Re: mm: mmap_sem lock assertion failure in __mlock_vma_pages_range

[Sasha Levin]

On 03/11/2014 04:07 PM, Davidlohr Bueso wrote:
> 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);

Yeah, this patch doesn't look like it's causing it, I guess it was more of a "you touched this
code last - do you still remember what's going on here?" :).

It's semi-odd because it seems like an obvious issue to hit with trinity but it's the first time
I've seen it and it's probably been there for a while (that BUG_ON is there from 2009).


Thanks,
Sasha
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