On 2/23/18 11:45 AM, Alexey Dobriyan wrote:
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.

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


I think it might be because:

        CPU A                                  CPU B
                                         read /proc/*/cmdline
acquire mmap_sem
munmap(300G)                 try to acquire mmap_sem --> go to sleep
release mmap_sem
                                          got mmap_sem
                                          release mmap_sem


The munmap might happen right before access_remote_vm(), but I just didn't run into it for the time being. It may be hit on another machine or with some changes to the test cases.

BTW, even the hung I met happened occassionally, not very often. So, the access_remote_vm() hit sounds less often. But, I agree it is still possible in theory.


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