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([2620:10d:c091:500::7:e1ca]) by smtp.gmail.com with ESMTPSA id af79cd13be357-7a4ff0e5afesm435612785a.87.2024.08.19.07.17.45 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 19 Aug 2024 07:17:46 -0700 (PDT) Message-ID: Date: Mon, 19 Aug 2024 10:17:45 -0400 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 4/6] mm: Introduce a pageflag for partially mapped folios To: Barry Song Cc: akpm@linux-foundation.org, linux-mm@kvack.org, hannes@cmpxchg.org, riel@surriel.com, shakeel.butt@linux.dev, roman.gushchin@linux.dev, yuzhao@google.com, david@redhat.com, ryan.roberts@arm.com, rppt@kernel.org, willy@infradead.org, cerasuolodomenico@gmail.com, ryncsn@gmail.com, corbet@lwn.net, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, kernel-team@meta.com References: <20240819023145.2415299-1-usamaarif642@gmail.com> <20240819023145.2415299-5-usamaarif642@gmail.com> Content-Language: en-US From: Usama Arif In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 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 >> --- >> 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? >> } >> 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? > Its not, I did it because I wanted to make it explicit that the folio is being initialized, similar to how before this INIT_LIST_HEAD(&folio->_deferred_list) is done here. There is no functional issue in removing it here, because I believe the flag is initialized to false from start. >> + } >> } >> >> 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