On Mon, Aug 19, 2024 at 8:29 PM Barry Song <baohua@xxxxxxxxxx> 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);
sorry for typo,
if (folio_test_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?
>
> > }
> > spin_unlock_irqrestore(&ds_queue->split_queue_lock, flags);
> > }
> >
> > -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
> > @@ -3486,14 +3489,19 @@ 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) {
> > + if (!folio_test_partially_mapped(folio)) {
> > + __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 cannot become non-partially mapped */
> > + VM_WARN_ON_FOLIO(folio_test_partially_mapped(folio), 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);
> > list_add_tail(&folio->_deferred_list, &ds_queue->split_queue);
> > ds_queue->split_queue_len++;
> > #ifdef CONFIG_MEMCG
> > @@ -3542,6 +3550,7 @@ 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);
>
> as above? Do we also need if(test) for split_huge_page_to_list_to_order()?
>
> > ds_queue->split_queue_len--;
> > }
> > if (!--sc->nr_to_scan)
> > diff --git a/mm/internal.h b/mm/internal.h
> > index 52f7fc4e8ac3..27cbb5365841 100644
> > --- a/mm/internal.h
> > +++ b/mm/internal.h
> > @@ -662,8 +662,10 @@ static inline void prep_compound_head(struct page *page, unsigned int order)
> > atomic_set(&folio->_entire_mapcount, -1);
> > atomic_set(&folio->_nr_pages_mapped, 0);
> > atomic_set(&folio->_pincount, 0);
> > - if (order > 1)
> > + if (order > 1) {
> > INIT_LIST_HEAD(&folio->_deferred_list);
> > + __folio_clear_partially_mapped(folio);
>
> if partially_mapped is true for a new folio, does it mean we already have
> a bug somewhere?
>
> How is it possible for a new folio to be partially mapped?
>
> > + }
> > }
> >
> > static inline void prep_compound_tail(struct page *head, int tail_idx)
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index e1ffd2950393..0fd95daecf9a 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -4669,7 +4669,8 @@ static void uncharge_folio(struct folio *folio, struct uncharge_gather *ug)
> > VM_BUG_ON_FOLIO(folio_test_lru(folio), folio);
> > VM_BUG_ON_FOLIO(folio_order(folio) > 1 &&
> > !folio_test_hugetlb(folio) &&
> > - !list_empty(&folio->_deferred_list), folio);
> > + !list_empty(&folio->_deferred_list) &&
> > + folio_test_partially_mapped(folio), folio);
> >
> > /*
> > * Nobody should be changing or seriously looking at
> > diff --git a/mm/migrate.c b/mm/migrate.c
> > index 2d2e65d69427..ef4a732f22b1 100644
> > --- a/mm/migrate.c
> > +++ b/mm/migrate.c
> > @@ -1735,7 +1735,8 @@ static int migrate_pages_batch(struct list_head *from,
> > * use _deferred_list.
> > */
> > if (nr_pages > 2 &&
> > - !list_empty(&folio->_deferred_list)) {
> > + !list_empty(&folio->_deferred_list) &&
> > + folio_test_partially_mapped(folio)) {
> > if (!try_split_folio(folio, split_folios, mode)) {
> > nr_failed++;
> > stats->nr_thp_failed += is_thp;
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 408ef3d25cf5..a145c550dd2a 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -957,8 +957,9 @@ static int free_tail_page_prepare(struct page *head_page, struct page *page)
> > break;
> > case 2:
> > /* the second tail page: deferred_list overlaps ->mapping */
> > - if (unlikely(!list_empty(&folio->_deferred_list))) {
> > - bad_page(page, "on deferred list");
> > + if (unlikely(!list_empty(&folio->_deferred_list) &&
> > + folio_test_partially_mapped(folio))) {
> > + bad_page(page, "partially mapped folio on deferred list");
> > goto out;
> > }
> > break;
> > diff --git a/mm/rmap.c b/mm/rmap.c
> > index a6b9cd0b2b18..4c330635aa4e 100644
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
> > @@ -1578,8 +1578,9 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
> > * Check partially_mapped first to ensure it is a large folio.
> > */
> > if (partially_mapped && folio_test_anon(folio) &&
> > - list_empty(&folio->_deferred_list))
> > - deferred_split_folio(folio);
> > + !folio_test_partially_mapped(folio))
> > + deferred_split_folio(folio, true);
> > +
> > __folio_mod_stat(folio, -nr, -nr_pmdmapped);
> >
> > /*
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 25e43bb3b574..25f4e8403f41 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -1233,7 +1233,8 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
> > * Split partially mapped folios right away.
> > * We can free the unmapped pages without IO.
> > */
> > - if (data_race(!list_empty(&folio->_deferred_list)) &&
> > + if (data_race(!list_empty(&folio->_deferred_list) &&
> > + folio_test_partially_mapped(folio)) &&
> > split_folio_to_list(folio, folio_list))
> > goto activate_locked;
> > }
> > --
> > 2.43.5
> >
>
> Thanks
> Barry
