Re: [PATCH v3] mm/oom: do not oom reap task with an unresolved robust futex

[Michal Hocko]

On Wed 02-03-22 12:26:45, Nico Pache wrote:
> 
> 
> On 3/2/22 09:24, Michal Hocko wrote:
> > Sorry, this has slipped through cracks.
> > 
> > On Mon 14-02-22 15:39:31, Nico Pache wrote:
> > [...]
> >> We've recently been discussing the following if statement in __oom_reap_task_mm:
> >> 	if (vma_is_anonymous(vma) || !(vma->vm_flags & VM_SHARED))
> >>
> >> Given the comment above it, and some of the upstream discussion the original
> >> RFC, we are struggling to see why this should be a `||` and not an `&&`. If we
> >> only want to reap anon memory and reaping shared memory can be dangerous is this
> >> statement incorrect?
> >>
> >> We have a patch queued up to make this change, but wanted to get your opinion on
> >> why this was originally designed this way in case we are missing something.
> > 
> > I do not really see why this would be wrong. Private file backed
> > mappings can contain a reapable memory as well. I do not see how this
> > would solve the futex issue.
> We were basing our discussion around the following comment:
> /*
>  * Only anonymous pages have a good chance to be dropped
>  * without additional steps which we cannot afford as we
>  * are OOM already.
>  *
>  * We do not even care about fs backed pages because all
>  * which are reclaimable have already been reclaimed and
>  * we do not want to block exit_mmap by keeping mm ref
>  * count elevated without a good reason.
>  */
> 
> So changing to an && would align the functionality with this comment by ignoring
> fs backed pages, and additionally it prevents shared mappings from being reaped.
> We have tested this change and found we can no longer reproduce the issue. In
> our case we allocate the mutex on a MAP_SHARED|MAP_ANONYMOUS mmap so the if-
> statement in question would no longer return true after the && change.
> 
> If it is the case that private fs backed pages matter perhaps we want something
> like this:
> 	if ((vma_is_anonymous(vma) && !(vma->vm_flags & VM_SHARED))
> 	||(!vma_is_anonymous(vma) && !(vma->vm_flags & VM_SHARED)))
> 
> or more simply:
> 	if(!(vma->vm_flags & VM_SHARED))
> 
> to exclude all VM_SHARED mappings.

I would have to think about that some more but I do not really see how
this is related to the futex issue. In other words what kind of problem
does this solve?

-- 
Michal Hocko
SUSE Labs