From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-ed1-f50.google.com (mail-ed1-f50.google.com [209.85.208.50]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id A5E461C303D for ; Thu, 24 Oct 2024 10:20:17 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.208.50 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1729765220; cv=none; b=kReip/zXXE+W4RVezJ5wVFmsd1E8gNfWycf/ijXHnVo2cdFJoTS4yi2473NqTnGRCqFId+Za8dwFptJF8C2GnmV+t2xrqQtHikLNXAXc6GP+AeWKMASckTrZi3BD0eOXkFQsw/8M5qeix6wkUW/+QKiM52uyzjMRr4B9Cf0XSXo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1729765220; c=relaxed/simple; bh=hQC9wdKV4vZAHUPwSLHAY9OjOwouynxOks2ORhe7DW4=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=N65OusLTWtjrwdXV895/30WH02aD2lGNZMBZQgoPV6N4WMkzPxondzsd+efGEGz6879y4/WVP4KPrKIbI72w/YRe8+Ub+Yh8Z8pNC4EFQdcL3TJseToRTjkL5VGTl8+qEe2v38kYjeW8sTR9GL18SKNZd68SOhYWmDgPJHoEmAY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com; spf=pass smtp.mailfrom=gmail.com; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b=QlPGooEU; arc=none smtp.client-ip=209.85.208.50 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=gmail.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="QlPGooEU" Received: by mail-ed1-f50.google.com with SMTP id 4fb4d7f45d1cf-5c937b5169cso1185029a12.1 for ; Thu, 24 Oct 2024 03:20:17 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20230601; t=1729765216; x=1730370016; darn=vger.kernel.org; h=content-transfer-encoding:in-reply-to:from:content-language :references:cc:to:subject:user-agent:mime-version:date:message-id :from:to:cc:subject:date:message-id:reply-to; bh=iA8XsUv9ZiRb6klI05HB7Oq4fmXPGR/w8amvD8JEpMA=; b=QlPGooEU2fRIvIu7iDRiNNPlecd/ag/IO8AW1spYit5taZspGPB0YCYzdLG59o3RCh CY3XGYYnSD1rPjMITXuK3qmGw8dqv6lvhx3OaMh4b2wU4+YlqL815gxC7vJ+qDqo1G0m knVNx5NPjsIyDY4fp6Am32I8R01Zkhavsvwbe7M9crOClZIUsLPp0Wut0NoYbT2oEdg2 8wDG5O96mFO9x0UrU1fR/b5rjM+MzaO+mlFzPt4ZYdlTqv8UPb7v0S3AP9r4eLRz7CAU yaai4eIQ+srrXLOVxvkuyIgstINQy/V/5lV9Hc84QyfvG9RAV4sOlRsrO1yZOVdv4nfj mOGg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1729765216; x=1730370016; h=content-transfer-encoding:in-reply-to:from:content-language :references:cc:to:subject:user-agent:mime-version:date:message-id :x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=iA8XsUv9ZiRb6klI05HB7Oq4fmXPGR/w8amvD8JEpMA=; b=qWr4ZVg0Ar4+/GBVt1BsJOKdC18UL66OLGWQcvGCUXY1e+KC7dtZvBZgdAUK2WcTUq Rv6R9ANLg3tjm3WJA0PM3kbwcvcyV5R/j9FKycPKLi9NU6iC/p4lxx3L018cVxYkX9U/ zMN6r3d9dhzHkbyG7910bkkCRwIOaLWAUYyBTmir+BFc6gMO9OU/cMhoAIE5yvTIpr/r xy83lFt+CVyb+ZMhFG/AZG6PIEzYC3VlgS5SMKpqiYLgh4GoNO5SwANTlSWUrhq8ciqG UjkSferx5foU/QNfe9hWqDf4g86tNVojU9DWK0tXz+QoiWXjKz6yuzljVxL6LTAFPF8Y W1GA== X-Forwarded-Encrypted: i=1; AJvYcCVOkxMWbfnJoGKFQKloyIQvrRKy+3/564yGNAI/BFG/8ZggSJXlcJq52RWGp9x7PfqEkTYlLxxHHKfW4Zk=@vger.kernel.org X-Gm-Message-State: AOJu0Ywm522IE7cP8UJNCx2cJ+x/hKcLX6Z9XDec42nUycnaChr4gYFD JLsWp3VtOaI1pNr0W0qmDcjiqUMQo/ceB9Tm5ePeDAJh58BSK6YG X-Google-Smtp-Source: AGHT+IGUsj/cRGtaDvz+F0J9nNmkVXRZfqrOpZd7s6gYfKwD8/kx33LqFr/jokfnX0ukDrqnm18jLg== X-Received: by 2002:a05:6402:234b:b0:5ca:18c9:f382 with SMTP id 4fb4d7f45d1cf-5cba1fdb9f2mr1766787a12.3.1729765215369; Thu, 24 Oct 2024 03:20:15 -0700 (PDT) Received: from ?IPV6:2a03:83e0:1126:4:eb:d0d0:c7fd:c82c? ([2620:10d:c092:500::6:222b]) by smtp.gmail.com with ESMTPSA id 4fb4d7f45d1cf-5cb66c6b9d1sm5741285a12.70.2024.10.24.03.20.14 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 24 Oct 2024 03:20:14 -0700 (PDT) Message-ID: <82d32616-9135-4b30-9e91-b190a03bcb54@gmail.com> Date: Thu, 24 Oct 2024 11:20:14 +0100 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 hotfix 1/2] mm/thp: fix deferred split queue not partially_mapped To: Hugh Dickins , Andrew Morton Cc: Yang Shi , Wei Yang , "Kirill A. Shutemov" , Matthew Wilcox , David Hildenbrand , Johannes Weiner , Baolin Wang , Barry Song , Kefeng Wang , Ryan Roberts , Nhat Pham , Zi Yan , Chris Li , linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <760237a3-69d6-9197-432d-0306d52c048a@google.com> Content-Language: en-US From: Usama Arif In-Reply-To: <760237a3-69d6-9197-432d-0306d52c048a@google.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 24/10/2024 05:10, Hugh Dickins wrote: > Recent changes are putting more pressure on THP deferred split queues: > under load revealing long-standing races, causing list_del corruptions, > "Bad page state"s and worse (I keep BUGs in both of those, so usually > don't get to see how badly they end up without). The relevant recent > changes being 6.8's mTHP, 6.10's mTHP swapout, and 6.12's mTHP swapin, > improved swap allocation, and underused THP splitting. > > The new unlocked list_del_init() in deferred_split_scan() is buggy. > I gave bad advice, it looks plausible since that's a local on-stack > list, but the fact is that it can race with a third party freeing or > migrating the preceding folio (properly unqueueing it with refcount 0 > while holding split_queue_lock), thereby corrupting the list linkage. > > The obvious answer would be to take split_queue_lock there: but it has > a long history of contention, so I'm reluctant to add to that. Instead, > make sure that there is always one safe (raised refcount) folio before, > by delaying its folio_put(). (And of course I was wrong to suggest > updating split_queue_len without the lock: leave that until the splice.) > Thanks for this, I imagine this was quite tricky to debug. Avoiding taking the split_queue_lock by keeping reference of the preceding folio is really nice. And I should have spotted in the original patch that split_queue_len shouldn't be changed without holding split_queue_lock. Acked-by: Usama Arif > And remove two over-eager partially_mapped checks, restoring those tests > to how they were before: if uncharge_folio() or free_tail_page_prepare() > finds _deferred_list non-empty, it's in trouble whether or not that folio > is partially_mapped (and the flag was already cleared in the latter case). > > Fixes: dafff3f4c850 ("mm: split underused THPs") > Signed-off-by: Hugh Dickins > --- > mm/huge_memory.c | 21 +++++++++++++++++---- > mm/memcontrol.c | 3 +-- > mm/page_alloc.c | 5 ++--- > 3 files changed, 20 insertions(+), 9 deletions(-) > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index 2fb328880b50..a1d345f1680c 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -3718,8 +3718,8 @@ static unsigned long deferred_split_scan(struct shrinker *shrink, > struct deferred_split *ds_queue = &pgdata->deferred_split_queue; > unsigned long flags; > LIST_HEAD(list); > - struct folio *folio, *next; > - int split = 0; > + struct folio *folio, *next, *prev = NULL; > + int split = 0, removed = 0; > > #ifdef CONFIG_MEMCG > if (sc->memcg) > @@ -3775,15 +3775,28 @@ static unsigned long deferred_split_scan(struct shrinker *shrink, > */ > if (!did_split && !folio_test_partially_mapped(folio)) { > list_del_init(&folio->_deferred_list); > - ds_queue->split_queue_len--; > + 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); > } > - folio_put(folio); > + if (folio) > + folio_put(folio); > } > > 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); > + > /* > * Stop shrinker if we didn't split any page, but the queue is empty. > * This can happen if pages were freed under us. > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index 7845c64a2c57..2703227cce88 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -4631,8 +4631,7 @@ 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_test_partially_mapped(folio), folio); > + !list_empty(&folio->_deferred_list), folio); > > /* > * Nobody should be changing or seriously looking at > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index 8afab64814dc..4b21a368b4e2 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -961,9 +961,8 @@ 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) && > - folio_test_partially_mapped(folio))) { > - bad_page(page, "partially mapped folio on deferred list"); > + if (unlikely(!list_empty(&folio->_deferred_list))) { > + bad_page(page, "on deferred list"); > goto out; > } > break;