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Wed, 30 Apr 2025 07:37:18 -0700 (PDT) Date: Wed, 30 Apr 2025 10:37:14 -0400 From: Johannes Weiner To: Muchun Song Cc: mhocko@kernel.org, roman.gushchin@linux.dev, shakeel.butt@linux.dev, muchun.song@linux.dev, akpm@linux-foundation.org, david@fromorbit.com, zhengqi.arch@bytedance.com, yosry.ahmed@linux.dev, nphamcs@gmail.com, chengming.zhou@linux.dev, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, linux-mm@kvack.org, hamzamahfooz@linux.microsoft.com, apais@linux.microsoft.com, Hugh Dickins Subject: Re: [PATCH RFC 07/28] mm: thp: use folio_batch to handle THP splitting in deferred_split_scan() Message-ID: <20250430143714.GA2020@cmpxchg.org> References: <20250415024532.26632-1-songmuchun@bytedance.com> <20250415024532.26632-8-songmuchun@bytedance.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250415024532.26632-8-songmuchun@bytedance.com> On Tue, Apr 15, 2025 at 10:45:11AM +0800, Muchun Song wrote: > 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. > > 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. 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 aim to reparent LRU folios during memcg offline to > eliminate dying memory cgroups. This patch prepares for using > folio_split_queue_lock_irqsave() as folio memcg may change then. > > Signed-off-by: Muchun Song This is a very nice simplification. And getting rid of the stack list and its subtle implication on all the various current and future list_empty(&folio->_deferred_list) checks should be much more robust. However, I think there is one snag related to this: > --- > mm/huge_memory.c | 69 +++++++++++++++++++++--------------------------- > 1 file changed, 30 insertions(+), 39 deletions(-) > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index 70820fa75c1f..d2bc943a40e8 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -4220,40 +4220,47 @@ 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; > > #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) { > + done = false; > + break; > + } > } > split_queue_unlock_irqrestore(ds_queue, flags); > > - list_for_each_entry_safe(folio, next, &list, _deferred_list) { > + for (i = 0; i < folio_batch_count(&fbatch); i++) { > bool did_split = false; > bool underused = false; > + struct deferred_split *fqueue; > > + folio = fbatch.folios[i]; > if (!folio_test_partially_mapped(folio)) { > underused = thp_underused(folio); > if (!underused) > @@ -4269,39 +4276,23 @@ static unsigned long deferred_split_scan(struct shrinker *shrink, > } > folio_unlock(folio); > next: > + if (did_split || !folio_test_partially_mapped(folio)) > + continue; There IS a list_empty() check in the splitting code that we actually relied on, for cleaning up the partially_mapped state and counter: !list_empty(&folio->_deferred_list)) { ds_queue->split_queue_len--; 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); With the folios isolated up front, it looks like you need to handle this from the shrinker. Otherwise this looks correct to me. But this code is subtle, I would feel much better if Hugh (CC-ed) could take a look as well. Thanks! > /* > - * split_folio() removes folio from list on success. > * Only add back to the queue if folio is partially mapped. > * If thp_underused returns false, or if split_folio fails > * in the case it was underused, then consider it used and > * don't add it back to split_queue. > */ > - if (did_split) { > - ; /* folio already removed from list */ > - } else if (!folio_test_partially_mapped(folio)) { > - list_del_init(&folio->_deferred_list); > - removed++; > - } else { > - /* > - * That unlocked list_del_init() above would be unsafe, > - * unless its folio is separated from any earlier folios > - * left on the list (which may be concurrently unqueued) > - * by one safe folio with refcount still raised. > - */ > - swap(folio, prev); > - } > - if (folio) > - folio_put(folio); > + fqueue = folio_split_queue_lock_irqsave(folio, &flags); > + list_add_tail(&folio->_deferred_list, &fqueue->split_queue); > + fqueue->split_queue_len++; > + split_queue_unlock_irqrestore(fqueue, flags); > } > + folios_put(&fbatch); > > - spin_lock_irqsave(&ds_queue->split_queue_lock, flags); > - list_splice_tail(&list, &ds_queue->split_queue); > - ds_queue->split_queue_len -= removed; > - split_queue_unlock_irqrestore(ds_queue, flags); > - > - if (prev) > - folio_put(prev); > - > + if (!done) > + goto retry; > /* > * Stop shrinker if we didn't split any page, but the queue is empty. > * This can happen if pages were freed under us. > -- > 2.20.1