Re: [RFC PATCH v4 8/8] hugetlb: use new vma_lock for pmd sharing synchronization

Mike Kravetz
2022-07-29T17:41:46Z

On 07/29/22 09:41, Miaohe Lin wrote:
> On 2022/7/29 1:47, Mike Kravetz wrote:
> > On 07/28/22 14:51, Miaohe Lin wrote:
> snip
> >>
> >> Do we need to check &mm->mm_users == 0 here in case the address_space of corresponding process
> >> has exited? In this case, mmdrop will drop the last reference and free the skipped_mm. So we will
> >> use skipped_mm below after it's freed?
> >>
> >
> > Good point!
> > I think we need to wait to drop since we want to hold the read lock.
> > Will update.
> >
> >>> + vma = find_vma(skipped_mm, skipped_vm_start);
> >>> + if (!vma || vma->vm_file->f_mapping != mapping ||
> >>
> >> If skipped_vm_start is unmapped and remapped as a anon vma before we taking the mmap_read_lock,
> >> vma->vm_file will be NULL?
> >>
> >
> > IIUC, vma->vm_file will always be set even for an anon vma. The fault
> > code depends on this. See beginning of hugetlb_fault() where we
> > unconditionally do:
> >
> > mapping = vma->vm_file->f_mapping;
>
> What if vma is non-hugetlb anon vma?
>

Right. The first check after !vma should be for hugetlb as we do not
care if non-hugetlb vmas and want to quit. Something like,

vma = find_vma(skipped_mm, skipped_vm_start);
if (!vma || !is_vm_hugetlb_page(vma) ||

--
Mike Kravetz