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[192.26.29.3]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-49b94dc0f57sm91854115e9.2.2026.08.28.14.07.08 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 28 Aug 2026 14:07:09 -0700 (PDT) Mime-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=UTF-8 Date: Fri, 28 Aug 2026 23:07:08 +0200 Message-Id: From: "Kumar Kartikeya Dwivedi" To: "Shakeel Butt" , "Hui Zhu" Cc: "Roman Gushchin" , "JP Kobryn" , "Andrew Morton" , "Andrii Nakryiko" , "Eduard Zingerman" , "Ihor Solodrai" , "Alexei Starovoitov" , "Daniel Borkmann" , "Martin KaFai Lau" , "Song Liu" , "Yonghong Song" , "Jiri Olsa" , "Emil Tsalapatis" , "Shuah Khan" , "Barry Song" , "Geliang Tang" , , , , , "Hui Zhu" Subject: Re: [PATCH bpf-next v5 1/2] mm/bpf: Add bpf_proactive_reclaim kfunc X-Mailer: aerc 0.21.0 References: <5dfdc7800469eac4e9a240f2422ba65d4ef4c4ba.1787826402.git.zhuhui@kylinos.cn> In-Reply-To: X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 346F614000C X-Stat-Signature: reu8abaziohjqzxpq117xezpduksz7r8 X-Rspam-User: X-HE-Tag: 1787951231-205278 X-HE-Meta: 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 +ny3Ztnb 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri Aug 28, 2026 at 9:53 PM CEST, Shakeel Butt wrote: > On Thu, Aug 27, 2026 at 06:36:29PM +0800, Hui Zhu wrote: >> From: Hui Zhu >> >> Add bpf_proactive_reclaim(), a sleepable kfunc which performs one >> proactive reclaim pass on a given memory cgroup, similar to a write >> to memory.reclaim but without retrying until the target is reached. >> >> The kfunc refuses to reclaim if the calling task is already in a >> reclaim context, as a nested reclaim would corrupt the outer reclaim >> state. >> >> Signed-off-by: Hui Zhu >> --- >> mm/bpf_memcontrol.c | 46 +++++++++++++++++++++++++++++++++++++++++++++ >> 1 file changed, 46 insertions(+) >> >> diff --git a/mm/bpf_memcontrol.c b/mm/bpf_memcontrol.c >> index 716df49d7647..297ff7f05042 100644 >> --- a/mm/bpf_memcontrol.c >> +++ b/mm/bpf_memcontrol.c >> @@ -6,6 +6,7 @@ >> */ >> >> #include >> +#include >> #include >> >> __bpf_kfunc_start_defs(); >> @@ -159,6 +160,49 @@ __bpf_kfunc void bpf_mem_cgroup_flush_stats(struct = mem_cgroup *memcg) >> mem_cgroup_flush_stats(memcg); >> } >> >> +/* >> + * Reclaim must not recurse: try_to_free_mem_cgroup_pages() overwrites >> + * current->reclaim_state, so a nested call would corrupt the outer >> + * reclaim state. Reclaim windows are marked with PF_MEMALLOC; >> + * reclaim_state is also checked because it is installed slightly >> + * before PF_MEMALLOC. >> + */ >> +static bool bpf_in_reclaim_context(void) >> +{ >> + return (current->flags & PF_MEMALLOC) || current->reclaim_state; >> +} >> + >> +/** >> + * bpf_proactive_reclaim - proactively reclaim memory from a memory >> + * cgroup >> + * @memcg: the target memory cgroup to reclaim from >> + * @size: the amount of memory to reclaim, in bytes >> + * >> + * Trigger one proactive reclaim pass on @memcg, similar to a write to >> + * memory.reclaim, but without retrying until @size is reached. >> + * Must not be called with a filesystem lock held: the reclaim path >> + * may deadlock on it via filesystem shrinkers. >> + * >> + * Return: The amount of memory reclaimed, in bytes, or 0 if @size is >> + * smaller than a page or the task is already in a reclaim context. >> + */ >> +__bpf_kfunc unsigned long bpf_proactive_reclaim(struct mem_cgroup *memc= g, >> + unsigned long size) >> +{ >> + unsigned long nr_reclaimed; >> + >> + if (size < PAGE_SIZE || unlikely(bpf_in_reclaim_context())) >> + return 0; > > I have been thinking about this more and more and after looking at the re= asoning > behind your check current->reclaim_state and also Sashiko's comment on NO= IO/NOFS > contexts, I am more convinced that this kfunc can not be a simple sleepab= le > function. We need more than that. We need clean process context as well s= imilar > to the userspace poking memory.reclaim. Something like kthread or workque= ue. > > Kumar & Andrii, is there a way to restrict a kfunc to only be called from > special BPF threads/workqueues? Is there some similar concept in BPF worl= d? > Yeah, I think the concern is valid. E.g. inode_rmdir() is sleepable but wil= l be problematic here, I think. My first instinct was if bpf_in_reclaim_context(= ) + nofs/noio save-restore might provide enough protection to let it be callabl= e from generic sleepable contexts, but I guess that will disable invocation o= f filesystem shrinkers unconditionally. So my suggestion would be to fix the context to BPF_PROG_TYPE_SYSCALL. Ther= e, we should be able to init and schedule timers which poll specific state and ar= ms wq execution etc. It then remains invocable from sleepable async contexts (wq, task_work) or the syscall program, all of which should be ok. Once BPF kthr= ead lands we can let it be callable from those threads as well, but that is for later. > [...]