hugetlbfs: remove meaningless variable avoid_reserve

The variable avoid_reserve is meaningless because we never changed its
value and just passed it to alloc_huge_page().  So remove it to make code
more clear that in hugetlbfs_fallocate, we never avoid reserve when alloc
hugepage yet.  Also add a comment offered by Mike Kravetz to explain this.

Link: https://lkml.kernel.org/r/20210120071508.9078-1-linmiaohe@huawei.comSigned-off-by: NMiaohe Lin <linmiaohe@huawei.com>
Reviewed-by: NDavid Hildenbrand <david@redhat.com>
Reviewed-by: NMike Kravetz <mike.kravetz@oracle.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

fs/hugetlbfs/inode.c

@@ -680,7 +680,6 @@ static long hugetlbfs_fallocate(struct file *file, int mode, loff_t offset,
 		 */
 		struct page *page;
 		unsigned long addr;
-		int avoid_reserve = 0;
 
 		cond_resched();
 
@@ -716,8 +715,15 @@ static long hugetlbfs_fallocate(struct file *file, int mode, loff_t offset,
 			continue;
 		}
 
-		/* Allocate page and add to page cache */
-		page = alloc_huge_page(&pseudo_vma, addr, avoid_reserve);
+		/*
+		 * Allocate page without setting the avoid_reserve argument.
+		 * There certainly are no reserves associated with the
+		 * pseudo_vma.  However, there could be shared mappings with
+		 * reserves for the file at the inode level.  If we fallocate
+		 * pages in these areas, we need to consume the reserves
+		 * to keep reservation accounting consistent.
+		 */
+		page = alloc_huge_page(&pseudo_vma, addr, 0);
 		hugetlb_drop_vma_policy(&pseudo_vma);
 		if (IS_ERR(page)) {
 			mutex_unlock(&hugetlb_fault_mutex_table[hash]);