On 09/19/15 15:24, Linus Torvalds wrote:

On Sat, Sep 19, 2015 at 8:03 AM, Oleg Nesterov [off-list ref] wrote:

quoted

+
+static void oom_unmap_func(struct work_struct *work)
+{
+       struct mm_struct *mm = xchg(&oom_unmap_mm, NULL);
+
+       if (!atomic_inc_not_zero(&mm->mm_users))
+               return;
+
+       // If this is not safe we can do use_mm() + unuse_mm()
+       down_read(&mm->mmap_sem);

I don't think this is safe.

What makes you sure that we might not deadlock on the mmap_sem here?
For all we know, the process that is going out of memory is in the
middle of a mmap(), and already holds the mmap_sem for writing. No?

Potentially stupid question that others may be asking: Is it legal to 
return EINTR from mmap() to let a SIGKILL from the OOM handler punch the 
task out of the kernel and back to userspace?

(sorry for the dupe btw, new email client snuck in html and I got bounced)

So at the very least that needs to be a trylock, I think. And I'm not
sure zap_page_range() is ok with the mmap_sem only held for reading.
Normally our rule is that you can *populate* the page tables
concurrently, but you can't tear the down.

                 Linus

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