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From:   Kairui Song <ryncsn@gmail.com>
To:     linux-mm@kvack.org
Cc:     linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Miaohe Lin <linmiaohe@huawei.com>,
        David Hildenbrand <david@redhat.com>,
        "Huang, Ying" <ying.huang@intel.com>,
        Hugh Dickins <hughd@google.com>,
        Matthew Wilcox <willy@infradead.org>,
        Kairui Song <kasong@tencent.com>
Subject: [PATCH v2 1/4] swapfile: get rid of volatile and avoid redundant read
Date:   Tue, 20 Dec 2022 02:58:37 +0800
Message-Id: <20221219185840.25441-2-ryncsn@gmail.com>
In-Reply-To: <20221219185840.25441-1-ryncsn@gmail.com>
References: <20221219185840.25441-1-ryncsn@gmail.com>
Reply-To: Kairui Song <kasong@tencent.com>
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From: Kairui Song <kasong@tencent.com>

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

Signed-off-by: Kairui Song <kasong@tencent.com>
Reviewed-by: "Huang, Ying" <ying.huang@intel.com>
---
 mm/swapfile.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 908a529bca12..6d3f60bd383b 100644
--- a/mm/swapfile.c
+++ b/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;
 		}
 
-- 
2.35.2