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

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

On Tue, Jul 16, 2013 at 11:17:23AM +0530, Aneesh Kumar K.V wrote:
> Joonsoo Kim <iamjoonsoo.kim@lge.com> writes:
> 
> > 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.
> 
> 
> One process mmap with MAP_SHARED and another one with MAP_SHARED | MAP_NORESERVE
> Now the first process will result in reserving the pages from the hugtlb
> pool. Now if the second process try to dequeue huge page and we don't
> have free space we will fail because
> 
> vma_has_reservers will now return zero because VM_NORESERVE is set 
> and we can have (h->free_huge_pages - h->resv_huge_pages) == 0;

I think that this behavior is correct, because a user who mapped with
VM_NORESERVE should not think their allocation always succeed. With patch 9,
he can be ensured to succeed, but I think it is side-effect.

> The below hunk in your patch 9 handles that
> 
>  +	if (vma->vm_flags & VM_NORESERVE) {
>  +		/*
>  +		 * This address is already reserved by other process(chg == 0),
>  +		 * so, we should decreament reserved count. Without
>  +		 * decreamenting, reserve count is remained after releasing
>  +		 * inode, because this allocated page will go into page cache
>  +		 * and is regarded as coming from reserved pool in releasing
>  +		 * step. Currently, we don't have any other solution to deal
>  +		 * with this situation properly, so add work-around here.
>  +		 */
>  +		if (vma->vm_flags & VM_MAYSHARE && chg == 0)
>  +			return 1;
>  +		else
>  +			return 0;
>  +	}
> 
> so may be both of these should be folded ?

I think that these patches should not be folded, because these handle
two separate issues. Reserve count mismatch issue mentioned in patch 9
is not introduced by patch 7.

Thanks.

> 
> -aneesh
> 
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