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Wed, 27 Aug 2025 23:50:45 -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: From: Yafang Shao Date: Thu, 28 Aug 2025 14:50:08 +0800 X-Gm-Features: Ac12FXxzAaUxjJkSIKj9IhilFeKPZNkJYuhj1_W1tJSrtzY4m65m7APoD6Uecqk Message-ID: Subject: Re: [PATCH v6 mm-new 03/10] mm: thp: add a new kfunc bpf_mm_get_task() To: Andrii Nakryiko Cc: Lorenzo Stoakes , 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: E778840011 X-Stat-Signature: r34isoaz4xcjpse4zhz5xrod4i4tawsj X-Rspam-User: X-HE-Tag: 1756363846-14529 X-HE-Meta: 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 qmotMZ4C 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 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 Thu, Aug 28, 2025 at 5:50=E2=80=AFAM Andrii Nakryiko wrote: > > On Wed, Aug 27, 2025 at 8:48=E2=80=AFAM 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 m= ust > > > 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? > > > > > > > > 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_= cgroup *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_rel= ease(). > > > > Hmmm so now bpf programs can cause kernel bugs by keeping a reference a= round? > > BPF verifier will reject any program that cannot guarantee that > bpf_task_release() will always be called. So there shouldn't be any > problem here. Thanks for the clarification. > > > > > 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 p= ossibly > > 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? > > > > > + * > > > + * Return: The associated task_struct on success, or NULL on failure= . Note that > > > + * this function depends on CONFIG_MEMCG being enabled - it will alw= ays 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); > > Question to Yafang, though. Instead of adding new kfunc just for this, > have you tried marking mm->owner as BTF_TYPE_SAFE_TRUSTED_OR_NULL, > which, if I understand correctly, would allow BPF program to just work > with `mm->owner` (after checking for NULL) directly. And then you can > just use existing bpf_task_acquire() good suggestion. will change it. > > > > > > + if (!task) > > > + goto out; > > > + if (!refcount_inc_not_zero(&task->rcu_users)) > > > + goto out; > > nit: just call bpf_task_acquire(), which will more obviously pair with > suggested bpf_task_release()? makes sense. --=20 Regards Yafang