Re: [PATCH 7/9] mm, hugetlb: add VM_NORESERVE check in vma_has_reserves()

[Joonsoo Kim <iamjoonsoo.kim@lge.com>]

On Mon, Jul 15, 2013 at 08:41:12PM +0530, Aneesh Kumar K.V wrote:
> Joonsoo Kim <iamjoonsoo.kim@lge.com> writes:
> 
> > If we map the region with MAP_NORESERVE and MAP_SHARED,
> > we can skip to check reserve counting and eventually we cannot be ensured
> > to allocate a huge page in fault time.
> > With following example code, you can easily find this situation.
> >
> > Assume 2MB, nr_hugepages = 100
> >
> >         fd = hugetlbfs_unlinked_fd();
> >         if (fd < 0)
> >                 return 1;
> >
> >         size = 200 * MB;
> >         flag = MAP_SHARED;
> >         p = mmap(NULL, size, PROT_READ|PROT_WRITE, flag, fd, 0);
> >         if (p == MAP_FAILED) {
> >                 fprintf(stderr, "mmap() failed: %s\n", strerror(errno));
> >                 return -1;
> >         }
> >
> >         size = 2 * MB;
> >         flag = MAP_ANONYMOUS | MAP_SHARED | MAP_HUGETLB | MAP_NORESERVE;
> >         p = mmap(NULL, size, PROT_READ|PROT_WRITE, flag, -1, 0);
> >         if (p == MAP_FAILED) {
> >                 fprintf(stderr, "mmap() failed: %s\n", strerror(errno));
> >         }
> >         p[0] = '0';
> >         sleep(10);
> >
> > During executing sleep(10), run 'cat /proc/meminfo' on another process.
> > You'll find a mentioned problem.
> >
> > Solution is simple. We should check VM_NORESERVE in vma_has_reserves().
> > This prevent to use a pre-allocated huge page if free count is under
> > the reserve count.
> 
> You have a problem with this patch, which i guess you are fixing in
> patch 9. Consider two process
> 
> a) MAP_SHARED  on fd
> b) MAP_SHARED | MAP_NORESERVE on fd
> 
> We should allow the (b) to access the page even if VM_NORESERVE is set
> and we are out of reserve space .

I can't get your point.
Please elaborate more on this.

Thanks.

> 
> so may be you should rearrange the patch ?
> 
> >
> > Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> >
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index 6c1eb9b..f6a7a4e 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -464,6 +464,8 @@ void reset_vma_resv_huge_pages(struct vm_area_struct *vma)
> >  /* Returns true if the VMA has associated reserve pages */
> >  static int vma_has_reserves(struct vm_area_struct *vma)
> >  {
> > +	if (vma->vm_flags & VM_NORESERVE)
> > +		return 0;
> >  	if (vma->vm_flags & VM_MAYSHARE)
> >  		return 1;
> >  	if (is_vma_resv_set(vma, HPAGE_RESV_OWNER))
> > -- 
> > 1.7.9.5
> 
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