Re: [PATCH 6/8] hugetlb: add vma based lock for pmd sharing

Mike Kravetz
2022-09-07T20:52:18Z

On 08/29/22 15:24, Mike Kravetz wrote:
> On 08/27/22 17:30, Miaohe Lin wrote:
> > On 2022/8/25 1:57, Mike Kravetz wrote:
> > > Allocate a rw semaphore and hang off vm_private_data for
> > > synchronization use by vmas that could be involved in pmd sharing. Only
> > > add infrastructure for the new lock here. Actual use will be added in
> > > subsequent patch.
> > >
> > > Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
> >
> > <snip>
> >
> > > +static void hugetlb_vma_lock_free(struct vm_area_struct *vma)
> > > +{
> > > + /*
> > > + * Only present in sharable vmas. See comment in
> > > + * __unmap_hugepage_range_final about the neeed to check both
> >
> > s/neeed/need/
> >
> > > + * VM_SHARED and VM_MAYSHARE in free path
> >
> > I think there might be some wrong checks around this patch. As above comment said, we
> > need to check both flags, so we should do something like below instead?
> >
> > if (!(vma->vm_flags & (VM_MAYSHARE | VM_SHARED) == (VM_MAYSHARE | VM_SHARED)))
> >
> > > + */
>
> Thanks. I will update.
>
> > > + if (!vma || !(vma->vm_flags & (VM_MAYSHARE | VM_SHARED)))
> > > + return;

I think you misunderstood the comment which I admit was not very clear. And,
I misunderstood your suggestion. I believe the code is correct as it. Here
is the proposed update comment/code:

/*
* Only present in sharable vmas. See comment in
* __unmap_hugepage_range_final about how VM_SHARED could
* be set without VM_MAYSHARE. As a result, we need to
* check if either is set in the free path.
*/
if (!vma || !(vma->vm_flags & (VM_MAYSHARE | VM_SHARED)))
return;

Hopefully, that makes more sense.
--
Mike Kravetz