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([2620:10d:c091:500::4:4b1f]) by smtp.gmail.com with ESMTPSA id d75a77b69052e-4536a07667fsm51564551cf.93.2024.08.20.12.35.15 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 20 Aug 2024 12:35:15 -0700 (PDT) Message-ID: <953d398d-58be-41c6-bf30-4c9df597de77@gmail.com> Date: Tue, 20 Aug 2024 15:35:14 -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 , akpm@linux-foundation.org Cc: 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> <9a58e794-2156-4a9f-a383-1cdfc07eee5e@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 22:55, Barry Song wrote: > On Tue, Aug 20, 2024 at 9:34 AM Barry Song wrote: >> >> On Tue, Aug 20, 2024 at 8:16 AM Usama Arif wrote: >>> >>> >>> >>> On 19/08/2024 20:00, Barry Song wrote: >>>> On Tue, Aug 20, 2024 at 2:17 AM Usama Arif wrote: >>>>> >>>>> >>>>> >>>>> 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? >>>> >>>> In the pmd-order case, the majority of folios are not partially mapped. >>>> Unconditional writes will trigger cache synchronization across all >>>> CPUs (related to the MESI protocol), making them more costly. By >>>> using conditional writes, such as "if(test) write," we can avoid >>>> most unnecessary writes, which is much more efficient. Additionally, >>>> we only need to manage nr_split_deferred when the condition >>>> is met. We are carefully evaluating all scenarios to determine >>>> if modifications to the partially_mapped flag are necessary. >>>> >>> >>> >>> Hmm okay, as you said its needed for nr_split_deferred anyways. Something like below is ok to fold in? >>> >>> commit 4ae9e2067346effd902b342296987b97dee29018 (HEAD) >>> Author: Usama Arif >>> Date: Mon Aug 19 21:07:16 2024 +0100 >>> >>> mm: Introduce a pageflag for partially mapped folios fix >>> >>> Test partially_mapped flag before clearing it. This should >>> avoid unnecessary writes and will be needed in the nr_split_deferred >>> series. >>> >>> Signed-off-by: Usama Arif >>> >>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c >>> index 5d67d3b3c1b2..ccde60aaaa0f 100644 >>> --- a/mm/huge_memory.c >>> +++ b/mm/huge_memory.c >>> @@ -3479,7 +3479,8 @@ 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); >>> + if (folio_test_partially_mapped(folio)) >>> + __folio_clear_partially_mapped(folio); >>> } >>> spin_unlock_irqrestore(&ds_queue->split_queue_lock, flags); >>> } >>> @@ -3610,7 +3611,8 @@ 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); >>> + if (folio_test_partially_mapped(folio)) >>> + __folio_clear_partially_mapped(folio); >>> ds_queue->split_queue_len--; >>> } >>> if (!--sc->nr_to_scan) >>> >> >> Do we also need if (folio_test_partially_mapped(folio)) in >> split_huge_page_to_list_to_order()? >> >> I recall that in Yu Zhao's TAO, there’s a chance of splitting (shattering) >> non-partially-mapped folios. To be future-proof, we might want to handle >> both cases equally. > > we recall we also have a real case which can split entirely_mapped > folio: > > mm: huge_memory: enable debugfs to split huge pages to any order > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=fc4d182316bd5309b4066fd9ef21529ea397a7d4 > >> >> By the way, we might not need to clear the flag for a new folio. This differs >> from the init_list, which is necessary. If a new folio has the partially_mapped >> flag, it indicates that we failed to clear it when freeing the folio to >> the buddy system, which is a bug we need to fix in the free path. >> >> Thanks >> Barry I believe the below fixlet should address all concerns: >From 95492a51b1929ea274b4e5b78fc74e7736645d58 Mon Sep 17 00:00:00 2001 From: Usama Arif Date: Mon, 19 Aug 2024 21:07:16 +0100 Subject: [PATCH] mm: Introduce a pageflag for partially mapped folios fix Test partially_mapped flag before clearing it. This should avoid unnecessary writes and will be needed in the nr_split_deferred series. Also no need to clear partially_mapped prepping compound head, as it should start with already being cleared. Signed-off-by: Usama Arif --- include/linux/page-flags.h | 2 +- mm/huge_memory.c | 9 ++++++--- mm/internal.h | 4 +--- 3 files changed, 8 insertions(+), 7 deletions(-) diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h index c3bb0e0da581..f1602695daf2 100644 --- a/include/linux/page-flags.h +++ b/include/linux/page-flags.h @@ -1182,7 +1182,7 @@ static __always_inline void __ClearPageAnonExclusive(struct page *page) */ #define PAGE_FLAGS_SECOND \ (0xffUL /* order */ | 1UL << PG_has_hwpoisoned | \ - 1UL << PG_large_rmappable) + 1UL << PG_large_rmappable | 1UL << PG_partially_mapped) #define PAGE_FLAGS_PRIVATE \ (1UL << PG_private | 1UL << PG_private_2) diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 5d67d3b3c1b2..402b9d933de0 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -3422,7 +3422,8 @@ 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); + if (folio_test_partially_mapped(folio)) + __folio_clear_partially_mapped(folio); } spin_unlock(&ds_queue->split_queue_lock); if (mapping) { @@ -3479,7 +3480,8 @@ 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); + if (folio_test_partially_mapped(folio)) + __folio_clear_partially_mapped(folio); } spin_unlock_irqrestore(&ds_queue->split_queue_lock, flags); } @@ -3610,7 +3612,8 @@ 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); + if (folio_test_partially_mapped(folio)) + __folio_clear_partially_mapped(folio); ds_queue->split_queue_len--; } if (!--sc->nr_to_scan) diff --git a/mm/internal.h b/mm/internal.h index 27cbb5365841..52f7fc4e8ac3 100644 --- a/mm/internal.h +++ b/mm/internal.h @@ -662,10 +662,8 @@ 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); - } } static inline void prep_compound_tail(struct page *head, int tail_idx) -- 2.43.5