From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-184.mta0.migadu.com (out-184.mta0.migadu.com [91.218.175.184]) (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 836CD225397 for ; Mon, 29 Sep 2025 07:54:48 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.184 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1759132490; cv=none; b=kFSdJSCrOOS/s/z+kBp4K9hp8zbbKXlm5EI6X/pJ16sK62t5DDwQUNCgGLGDz3BVcRzMCAk9n3dlCRvL1VRMQcqpN/ZE2aGJz5cloqfMt4x38nr+YHR6fgwbRYe0Y5j4AzeMTzjcYwAtmax5O0Ox8J/2BVJ4x8zFE0OajM8Aia8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1759132490; c=relaxed/simple; bh=gLjPI+Ty4V5UonNuIad7q0vN67Gxc9NIE8ov/yfxSvQ=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=nGh166YIq7IZJF1N/DUfxwMsXNnjo/GfFbTNtpyQFzsVlwNhiovDuP3/SNkZzGZgOAw5O7saHALRKdl4jTItbR6cqxM3rUUlRUFyxALSEVeC13CCPE5f97pI/JheFwxgV4IQiW/8q5MdjrfThQMcOr5fZwNipxXQW5vfwRjAK7M= 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=Yx1Z/ysK; arc=none smtp.client-ip=91.218.175.184 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="Yx1Z/ysK" Message-ID: <4d13ffd1-25a5-44f7-9d7d-baa8bc576c04@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1759132476; 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=UVpToKISTa8VE/h8x/0KonSGNktaLtqCR9k5a/zcWWQ=; b=Yx1Z/ysKsp9m+j/fouRP9o5rGGuzMMG87cOpe8a9FM3eXbntfj1TuuB8NDst86aPvi9rID 6AekzqTQ/Klq9XgQFi/rCzRgv86y7EJrNzUzccesAzKIdf4NLY6NNoXQjxtjKsc+AG8Hjt QG+DtlJxBaBF7w72FO2t67L6JTJebrc= Date: Mon, 29 Sep 2025 15:54:26 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH v3 4/4] mm: thp: reparent the split queue during memcg offline To: Muchun Song Cc: hannes@cmpxchg.org, hughd@google.com, mhocko@suse.com, roman.gushchin@linux.dev, shakeel.butt@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 References: <2ddd0c184829e65c5b3afa34e93599783e7af3d4.1759056506.git.zhengqi.arch@bytedance.com> <2EC0CBCD-73FD-400A-921A-EAB45B21ACB8@linux.dev> <08a4f0b2-1735-4e3b-9f61-d55e45e8ec86@linux.dev> <1A84CFB1-FB4F-4630-A40C-73CDE7CA8C21@linux.dev> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Qi Zheng In-Reply-To: <1A84CFB1-FB4F-4630-A40C-73CDE7CA8C21@linux.dev> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT On 9/29/25 3:38 PM, Muchun Song wrote: > > >> On Sep 29, 2025, at 15:22, Qi Zheng wrote: >> >> >> >> On 9/29/25 2:20 PM, Muchun Song wrote: >>>> On Sep 28, 2025, at 19:45, Qi Zheng wrote: >>>> >>>> From: Qi Zheng >>>> >>>> Similar to list_lru, the split queue is relatively independent and does >>>> not need to be reparented along with objcg and LRU folios (holding >>>> objcg lock and lru lock). So let's apply the same mechanism as list_lru >>>> to reparent the split queue separately when memcg is offine. >>>> >>>> This is also a preparation for reparenting LRU folios. >>>> >>>> Signed-off-by: Qi Zheng >>>> --- >>>> include/linux/huge_mm.h | 4 ++++ >>>> mm/huge_memory.c | 46 +++++++++++++++++++++++++++++++++++++++++ >>>> mm/memcontrol.c | 1 + >>>> 3 files changed, 51 insertions(+) >>>> >>>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h >>>> index f327d62fc9852..0c211dcbb0ec1 100644 >>>> --- a/include/linux/huge_mm.h >>>> +++ b/include/linux/huge_mm.h >>>> @@ -417,6 +417,9 @@ static inline int split_huge_page(struct page *page) >>>> return split_huge_page_to_list_to_order(page, NULL, ret); >>>> } >>>> void deferred_split_folio(struct folio *folio, bool partially_mapped); >>>> +#ifdef CONFIG_MEMCG >>>> +void reparent_deferred_split_queue(struct mem_cgroup *memcg); >>>> +#endif >>>> >>>> void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd, >>>> unsigned long address, bool freeze); >>>> @@ -611,6 +614,7 @@ static inline int try_folio_split(struct folio *folio, struct page *page, >>>> } >>>> >>>> static inline void deferred_split_folio(struct folio *folio, bool partially_mapped) {} >>>> +static inline void reparent_deferred_split_queue(struct mem_cgroup *memcg) {} >>>> #define split_huge_pmd(__vma, __pmd, __address) \ >>>> do { } while (0) >>>> >>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c >>>> index bb32091e3133e..5fc0caca71de0 100644 >>>> --- a/mm/huge_memory.c >>>> +++ b/mm/huge_memory.c >>>> @@ -1094,9 +1094,22 @@ static struct deferred_split *folio_split_queue_lock(struct folio *folio) >>>> struct deferred_split *queue; >>>> >>>> memcg = folio_memcg(folio); >>>> +retry: >>>> queue = memcg ? &memcg->deferred_split_queue : >>>> &NODE_DATA(folio_nid(folio))->deferred_split_queue; >>>> spin_lock(&queue->split_queue_lock); >>>> + /* >>>> + * Notice: >>>> + * 1. The memcg could be NULL if cgroup_disable=memory is set. >>>> + * 2. There is a period between setting CSS_DYING and reparenting >>>> + * deferred split queue, and during this period the THPs in the >>>> + * deferred split queue will be hidden from the shrinker side. >> >> The shrinker side can find this deferred split queue by traversing >> memcgs, so we should check CSS_DYING after we acquire child >> split_queue_lock in : >> >> deferred_split_scan >> --> spin_lock_irqsave(&ds_queue->split_queue_lock, flags); >> if (css_is_dying(&memcg->css)) >> --> retry to get parent split_queue_lock >> >> So during this period, we use parent split_queue_lock to protect >> child deferred split queue. It's a little weird, but it's safe. >> >>>> + */ >>>> + if (unlikely(memcg && css_is_dying(&memcg->css))) { >>>> + spin_unlock(&queue->split_queue_lock); >>>> + memcg = parent_mem_cgroup(memcg); >>>> + goto retry; >>>> + } >>>> >>>> return queue; >>>> } >>>> @@ -1108,9 +1121,15 @@ folio_split_queue_lock_irqsave(struct folio *folio, unsigned long *flags) >>>> struct deferred_split *queue; >>>> >>>> memcg = folio_memcg(folio); >>>> +retry: >>>> queue = memcg ? &memcg->deferred_split_queue : >>>> &NODE_DATA(folio_nid(folio))->deferred_split_queue; >>>> spin_lock_irqsave(&queue->split_queue_lock, *flags); >>>> + if (unlikely(memcg && css_is_dying(&memcg->css))) { >>>> + spin_unlock_irqrestore(&queue->split_queue_lock, *flags); >>>> + memcg = parent_mem_cgroup(memcg); >>>> + goto retry; >>>> + } >>>> >>>> return queue; >>>> } >>>> @@ -4275,6 +4294,33 @@ static unsigned long deferred_split_scan(struct shrinker *shrink, >>>> return split; >>>> } >>>> >>>> +#ifdef CONFIG_MEMCG >>>> +void reparent_deferred_split_queue(struct mem_cgroup *memcg) >>>> +{ >>>> + struct mem_cgroup *parent = parent_mem_cgroup(memcg); >>>> + struct deferred_split *ds_queue = &memcg->deferred_split_queue; >>>> + struct deferred_split *parent_ds_queue = &parent->deferred_split_queue; >>>> + int nid; >>>> + >>>> + spin_lock_irq(&ds_queue->split_queue_lock); >>>> + spin_lock_nested(&parent_ds_queue->split_queue_lock, SINGLE_DEPTH_NESTING); >>>> + >>>> + if (!ds_queue->split_queue_len) >>>> + goto unlock; >>>> + >>>> + list_splice_tail_init(&ds_queue->split_queue, &parent_ds_queue->split_queue); >>>> + parent_ds_queue->split_queue_len += ds_queue->split_queue_len; >>>> + ds_queue->split_queue_len = 0; >>>> + >>>> + for_each_node(nid) >>>> + set_shrinker_bit(parent, nid, shrinker_id(deferred_split_shrinker)); >>>> + >>>> +unlock: >>>> + spin_unlock(&parent_ds_queue->split_queue_lock); >>>> + spin_unlock_irq(&ds_queue->split_queue_lock); >>>> +} >>>> +#endif >>>> + >>>> #ifdef CONFIG_DEBUG_FS >>>> static void split_huge_pages_all(void) >>>> { >>>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c >>>> index e090f29eb03bd..d03da72e7585d 100644 >>>> --- a/mm/memcontrol.c >>>> +++ b/mm/memcontrol.c >>>> @@ -3887,6 +3887,7 @@ static void mem_cgroup_css_offline(struct cgroup_subsys_state *css) >>>> zswap_memcg_offline_cleanup(memcg); >>>> >>>> memcg_offline_kmem(memcg); >>>> + reparent_deferred_split_queue(memcg); >>> Since the dying flag of a memcg is not set under split_queue_lock, >>> two threads holding different split_queue_locks (e.g., one for the >>> parent memcg and one for the child) can concurrently manipulate the >>> same split-queue list of a folio. I think we should take the same >> >> If we ensure that we will check CSS_DYING every time we take the >> split_queue_lock, then the lock protecting deferred split queue >> must be the same lock. >> >> To be more clear, consider the following case: >> >> CPU0 CPU1 CPU2 >> >> folio_split_queue_lock >> --> get child queue and lock >> >> set CSS_DYING >> >> deferred_split_scan >> unlock child queue lock >> --> acquire child queue lock >> ***WE SHOULD CHECK CSS_DYING HERE*** >> >> >> reparent spilt queue >> >> The deferred_split_scan() is problematic now, I will fix it as follow: >> >> diff --git a/mm/huge_memory.c b/mm/huge_memory.c >> index 5fc0caca71de0..9f1f61e7e0c8e 100644 >> --- a/mm/huge_memory.c >> +++ b/mm/huge_memory.c >> @@ -4208,6 +4208,7 @@ static unsigned long deferred_split_scan(struct shrinker *shrink, >> struct folio *folio, *next; >> int split = 0, i; >> struct folio_batch fbatch; >> + struct mem_cgroup *memcg; >> >> #ifdef CONFIG_MEMCG >> if (sc->memcg) >> @@ -4217,6 +4218,11 @@ static unsigned long deferred_split_scan(struct shrinker *shrink, >> folio_batch_init(&fbatch); >> retry: >> spin_lock_irqsave(&ds_queue->split_queue_lock, flags); >> + if (sc->memcg && css_is_dying(&sc->memcg->css)) { > > There are more than one place where we check whether a memcg is dying, > it is better to introduce a helper like mem_cgroup_is_dying to do this > in memcontrol.h. OK. I will try to add a cleanup patch to do this. > >> + spin_unlock_irqrestore(&ds_queue->split_queue_lock, flags); > > Yes, we could fix this like this way. But I suggest we introduce another > helper like folio_split_queue_lock to do the similar retry logic. Every users > of split_queue_lock are supposed to use this new helper or folio_split_queue_lock > to get the lock. Yes, will do. > >> + memcg = parent_mem_cgroup(sc->memcg); >> + spin_lock_irqsave(&memcg->deferred_split_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) { >> >> Of course I'll add helper functions and do some cleanup. > > Yes. > >> >> Thanks, >> Qi >> >> >>> solution like list_lru does to fix this. >>> Muchun, >>> Thanks. >>>> reparent_shrinker_deferred(memcg); >>>> wb_memcg_offline(memcg); >>>> lru_gen_offline_memcg(memcg); >>>> -- >>>> 2.20.1 > >