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Mon, 8 Feb 2021 04:30:33 +0000 Subject: Re: [PATCH v4 bpf-next 1/4] bpf: introduce task_vma bpf_iter To: Song Liu , , , CC: , , , References: <20210204205002.4075937-1-songliubraving@fb.com> <20210204205002.4075937-2-songliubraving@fb.com> From: Yonghong Song Message-ID: Date: Sun, 7 Feb 2021 20:30:29 -0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.7.0 In-Reply-To: <20210204205002.4075937-2-songliubraving@fb.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [2620:10d:c090:400::5:ba14] X-ClientProxiedBy: MWHPR07CA0018.namprd07.prod.outlook.com (2603:10b6:300:116::28) To BYAPR15MB4088.namprd15.prod.outlook.com (2603:10b6:a02:c3::18) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from [IPv6:2620:10d:c085:21c1::1261] (2620:10d:c090:400::5:ba14) by MWHPR07CA0018.namprd07.prod.outlook.com (2603:10b6:300:116::28) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.3825.17 via Frontend Transport; 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It > can be used to print customized information similar to /proc//maps. > > Current /proc//maps and /proc//smaps provide information of > vma's of a process. However, these information are not flexible enough to > cover all use cases. For example, if a vma cover mixed 2MB pages and 4kB > pages (x86_64), there is no easy way to tell which address ranges are > backed by 2MB pages. task_vma solves the problem by enabling the user to > generate customize information based on the vma (and vma->vm_mm, > vma->vm_file, etc.). > > To access the vma safely in the BPF program, task_vma iterator holds > target mmap_lock while calling the BPF program. If the mmap_lock is > contended, task_vma unlocks mmap_lock between iterations to unblock the > writer(s). This lock contention avoidance mechanism is similar to the one > used in show_smaps_rollup(). > > Signed-off-by: Song Liu > --- > kernel/bpf/task_iter.c | 215 ++++++++++++++++++++++++++++++++++++++++- > 1 file changed, 214 insertions(+), 1 deletion(-) > > diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c > index 175b7b42bfc46..31e63b6c3d718 100644 > --- a/kernel/bpf/task_iter.c > +++ b/kernel/bpf/task_iter.c > @@ -286,9 +286,196 @@ static const struct seq_operations task_file_seq_ops = { > .show = task_file_seq_show, > }; > > +struct bpf_iter_seq_task_vma_info { > + /* The first field must be struct bpf_iter_seq_task_common. > + * this is assumed by {init, fini}_seq_pidns() callback functions. > + */ > + struct bpf_iter_seq_task_common common; > + struct task_struct *task; > + struct vm_area_struct *vma; > + u32 tid; > + unsigned long prev_vm_start; > + unsigned long prev_vm_end; > +}; > + > +enum bpf_task_vma_iter_find_op { > + task_vma_iter_first_vma, /* use mm->mmap */ > + task_vma_iter_next_vma, /* use curr_vma->vm_next */ > + task_vma_iter_find_vma, /* use find_vma() to find next vma */ > +}; > + > +static struct vm_area_struct * > +task_vma_seq_get_next(struct bpf_iter_seq_task_vma_info *info) > +{ > + struct pid_namespace *ns = info->common.ns; > + enum bpf_task_vma_iter_find_op op; > + struct vm_area_struct *curr_vma; > + struct task_struct *curr_task; > + u32 curr_tid = info->tid; > + > + /* If this function returns a non-NULL vma, it holds a reference to > + * the task_struct, and holds read lock on vma->mm->mmap_lock. > + * If this function returns NULL, it does not hold any reference or > + * lock. > + */ > + if (info->task) { > + curr_task = info->task; > + curr_vma = info->vma; > + /* In case of lock contention, drop mmap_lock to unblock > + * the writer. > + */ > + if (mmap_lock_is_contended(curr_task->mm)) { > + info->prev_vm_start = curr_vma->vm_start; > + info->prev_vm_end = curr_vma->vm_end; > + op = task_vma_iter_find_vma; > + mmap_read_unlock(curr_task->mm); > + if (mmap_read_lock_killable(curr_task->mm)) > + goto finish; > + } else { > + op = task_vma_iter_next_vma; > + } > + } else { > +again: > + curr_task = task_seq_get_next(ns, &curr_tid, true); > + if (!curr_task) { > + info->tid = curr_tid + 1; > + goto finish; > + } > + > + if (curr_tid != info->tid) { > + info->tid = curr_tid; > + op = task_vma_iter_first_vma; > + } else { > + op = task_vma_iter_find_vma; > + } > + > + if (!curr_task->mm) > + goto next_task; > + > + if (mmap_read_lock_killable(curr_task->mm)) > + goto finish; We hold a reference for curr_task here. Going to "finish" does not release the reference. > + } > + > + switch (op) { > + case task_vma_iter_first_vma: > + curr_vma = curr_task->mm->mmap; > + break; > + case task_vma_iter_next_vma: > + curr_vma = curr_vma->vm_next; > + break; > + case task_vma_iter_find_vma: > + /* We dropped mmap_lock so it is necessary to use find_vma > + * to find the next vma. This is similar to the mechanism > + * in show_smaps_rollup(). > + */ > + curr_vma = find_vma(curr_task->mm, info->prev_vm_end - 1); > + > + if (curr_vma && (curr_vma->vm_start == info->prev_vm_start)) > + curr_vma = curr_vma->vm_next; > + break; > + } > + if (!curr_vma) { > + mmap_read_unlock(curr_task->mm); > + goto next_task; > + } > + info->task = curr_task; > + info->vma = curr_vma; > + return curr_vma; > + > +next_task: > + put_task_struct(curr_task); > + info->task = NULL; > + curr_tid++; > + goto again; > + > +finish: > + info->task = NULL; > + info->vma = NULL; > + return NULL; > +} > + [...]