mm/hugetlb: add available_huge_pages() func

In hugetlb.c there are several places which compare the values of
'h->free_huge_pages' and 'h->resv_huge_pages', it looks a bit messy, so
add a new available_huge_pages() function to do these.

Link: https://lkml.kernel.org/r/20220922021929.98961-1-xhao@linux.alibaba.comSigned-off-by: Xin Hao <xhao@linux.alibaba.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/hugetlb.c

@@ -1191,6 +1191,11 @@ static struct page *dequeue_huge_page_nodemask(struct hstate *h, gfp_t gfp_mask,
 	return NULL;
 }
 
+static unsigned long available_huge_pages(struct hstate *h)
+{
+	return h->free_huge_pages - h->resv_huge_pages;
+}
+
 static struct page *dequeue_huge_page_vma(struct hstate *h,
 				struct vm_area_struct *vma,
 				unsigned long address, int avoid_reserve,
@@ -1207,12 +1212,11 @@ static struct page *dequeue_huge_page_vma(struct hstate *h,
 	 * have no page reserves. This check ensures that reservations are
 	 * not "stolen". The child may still get SIGKILLed
 	 */
-	if (!vma_has_reserves(vma, chg) &&
-			h->free_huge_pages - h->resv_huge_pages == 0)
+	if (!vma_has_reserves(vma, chg) && !available_huge_pages(h))
 		goto err;
 
 	/* If reserves cannot be used, ensure enough pages are in the pool */
-	if (avoid_reserve && h->free_huge_pages - h->resv_huge_pages == 0)
+	if (avoid_reserve && !available_huge_pages(h))
 		goto err;
 
 	gfp_mask = htlb_alloc_mask(h);
@@ -2124,7 +2128,7 @@ int dissolve_free_huge_page(struct page *page)
 	if (!page_count(page)) {
 		struct page *head = compound_head(page);
 		struct hstate *h = page_hstate(head);
-		if (h->free_huge_pages - h->resv_huge_pages == 0)
+		if (!available_huge_pages(h))
 			goto out;
 
 		/*
@@ -2311,7 +2315,7 @@ struct page *alloc_huge_page_nodemask(struct hstate *h, int preferred_nid,
 		nodemask_t *nmask, gfp_t gfp_mask)
 {
 	spin_lock_irq(&hugetlb_lock);
-	if (h->free_huge_pages - h->resv_huge_pages > 0) {
+	if (available_huge_pages(h)) {
 		struct page *page;
 
 		page = dequeue_huge_page_nodemask(h, gfp_mask, preferred_nid, nmask);