linux-kernel - Re: [PATCH v2 3/4] mm: thp: use folio_batch to handle THP splitting in deferred_split

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <3ffda308-1210-4760-bac0-ba5b019c0e2a@redhat.com>
Date: Wed, 24 Sep 2025 14:31:57 +0200
From: David Hildenbrand <david@...hat.com>
To: Qi Zheng <zhengqi.arch@...edance.com>, hannes@...xchg.org,
 hughd@...gle.com, mhocko@...e.com, roman.gushchin@...ux.dev,
 shakeel.butt@...ux.dev, muchun.song@...ux.dev, lorenzo.stoakes@...cle.com,
 ziy@...dia.com, harry.yoo@...cle.com, baolin.wang@...ux.alibaba.com,
 Liam.Howlett@...cle.com, npache@...hat.com, ryan.roberts@....com,
 dev.jain@....com, baohua@...nel.org, lance.yang@...ux.dev,
 akpm@...ux-foundation.org
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 cgroups@...r.kernel.org, Muchun Song <songmuchun@...edance.com>
Subject: Re: [PATCH v2 3/4] mm: thp: use folio_batch to handle THP splitting
 in deferred_split_scan()

On 23.09.25 11:16, Qi Zheng wrote:
> From: Muchun Song <songmuchun@...edance.com>
> 
> 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 ("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 <songmuchun@...edance.com>
> Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
> ---

Nothing jumped at me

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers

David / dhildenb