On 09/20/15 11:05, Linus Torvalds wrote:

On Sun, Sep 20, 2015 at 5:56 AM, Oleg Nesterov [off-list ref] wrote:

quoted

In this case the workqueue thread will block.

What workqueue thread?

    pagefault_out_of_memory ->
       out_of_memory ->
          oom_kill_process

as far as I can tell, this can be called by any task. Now, that
pagefault case should only happen when the page fault comes from user
space, but we also have

    __alloc_pages_slowpath ->
       __alloc_pages_may_oom ->
          out_of_memory ->
             oom_kill_process

which can be called from just about any context (but atomic
allocations will never get here, so it can schedule etc).

I think in this case the oom killer should just slap a SIGKILL on the 
task and then back out, and whatever needed the memory should just wait 
patiently for the sacrificial lamb to commit seppuku.

Which, btw, we should IMO encourage ASAP in the context of the lamb by 
having anything potentially locky or semaphory pay attention to if the 
task in question has a fatal signal pending, and if so, drop everything 
and run like hell so that the task can cough up any locks or semaphores.

So what's your point? Explain again just how do you guarantee that you
can take the mmap_sem.

                        Linus

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Also, I observed that a task in the middle of dumping core doesn't 
respond to signals while it's dumping, and I would guess that might be 
the case even if the task receives a SIGKILL from the OOM handler.  Just 
a potential observation.

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