Re: [PATCH] mm/hugetlb: fix null ptr defer in hugetlb_vma_lock_write

[Mike Kravetz]

On 11/02/23 23:15, Rik van Riel wrote:
> On Thu, 2023-11-02 at 19:37 -0700, Mike Kravetz wrote:
> > On 11/02/23 19:24, Mike Kravetz wrote:
> > That qualification '(with resv_map)' caught my attention originally,
> > and
> > I thought about it again while looking into this.  We now cover the
> > common
> > cases, but there are still quite a few cases where resv_map is NULL
> > for
> > private mappings.  In such cases, the race between MADV_DONTNEED and
> > page
> > fault still exists.  Is that a concern?
> 
> Honestly, I'm not sure. In hugetlb_dup_vma_private, which is
> called at fork time, we have this comment:
> 
>          * - For MAP_PRIVATE mappings, this is the reserve map which
> does
>          *   not apply to children.  Faults generated by the children
> are
>          *   not guaranteed to succeed, even if read-only.
> 
> That suggests we already have no guarantee of faults
> succeeding after fork.

Right!

> 
> > 
> > With a bit more work we 'could' make sure every hugetlb vma has a
> > lock
> > to participate in this scheme.
> > 
> > Any thhoughts?
> 
> We can certainly close the race between MADV_DONTNEED
> and page faults for MAP_PRIVATE mappings in child processes,
> but that does not guarantee that we actually have hugetlb
> pages for those processes.
> 
> In short, I'm not sure :)

I sort of remember something Dave Hansen added years ago to help a customer
allocating LOTs of hugetlb pages dynamically.  I seem to recall that this
was to get better numa locality.  As a result, they did not use reservations.

I guess it sticks with me because it was/is a real example of a customer
choosing NOT to use reservations.  

I don't have any evidence that this is common.  My thought is to leave
it as is until someone complains.
-- 
Mike Kravetz