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Wed, 24 Sep 2025 23:12:02 -0700 (PDT) Message-ID: <46da5d33-20d5-4b32-bca5-466474424178@bytedance.com> Date: Thu, 25 Sep 2025 14:11:49 +0800 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 v2 4/4] mm: thp: reparent the split queue during memcg offline To: David Hildenbrand , 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, 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 Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org References: <55370bda7b2df617033ac12116c1712144bb7591.1758618527.git.zhengqi.arch@bytedance.com> From: Qi Zheng In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Hi David, On 9/24/25 8:38 PM, David Hildenbrand wrote: > On 23.09.25 11:16, Qi Zheng wrote: >> 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. >> >> 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. >> >> Signed-off-by: Qi Zheng >> --- >>   include/linux/huge_mm.h |  2 ++ >>   include/linux/mmzone.h  |  1 + >>   mm/huge_memory.c        | 39 +++++++++++++++++++++++++++++++++++++++ >>   mm/memcontrol.c         |  1 + >>   mm/mm_init.c            |  1 + >>   5 files changed, 44 insertions(+) >> >> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h >> index f327d62fc9852..a0d4b751974d2 100644 >> --- a/include/linux/huge_mm.h >> +++ b/include/linux/huge_mm.h >> @@ -417,6 +417,7 @@ 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); >> +void reparent_deferred_split_queue(struct mem_cgroup *memcg); >>   void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd, >>           unsigned long address, bool freeze); >> @@ -611,6 +612,7 @@ 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/include/linux/mmzone.h b/include/linux/mmzone.h >> index 7fb7331c57250..f3eb81fee056a 100644 >> --- a/include/linux/mmzone.h >> +++ b/include/linux/mmzone.h >> @@ -1346,6 +1346,7 @@ struct deferred_split { >>       spinlock_t split_queue_lock; >>       struct list_head split_queue; >>       unsigned long split_queue_len; >> +    bool is_dying; > > It's a bit weird to query whether the "struct deferred_split" is dying. > Shouldn't this be a memcg property? (and in particular, not exist for There is indeed a CSS_DYING flag. But we must modify 'is_dying' under the protection of the split_queue_lock, otherwise the folio may be added back to the deferred_split of child memcg. > the pglist_data part where it might not make sense at all?). Maybe: #ifdef CONFIG_MEMCG bool is_dying; #endif > >>   }; >>   #endif >> diff --git a/mm/huge_memory.c b/mm/huge_memory.c >> index 48b51e6230a67..de7806f759cba 100644 >> --- a/mm/huge_memory.c >> +++ b/mm/huge_memory.c >> @@ -1094,9 +1094,15 @@ 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); >> +    if (unlikely(queue->is_dying == true)) { > > if (unlikely(queue->is_dying)) Will do. > >> +        spin_unlock(&queue->split_queue_lock); >> +        memcg = parent_mem_cgroup(memcg); >> +        goto retry; >> +    } >>       return queue; >>   } >> @@ -1108,9 +1114,15 @@ 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(queue->is_dying == true)) { > > if (unlikely(queue->is_dying)) Will do. > >> +        spin_unlock_irqrestore(&queue->split_queue_lock, *flags); >> +        memcg = parent_mem_cgroup(memcg); >> +        goto retry; >> +    } >>       return queue; >>   } > > Nothing else jumped at me, but I am not a memcg expert :) Thanks, Qi >