swapfile: get rid of volatile and avoid redundant read

Patch series "Clean up and fixes for swap", v2.

This series cleans up some code paths, saves a few cycles and reduces the
object size by a bit.  It also fixes some rare race issue with statistics.


This patch (of 4):

Convert a volatile variable to more readable READ_ONCE.  And this actually
avoids the code from reading the variable twice redundantly when it races.

Link: https://lkml.kernel.org/r/20221219185840.25441-1-ryncsn@gmail.com
Link: https://lkml.kernel.org/r/20221219185840.25441-2-ryncsn@gmail.comSigned-off-by: Kairui Song <kasong@tencent.com>
Reviewed-by: "Huang, Ying" <ying.huang@intel.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/swapfile.c

@@ -1835,13 +1835,13 @@ static int unuse_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
 	pte_t *pte;
 	struct swap_info_struct *si;
 	int ret = 0;
-	volatile unsigned char *swap_map;
 
 	si = swap_info[type];
 	pte = pte_offset_map(pmd, addr);
 	do {
 		struct folio *folio;
 		unsigned long offset;
+		unsigned char swp_count;
 
 		if (!is_swap_pte(*pte))
 			continue;
@@ -1852,7 +1852,6 @@ static int unuse_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
 
 		offset = swp_offset(entry);
 		pte_unmap(pte);
-		swap_map = &si->swap_map[offset];
 		folio = swap_cache_get_folio(entry, vma, addr);
 		if (!folio) {
 			struct page *page;
@@ -1869,8 +1868,10 @@ static int unuse_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
 				folio = page_folio(page);
 		}
 		if (!folio) {
-			if (*swap_map == 0 || *swap_map == SWAP_MAP_BAD)
+			swp_count = READ_ONCE(si->swap_map[offset]);
+			if (swp_count == 0 || swp_count == SWAP_MAP_BAD)
 				goto try_next;
+
 			return -ENOMEM;
 		}