linux-kernel - Re: [External] Re: [PATCH v2] fs/proc/kcore.c: add mmap interface

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Message-ID: <be411abf-1794-521a-8c79-0a3cbea4d3bd@bytedance.com>
Date:   Wed, 2 Jun 2021 10:31:44 +0800
From:   zhoufeng <zhoufeng.zf@...edance.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     adobriyan@...il.com, rppt@...nel.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, songmuchun@...edance.com,
        zhouchengming@...edance.com, chenying.kernel@...edance.com,
        zhengqi.arch@...edance.com
Subject: Re: [External] Re: [PATCH v2] fs/proc/kcore.c: add mmap interface



在 2021/6/2 上午10:22, Andrew Morton 写道:
> On Tue,  1 Jun 2021 16:22:41 +0800 Feng zhou <zhoufeng.zf@...edance.com> wrote:
> 
>> From: ZHOUFENG <zhoufeng.zf@...edance.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;
> 

   Thank you very much.