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From:   Miaohe Lin <linmiaohe@huawei.com>
To:     <akpm@linux-foundation.org>
CC:     <david@redhat.com>, <ying.huang@intel.com>,
        <songmuchun@bytedance.com>, <quic_qiancai@quicinc.com>,
        <linux-mm@kvack.org>, <linux-kernel@vger.kernel.org>,
        <linmiaohe@huawei.com>
Subject: [PATCH v3 1/2] mm/swapfile: fix possible data races of inuse_pages
Date:   Sat, 25 Jun 2022 17:33:45 +0800
Message-ID: <20220625093346.48894-2-linmiaohe@huawei.com>
In-Reply-To: <20220625093346.48894-1-linmiaohe@huawei.com>
References: <20220625093346.48894-1-linmiaohe@huawei.com>
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si->inuse_pages could still be accessed concurrently now. The plain reads
outside si->lock critical section, i.e. swap_show and si_swapinfo, which
results in data races. READ_ONCE and WRITE_ONCE is used to fix such data
races. Note these data races should be ok because they're just used for
showing swap info.

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
---
 mm/swapfile.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index edc3420d30e7..5c8681a3f1d9 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -695,7 +695,7 @@ static void swap_range_alloc(struct swap_info_struct *si, unsigned long offset,
 		si->lowest_bit += nr_entries;
 	if (end == si->highest_bit)
 		WRITE_ONCE(si->highest_bit, si->highest_bit - nr_entries);
-	si->inuse_pages += nr_entries;
+	WRITE_ONCE(si->inuse_pages, si->inuse_pages + nr_entries);
 	if (si->inuse_pages == si->pages) {
 		si->lowest_bit = si->max;
 		si->highest_bit = 0;
@@ -732,7 +732,7 @@ static void swap_range_free(struct swap_info_struct *si, unsigned long offset,
 			add_to_avail_list(si);
 	}
 	atomic_long_add(nr_entries, &nr_swap_pages);
-	si->inuse_pages -= nr_entries;
+	WRITE_ONCE(si->inuse_pages, si->inuse_pages - nr_entries);
 	if (si->flags & SWP_BLKDEV)
 		swap_slot_free_notify =
 			si->bdev->bd_disk->fops->swap_slot_free_notify;
@@ -2641,7 +2641,7 @@ static int swap_show(struct seq_file *swap, void *v)
 	}
 
 	bytes = si->pages << (PAGE_SHIFT - 10);
-	inuse = si->inuse_pages << (PAGE_SHIFT - 10);
+	inuse = READ_ONCE(si->inuse_pages) << (PAGE_SHIFT - 10);
 
 	file = si->swap_file;
 	len = seq_file_path(swap, file, " \t\n\\");
@@ -3260,7 +3260,7 @@ void si_swapinfo(struct sysinfo *val)
 		struct swap_info_struct *si = swap_info[type];
 
 		if ((si->flags & SWP_USED) && !(si->flags & SWP_WRITEOK))
-			nr_to_be_unused += si->inuse_pages;
+			nr_to_be_unused += READ_ONCE(si->inuse_pages);
 	}
 	val->freeswap = atomic_long_read(&nr_swap_pages) + nr_to_be_unused;
 	val->totalswap = total_swap_pages + nr_to_be_unused;
-- 
2.23.0