Re: [PATCH] mm/hugetlbfs: Unmap pages if page fault raced with hole punch

[Andrew Morton]

On Wed,  6 Jan 2016 14:37:04 -0800 Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:

> Page faults can race with fallocate hole punch.  If a page fault happens
> between the unmap and remove operations, the page is not removed and
> remains within the hole.  This is not the desired behavior.  The race
> is difficult to detect in user level code as even in the non-race
> case, a page within the hole could be faulted back in before fallocate
> returns.  If userfaultfd is expanded to support hugetlbfs in the future,
> this race will be easier to observe.
> 
> If this race is detected and a page is mapped, the remove operation
> (remove_inode_hugepages) will unmap the page before removing.  The unmap
> within remove_inode_hugepages occurs with the hugetlb_fault_mutex held
> so that no other faults will be processed until the page is removed.
> 
> The (unmodified) routine hugetlb_vmdelete_list was moved ahead of
> remove_inode_hugepages to satisfy the new reference.
> 
> ...
>
> --- a/fs/hugetlbfs/inode.c
> +++ b/fs/hugetlbfs/inode.c
>
> ...
>
> @@ -395,37 +431,43 @@ static void remove_inode_hugepages(struct inode *inode, loff_t lstart,
>  							mapping, next, 0);
>  			mutex_lock(&hugetlb_fault_mutex_table[hash]);
>  
> -			lock_page(page);
> -			if (likely(!page_mapped(page))) {

hm, what are the locking requirements for page_mapped()?

> -				bool rsv_on_error = !PagePrivate(page);
> -				/*
> -				 * We must free the huge page and remove
> -				 * from page cache (remove_huge_page) BEFORE
> -				 * removing the region/reserve map
> -				 * (hugetlb_unreserve_pages).  In rare out
> -				 * of memory conditions, removal of the
> -				 * region/reserve map could fail.  Before
> -				 * free'ing the page, note PagePrivate which
> -				 * is used in case of error.
> -				 */
> -				remove_huge_page(page);

And remove_huge_page().

> -				freed++;
> -				if (!truncate_op) {
> -					if (unlikely(hugetlb_unreserve_pages(
> -							inode, next,
> -							next + 1, 1)))
> -						hugetlb_fix_reserve_counts(
> -							inode, rsv_on_error);
> -				}
>
> ...
>