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[2003:d8:2f2a:e200:f98f:8d71:83f:f88]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3ee106fd0edsm18355715f8f.53.2025.09.22.01.43.34 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 22 Sep 2025 01:43:35 -0700 (PDT) Message-ID: <40772b34-30c8-4f16-833c-34fdd7c69176@redhat.com> Date: Mon, 22 Sep 2025 10:43:34 +0200 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 3/4] mm: thp: use folio_batch to handle THP splitting in deferred_split_scan() To: Qi Zheng , hannes@cmpxchg.org, hughd@google.com, mhocko@suse.com, roman.gushchin@linux.dev, shakeel.butt@linux.dev, muchun.song@linux.dev, lorenzo.stoakes@oracle.com, ziy@nvidia.com, baolin.wang@linux.alibaba.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, lance.yang@linux.dev, akpm@linux-foundation.org Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, Muchun Song References: <3db5da29d767162a006a562963eb52df9ce45a51.1758253018.git.zhengqi.arch@bytedance.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 19.09.25 05:46, Qi Zheng wrote: > From: Muchun Song > > The maintenance of the folio->_deferred_list is intricate because it's > reused in a local list. > > Here are some peculiarities: > > 1) When a folio is removed from its split queue and added to a local > on-stack list in deferred_split_scan(), the ->split_queue_len isn't > updated, leading to an inconsistency between it and the actual > number of folios in the split queue. deferred_split_count() will now return "0" even though there might be concurrent scanning going on. I assume that's okay because we are not returning SHRINK_EMPTY (which is a difference). > > 2) When the folio is split via split_folio() later, it's removed from > the local list while holding the split queue lock. At this time, > this lock protects the local list, not the split queue. > > 3) To handle the race condition with a third-party freeing or migrating > the preceding folio, we must ensure there's always one safe (with > raised refcount) folio before by delaying its folio_put(). More > details can be found in commit e66f3185fa04 ("mm/thp: fix deferred > split queue not partially_mapped"). It's rather tricky. > > We can use the folio_batch infrastructure to handle this clearly. In this > case, ->split_queue_len will be consistent with the real number of folios > in the split queue. If list_empty(&folio->_deferred_list) returns false, > it's clear the folio must be in its split queue (not in a local list > anymore). > > In the future, we will reparent LRU folios during memcg offline to > eliminate dying memory cgroups, which requires reparenting the split queue > to its parent first. So this patch prepares for using > folio_split_queue_lock_irqsave() as the memcg may change then. > > Signed-off-by: Muchun Song > Signed-off-by: Qi Zheng > --- > mm/huge_memory.c | 88 +++++++++++++++++++++++------------------------- > 1 file changed, 42 insertions(+), 46 deletions(-) > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index d34516a22f5bb..ab16da21c94e0 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -3760,21 +3760,22 @@ static int __folio_split(struct folio *folio, unsigned int new_order, > struct lruvec *lruvec; > int expected_refs; > > - if (folio_order(folio) > 1 && > - !list_empty(&folio->_deferred_list)) { > - ds_queue->split_queue_len--; > + if (folio_order(folio) > 1) { > + if (!list_empty(&folio->_deferred_list)) { > + ds_queue->split_queue_len--; > + /* > + * Reinitialize page_deferred_list after removing the > + * page from the split_queue, otherwise a subsequent > + * split will see list corruption when checking the > + * page_deferred_list. > + */ > + list_del_init(&folio->_deferred_list); > + } > if (folio_test_partially_mapped(folio)) { > folio_clear_partially_mapped(folio); > mod_mthp_stat(folio_order(folio), > MTHP_STAT_NR_ANON_PARTIALLY_MAPPED, -1); > } > - /* > - * Reinitialize page_deferred_list after removing the > - * page from the split_queue, otherwise a subsequent > - * split will see list corruption when checking the > - * page_deferred_list. > - */ > - list_del_init(&folio->_deferred_list); > } BTW I am not sure about holding the split_queue_lock before freezing the refcount (comment above the freeze): freezing should properly sync against the folio_try_get(): one of them would fail. So not sure if that is still required. But I recall something nasty regarding that :) > split_queue_unlock(ds_queue); > if (mapping) { > @@ -4173,40 +4174,48 @@ static unsigned long deferred_split_scan(struct shrinker *shrink, > struct pglist_data *pgdata = NODE_DATA(sc->nid); > struct deferred_split *ds_queue = &pgdata->deferred_split_queue; > unsigned long flags; > - LIST_HEAD(list); > - struct folio *folio, *next, *prev = NULL; > - int split = 0, removed = 0; > + struct folio *folio, *next; > + int split = 0, i; > + struct folio_batch fbatch; > + bool done; Is "done" really required? Can't we just use sc->nr_to_scan tos ee if there is work remaining to be done so we retry? > > #ifdef CONFIG_MEMCG > if (sc->memcg) > ds_queue = &sc->memcg->deferred_split_queue; > #endif > > + folio_batch_init(&fbatch); > +retry: > + done = true; > spin_lock_irqsave(&ds_queue->split_queue_lock, flags); > /* Take pin on all head pages to avoid freeing them under us */ > list_for_each_entry_safe(folio, next, &ds_queue->split_queue, > _deferred_list) { > if (folio_try_get(folio)) { > - list_move(&folio->_deferred_list, &list); > - } else { > + folio_batch_add(&fbatch, folio); > + } else if (folio_test_partially_mapped(folio)) { > /* We lost race with folio_put() */ > - if (folio_test_partially_mapped(folio)) { > - folio_clear_partially_mapped(folio); > - mod_mthp_stat(folio_order(folio), > - MTHP_STAT_NR_ANON_PARTIALLY_MAPPED, -1); > - } > - list_del_init(&folio->_deferred_list); > - ds_queue->split_queue_len--; > + folio_clear_partially_mapped(folio); > + mod_mthp_stat(folio_order(folio), > + MTHP_STAT_NR_ANON_PARTIALLY_MAPPED, -1); > } > + list_del_init(&folio->_deferred_list); > + ds_queue->split_queue_len--; > if (!--sc->nr_to_scan) > break; > + if (folio_batch_space(&fbatch) == 0) { Nit: if (!folio_batch_space(&fbatch)) { -- Cheers David / dhildenb