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Message-ID: <CAGsJ_4yKuvMSazWABXqaeRr84hLEubET0nCUhPFYHQnfR4Tm8w@mail.gmail.com>
Date: Mon, 19 Aug 2024 20:29:36 +1200
From: Barry Song <baohua@...nel.org>
To: Usama Arif <usamaarif642@...il.com>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org, hannes@...xchg.org,
riel@...riel.com, shakeel.butt@...ux.dev, roman.gushchin@...ux.dev,
yuzhao@...gle.com, david@...hat.com, ryan.roberts@....com, rppt@...nel.org,
willy@...radead.org, cerasuolodomenico@...il.com, ryncsn@...il.com,
corbet@....net, linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
kernel-team@...a.com
Subject: Re: [PATCH v4 4/6] mm: Introduce a pageflag for partially mapped folios
Hi Usama,
I feel it is much better now! thanks!
On Mon, Aug 19, 2024 at 2:31 PM Usama Arif <usamaarif642@...il.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@...il.com>
> ---
> 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?
> }
> 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
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