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Wed, 27 Aug 2025 23:48:10 -0700 (PDT) MIME-Version: 1.0 References: <20250826071948.2618-1-laoar.shao@gmail.com> <20250826071948.2618-4-laoar.shao@gmail.com> <5fb8bd8d-cdd9-42e0-b62d-eb5a517a35c2@lucifer.local> In-Reply-To: <5fb8bd8d-cdd9-42e0-b62d-eb5a517a35c2@lucifer.local> From: Yafang Shao Date: Thu, 28 Aug 2025 14:47:34 +0800 X-Gm-Features: Ac12FXxFTwzSzvJhj5UIbCdO1z68biL2fU9bhU24okrE4ErExRxkOpwYSA6y-j0 Message-ID: Subject: Re: [PATCH v6 mm-new 03/10] mm: thp: add a new kfunc bpf_mm_get_task() To: Lorenzo Stoakes Cc: akpm@linux-foundation.org, david@redhat.com, ziy@nvidia.com, baolin.wang@linux.alibaba.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, hannes@cmpxchg.org, usamaarif642@gmail.com, gutierrez.asier@huawei-partners.com, willy@infradead.org, ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org, ameryhung@gmail.com, rientjes@google.com, corbet@lwn.net, bpf@vger.kernel.org, linux-mm@kvack.org, linux-doc@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 3F6008000E X-Stat-Signature: 9n6a6us1mg99gbcgqsrqrc7oerd9cz3w X-Rspam-User: X-HE-Tag: 1756363691-570147 X-HE-Meta: U2FsdGVkX1+aaG5/rqXomBZ2BCnr/7uBlUVqy3EWF7Fdtb7Q7qbdKa5Pgu1LHrEjP2PdTpNKLt+HgeKd4A3SzKTQDgRBGiApREBmHGkLa7AdZbZgR2QCVPP2MBK9lnOvdyYy83hjNeF4AJfk/m52BNHqp8J7m4zaWrPjFm5MdjIdn7n8gJBOvx/UOV1qTjJBTqLTShOFhWm5tBYJ4fM20RUZ4E2JXL2uG1UeBbe2Hea0Wht+PUzHyNL0QdU5PcQoBaqJqh2e10rdxvfJ9QaUx9anUIGX2T/Yjo/JCPiuSOTUNC3kwa5MTmPGunXHSPPhJXw7V0CZFTS11ojY2e/nV2q1qZey1QGV12myHMU++pNiCMFEPpAI5l3PcxhgoOBEyAzvk0uOVngRIY61TXfVh0RFHB8SwcWvYBFDyFgg/1fENEi5jxxVRfaGH++BahwrSeyXbxPt5lRtrYVhn9dcOY4RzhAB569KsIE6Srl1B27siOwaoTttnFW68SJNZhgHpTCrTwftTIr9O12sF00bntHMepMlfGBfVqnkKTWE84P8tth9bX4rSjDMYTKrABzJRCZbdffeeFRrBWJS7i2RxHYoZsXn61Kw8N3/Je1QkOwZ1lMrOmEfpGT7H844bLO0PtvMIP+DR82qoqpQy3X38xkyx4LH22t34RT42AyNFB0rMb811/++O/SNYvGlAaK0Du7UPKD3DpwDdg4JQBzGyWGMkQD8ewEP36ECk1G9muf1bvcf3fw16HkBN1RXjNQC6nov69eImM7PUAGDyQ+/taDvPg0yyC29jhEemFAH62RH6/hyLHR8o3zKcyatDLX+Qvo9HL+vvfnUz1R6YiXFPI7qqu1eRR25Q+epquZZigAbo2qGtCvQYKVakKT56AVMvlnzWOv4kv1lEz2U2Ywtvb4ORNArVgNDWvekFInk4KR2dpMrXYRKRXz3A/8vS5o3joU7VCbo1NzYch2SUQm tOHBDXTC POKAaeIxfP4FwyjOWhQ+0go4b6lyleaTgge9TftkMym8xQcEih54YCOHzvKXsBp1EeBg/ok9o26VUiBeUfp46DoieHbv+Gr2ODCD3CvFLK/tUKgLJvGgD5QdifeAzVcyvmm/4AJy7LnGpJKtiCZRNHBeSZC2dRWfrlx+Pp2TC1dNM/HFD4/GdHdmPtrCXeEl91Rn7zRy4nmfSl/Guh6XFQ4xqR0MMF6VWZ02Zo6y/N7oa9nyDUgLjKP0n/ccUyPhTvI6vQ/rGkWKlHlaIQvAKt6jwZlMUwdQ7Wl429FDJYV5IouEvAAcLMqMXpTdxx9yGsvHyrZF+zehnS0rpxSIkQrqQx0/UUzHBrX3fgjLwGQvVSUl/1p3tjt6JlXq97uiOOQE0AApG699xt9xyskZKCoeeQx/1pRMwDIIrhWPn+HyEwowoITRuIAP1CNqIohOJRZ2C X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, Aug 27, 2025 at 11:42=E2=80=AFPM Lorenzo Stoakes wrote: > > On Tue, Aug 26, 2025 at 03:19:41PM +0800, Yafang Shao wrote: > > We will utilize this new kfunc bpf_mm_get_task() to retrieve the > > associated task_struct from the given @mm. The obtained task_struct mus= t > > be released by calling bpf_task_release() as a paired operation. > > You're basically describing the patch you're not saying why - yeah you're > getting a task struct from an mm (only if CONFIG_MEMCG which you don't > mention here), but not for what purpose you intend to use this? For example, we could retrieve task->comm or other attributes and make decisions based on that information. I=E2=80=99ll provide a clearer description in the next revision. > > > > > Signed-off-by: Yafang Shao > > --- > > mm/bpf_thp.c | 34 ++++++++++++++++++++++++++++++++++ > > 1 file changed, 34 insertions(+) > > > > diff --git a/mm/bpf_thp.c b/mm/bpf_thp.c > > index b757e8f425fd..46b3bc96359e 100644 > > --- a/mm/bpf_thp.c > > +++ b/mm/bpf_thp.c > > @@ -205,11 +205,45 @@ __bpf_kfunc void bpf_put_mem_cgroup(struct mem_cg= roup *memcg) > > #endif > > } > > > > +/** > > + * bpf_mm_get_task - Get the task struct associated with a mm_struct. > > + * @mm: The mm_struct to query > > + * > > + * The obtained task_struct must be released by calling bpf_task_relea= se(). > > Hmmm so now bpf programs can cause kernel bugs by keeping a reference aro= und? > > This feels extremely dodgy, I don't like this at all. > > I thought the whole point of BPF was that this kind of thing couldn't pos= sibly > happen? > > Or would this be a kernel bug? > > If a bpf program can lead to a refcount not being put, this is not > upstreamable surely? As explained by Andrii, the BPF verifier can protect it. > > > + * > > + * Return: The associated task_struct on success, or NULL on failure. = Note that > > + * this function depends on CONFIG_MEMCG being enabled - it will alway= s return > > + * NULL if CONFIG_MEMCG is not configured. > > + */ > > +__bpf_kfunc struct task_struct *bpf_mm_get_task(struct mm_struct *mm) > > +{ > > +#ifdef CONFIG_MEMCG > > + struct task_struct *task; > > + > > + if (!mm) > > + return NULL; > > + rcu_read_lock(); > > + task =3D rcu_dereference(mm->owner); > > > + if (!task) > > + goto out; > > + if (!refcount_inc_not_zero(&task->rcu_users)) > > + goto out; > > + > > + rcu_read_unlock(); > > + return task; > > + > > +out: > > + rcu_read_unlock(); > > +#endif > > This #ifdeffery is horrid, can we please just have separate functions ins= tead of > inside the one? Thanks. > > > + return NULL; > > So we can't tell the difference between this failling due to CONFIG_MEMCG > not being set (in which case it will _always_ fail) or we couldn't get a > task or we couldn't get a refcount on the task. > > Maybe this doesn't matter since perhaps we are only using this if > CONFIG_MEMCG but in that case why even expose this if !CONFIG_MEMCG? > As suggested by Andrii, I will remove this kfunc and mark mm->owner as BTF_TYPE_SAFE_TRUSTED_OR_NULL. Thanks for your comments. --=20 Regards Yafang