Re: mm: mmap_sem lock assertion failure in __mlock_vma_pages_range

[Davidlohr Bueso]

On Tue, 2014-03-11 at 15:39 -0400, Sasha Levin wrote:
> Hi all,
> 
> I've ended up deleting the log file by mistake, but this bug does seem to be important
> so I'd rather not wait before the same issue is triggered again.
> 
> The call chain is:
> 
> 	mlock (mm/mlock.c:745)
> 		__mm_populate (mm/mlock.c:700)
> 			__mlock_vma_pages_range (mm/mlock.c:229)
> 				VM_BUG_ON(!rwsem_is_locked(&mm->mmap_sem));

So __mm_populate() is only called by mlock(2) and this VM_BUG_ON seems
wrong as we call it without the lock held:

	up_write(&current->mm->mmap_sem);
	if (!error)
		error = __mm_populate(start, len, 0);
	return error;
}

> 
> It seems to be a rather simple trace triggered from userspace. The only recent patch
> in the area (that I've noticed) was "mm/mlock: prepare params outside critical region".
> I've reverted it and trying to testing without it.

Odd, this patch should definitely *not* cause this. In any case every
operation removed from the critical region is local to the function:

	lock_limit = rlimit(RLIMIT_MEMLOCK);
	lock_limit >>= PAGE_SHIFT;
	locked = len >> PAGE_SHIFT;

	down_write(&current->mm->mmap_sem);

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