From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-174.mta1.migadu.com (out-174.mta1.migadu.com [95.215.58.174]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id D9DB8145B3E for ; Mon, 13 Oct 2025 07:28:51 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.174 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1760340533; cv=none; b=UZ1dOxC7n4s4V7Ml4EuVoZly+IWlq57ZYvVpDhBOMQxbvxV8+ED8QQHrpNpQkLMLj5TXDsibh3XCKyehQg/WyZbt1KYTkYoKjtY52xUiFLvbAMoknZjgEdSoOTawPduttJNUdcNZ/XV6DqUGZvA8Kdxf0ZsNLllUaHnagm3aIwU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1760340533; c=relaxed/simple; bh=m/1YodZeSj5G3x4jAf6N3F7pdgV0hahLXHdLENXg8Xg=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Ko/rfeJ/3Tvf0ulTk8twZChYhtVrlYbu5wBeelIxbGHN0nuC5Aklpko4T7LJxRV6u8vfUaYlNozHa0JAs3ru9VnB/Wr6+wOCDTjogSkMOxlt7XEmuI3Avvmcn4EdM3NCByDiV8A1MNrZsgCJWRbd3Enz7VIyz7V1OCdgi+m/kwE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=O1Zjb7eQ; arc=none smtp.client-ip=95.215.58.174 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="O1Zjb7eQ" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1760340519; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=IkLKvSUaQ4BnGmUhizr6OnCKnUnuAC5ZuO6XhuLGhRQ=; b=O1Zjb7eQu5+hY64dyZyuiEp94+2tjMc+TNk3zePZG8N6CXTZ9bs0B6nz5bcpd/RRprCWyi k2CWFu4wddIDJRoOQUQZIyP/lCul2iotxNRRoGS2Rd2lTtDFfvercH+HFzJQ/FAxPIEwCq T2orOfnrwXuArjHFGynllIFgiY7dmmU= Date: Mon, 13 Oct 2025 15:28:28 +0800 Precedence: bulk X-Mailing-List: cgroups@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH v4 3/4] mm: thp: use folio_batch to handle THP splitting in deferred_split_scan() To: Shakeel Butt Cc: hannes@cmpxchg.org, hughd@google.com, mhocko@suse.com, roman.gushchin@linux.dev, muchun.song@linux.dev, david@redhat.com, lorenzo.stoakes@oracle.com, ziy@nvidia.com, harry.yoo@oracle.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, linux-mm@kvack.org, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, Muchun Song , Qi Zheng References: <304df1ad1e8180e102c4d6931733bcc77774eb9e.1759510072.git.zhengqi.arch@bytedance.com> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Qi Zheng In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT Hi Shakeel, On 10/7/25 7:16 AM, Shakeel Butt wrote: > On Sat, Oct 04, 2025 at 12:53:17AM +0800, 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. >> >> 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. > > I think the above text needs some massaging. Rather than saying lock > protects the local list, I think, it would be better to say that the > lock is not needed as it is not protecting anything. Make sense, will do. > >> >> 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 >> Reviewed-by: Zi Yan >> Acked-by: David Hildenbrand > > One nit below. > > Acked-by: Shakeel Butt Thanks! > >> --- >> mm/huge_memory.c | 85 ++++++++++++++++++++++-------------------------- >> 1 file changed, 39 insertions(+), 46 deletions(-) >> >> diff --git a/mm/huge_memory.c b/mm/huge_memory.c >> index 134666503440d..59ddebc9f3232 100644 >> --- a/mm/huge_memory.c >> +++ b/mm/huge_memory.c >> @@ -3782,21 +3782,22 @@ 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); >> } >> split_queue_unlock(ds_queue); >> if (mapping) { >> @@ -4185,35 +4186,40 @@ static unsigned long deferred_split_scan(struct shrinker *shrink, >> { >> struct deferred_split *ds_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; >> >> + folio_batch_init(&fbatch); >> + >> +retry: >> ds_queue = split_queue_lock_irqsave(sc->nid, sc->memcg, &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)) >> + 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)) { >> /* >> * See try_to_map_unused_to_zeropage(): we cannot >> @@ -4236,38 +4242,25 @@ static unsigned long deferred_split_scan(struct shrinker *shrink, >> } >> folio_unlock(folio); >> next: >> + if (did_split || !folio_test_partially_mapped(folio)) >> + continue; >> /* >> - * 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); >> + fqueue = folio_split_queue_lock_irqsave(folio, &flags); >> + if (list_empty(&folio->_deferred_list)) { >> + list_add_tail(&folio->_deferred_list, &fqueue->split_queue); >> + fqueue->split_queue_len++; >> } >> - if (folio) >> - folio_put(folio); >> + split_queue_unlock_irqrestore(fqueue, flags); > > Is it possible to move this lock/list_add/unlock code chunk out of loop > and before the folios_put(). I think it would be possible if you tag the > corresponding index or have a separate bool array. It is also reasonable > to claim that the contention of this lock is not a concern for now. Considering the code complexity, perhaps we could wait until contention on this lock becomes a problem? Thanks, Qi > >> } >> + 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; >> - spin_unlock_irqrestore(&ds_queue->split_queue_lock, flags); >> - >> - if (prev) >> - folio_put(prev); >> + if (sc->nr_to_scan) >> + goto retry; >> >> /* >> * Stop shrinker if we didn't split any page, but the queue is empty. >> -- >> 2.20.1 >>