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SubjectRe: [PATCH v2] fs/proc/kcore.c: add mmap interface
On Tue,  1 Jun 2021 16:22:41 +0800 Feng zhou <zhoufeng.zf@bytedance.com> wrote:

> From: ZHOUFENG <zhoufeng.zf@bytedance.com>
>
> When we do the kernel monitor, use the DRGN
> (https://github.com/osandov/drgn) access to kernel data structures,
> found that the system calls a lot. DRGN is implemented by reading
> /proc/kcore. After looking at the kcore code, it is found that kcore
> does not implement mmap, resulting in frequent context switching
> triggered by read. Therefore, we want to add mmap interface to optimize
> performance. Since vmalloc and module areas will change with allocation
> and release, consistency cannot be guaranteed, so mmap interface only
> maps KCORE_TEXT and KCORE_RAM.
>
> ...
>
> +static int mmap_kcore(struct file *file, struct vm_area_struct *vma)
> +{
> + size_t size = vma->vm_end - vma->vm_start;
> + u64 start, pfn;
> + int nphdr;
> + size_t data_offset;
> + size_t phdrs_len, notes_len;
> + struct kcore_list *m = NULL;
> + int ret = 0;
> +
> + down_read(&kclist_lock);
> +
> + get_kcore_size(&nphdr, &phdrs_len, &notes_len, &data_offset);
> +
> + start = kc_offset_to_vaddr(((u64)vma->vm_pgoff << PAGE_SHIFT) -
> + ((data_offset >> PAGE_SHIFT) << PAGE_SHIFT));
> +
> + list_for_each_entry(m, &kclist_head, list) {
> + if (start >= m->addr && size <= m->size)
> + break;
> + }
> +
> + if (&m->list == &kclist_head) {
> + ret = -EINVAL;
> + goto out;
> + }
> +
> + if (vma->vm_flags & (VM_WRITE | VM_EXEC)) {
> + ret = -EPERM;
> + goto out;
> + }
> +
> + vma->vm_flags &= ~(VM_MAYWRITE | VM_MAYEXEC);
> + vma->vm_flags |= VM_MIXEDMAP;
> + vma->vm_ops = &kcore_mmap_ops;
> +
> + if (kern_addr_valid(start)) {
> + if (m->type == KCORE_RAM || m->type == KCORE_REMAP)

KCORE_REMAP was removed by
https://lkml.kernel.org/r/20210526093041.8800-2-david@redhat.com

I did this:

--- a/fs/proc/kcore.c~fs-proc-kcorec-add-mmap-interface-fix
+++ a/fs/proc/kcore.c
@@ -660,7 +660,7 @@ static int mmap_kcore(struct file *file,
vma->vm_ops = &kcore_mmap_ops;

if (kern_addr_valid(start)) {
- if (m->type == KCORE_RAM || m->type == KCORE_REMAP)
+ if (m->type == KCORE_RAM)
pfn = __pa(start) >> PAGE_SHIFT;
else if (m->type == KCORE_TEXT)
pfn = __pa_symbol(start) >> PAGE_SHIFT;
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