Re: [PATCH] fs: proc: use down_read_killable in proc_pid_cmdline_read()

[Alexey Dobriyan <adobriyan@gmail.com>]

On Fri, Feb 23, 2018 at 11:42:34AM -0800, Yang Shi wrote:
> 
> 
> 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.

Maybe.

But you need to explain why there is no backtrace several lines later:

	access_remote_vm
	__access_remote_vm
	down_read(&mm->mmap_sem)