Arif
2024-08-14T11:11:29Z

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:

-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 +3488,15 @@ 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);
if (list_empty(&folio->_deferred_list)) {
- if (folio_test_pmd_mappable(folio))
- count_vm_event(THP_DEFERRED_SPLIT_PAGE);
- count_mthp_stat(folio_order(folio), MTHP_STAT_SPLIT_DEFERRED);
+ if (partially_mapped) {
+ if (folio_test_pmd_mappable(folio))
+ count_vm_event(THP_DEFERRED_SPLIT_PAGE);
+ count_mthp_stat(folio_order(folio), MTHP_STAT_SPLIT_DEFERRED);
+ }
list_add_tail(&folio->_deferred_list, &ds_queue->split_queue);
ds_queue->split_queue_len++;
#ifdef CONFIG_MEMCG

> I am trying to rebase my NR_SPLIT_DEFERRED counter on top of your
> work, but this "clear" makes that job quite tricky. as I am not sure
> if this patch
> is going to clear the partially_mapped flag for folios on deferred_list.
>
> Otherwise, I can simply do the below whenever folio is leaving deferred_list
>
> ds_queue->split_queue_len--;
> list_del_init(&folio->_deferred_list);
> if (folio_test_clear_partially_mapped(folio))
> mod_mthp_stat(folio_order(folio),
> MTHP_STAT_NR_SPLIT_DEFERRED, -1);
>