linux-kernel - Re: [PATCH] fs: proc: use down_read_killable in proc_pid_cmdline

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Message-ID: <e8cdbc7d-45ef-ab1f-6df0-1d8ef1d4f1eb@linux.alibaba.com>
Date:   Fri, 23 Feb 2018 11:42:34 -0800
From:   Yang Shi <yang.shi@...ux.alibaba.com>
To:     Alexey Dobriyan <adobriyan@...il.com>
Cc:     akpm@...ux-foundation.org, mingo@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs: proc: use down_read_killable in
 proc_pid_cmdline_read()



On 2/23/18 11:33 AM, Alexey Dobriyan wrote:
> On Wed, Feb 21, 2018 at 03:13:10PM -0800, Yang Shi wrote:
>
>>>>> 2) access_remote_vm() et al will do the same ->mmap_sem, and
>>>> Yes, it does. But, __access_remote_vm() is called by access_process_vm()
>>>> too, which is used by much more places, i.e. ptrace, so I was not sure
>>>> if it is preferred to convert to killable version. So, I leave it untouched.
>>> Yeah, but ->mmap_sem is taken 3 times per /proc/*/cmdline read
>>> and your scalability tests should trigger next backtrace right away.
>> Yes, however, I didn't run into it if mmap_sem is acquired earlier.
>>
>> How about defining a killable version, like
>> __access_remote_vm_killable() which use down_read_killable(), then the
>> killable version can be used by proc/*/cmdline? There might be other
>> users in the future.
> It would be a disaster as interfaces multiply.

Might be not that bad. Just need add:

__access_remote_vm_killable()
access_remote_vm_killable()

Then access_process_vm() keep using __access_remote_vm(). And, use 
access_remote_vm_killable() in fs/proc/base. And, it looks the 
access_process_vm() calls in get_cmdline() can be changed to 
access_remote_vm() (non killable version).

Yang