Re: [PATCH 2/3] hugetlbfs: close race between MADV_DONTNEED and page fault

[Rik van Riel <riel@surriel.com>]

On Wed, 2023-10-04 at 20:19 -0700, Mike Kravetz wrote:
> On 10/03/23 23:25, riel@surriel.com wrote:
> > 
> > @@ -5457,11 +5460,12 @@ void __unmap_hugepage_range_final(struct
> > mmu_gather *tlb,
> >                  * someone else.
> >                  */
> >                 __hugetlb_vma_unlock_write_free(vma);
> > -               i_mmap_unlock_write(vma->vm_file->f_mapping);
> >         } else {
> > -               i_mmap_unlock_write(vma->vm_file->f_mapping);
> >                 hugetlb_vma_unlock_write(vma);
> >         }
> > +
> > +       if (vma->vm_file)
> > +               i_mmap_unlock_write(vma->vm_file->f_mapping);
> >  }
> 
> In the case of a mmap(hugetlbfs_file_mmap) error, the per-vma hugetlb
> lock will not be setup.  The hugetlb_vma_lock/unlock routines do not
> check for this as they were previously always called after the lock
> was
> set up.  So, we can now get:

Wait, the hugetlb_vma_(un)lock_{read,write} functions do
have checks for the presence of the lock:

void hugetlb_vma_lock_read(struct vm_area_struct *vma)
{
        if (__vma_shareable_lock(vma)) {
                struct hugetlb_vma_lock *vma_lock = vma-
>vm_private_data;

                down_read(&vma_lock->rw_sema);
        } else if (__vma_private_lock(vma)) {
                struct resv_map *resv_map = vma_resv_map(vma);

                down_read(&resv_map->rw_sema);
        }
}

Both __vma_shareable_lock and __vma_private_lock check that
vma->vm_private_data points at something.

Exactly what corner case am I missing here?

What leaves vma->vm_private_data pointing at something
invalid?

> 
> +++ b/mm/hugetlb.c
> @@ -5503,10 +5503,12 @@ void __unmap_hugepage_range(struct mmu_gather
> *tlb, struct vm_area_struct *vma,
>  void __hugetlb_zap_begin(struct vm_area_struct *vma,
>                          unsigned long *start, unsigned long *end)
>  {
> +       if (!vma->vm_file)      /* hugetlbfs_file_mmap error */
> +               return;
> +

This does not seem quite correct, because the locking is needed to
avoid the race between MADV_DONTNEED and the page fault path.


> Another way to resolve would be to fix up the hugetlb_vma_lock/unlock
> routines
> to check for and handle a null lock.

I thought I had that already. 

Does __vma_shareable_lock need to check for !vma->vm_file ?

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