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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Jan Kara <jack@suse.cz>,
        "zhangyi (F)" <yi.zhang@huawei.com>, Theodore Tso <tytso@mit.edu>
Subject: [PATCH 4.19 11/40] jbd2: improve comments about freeing data buffers whose page mapping is NULL
Date:   Mon, 20 Apr 2020 14:39:21 +0200
Message-Id: <20200420121454.697285897@linuxfoundation.org>
In-Reply-To: <20200420121444.178150063@linuxfoundation.org>
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From: zhangyi (F) <yi.zhang@huawei.com>

commit 780f66e59231fcf882f36c63f287252ee47cc75a upstream.

Improve comments in jbd2_journal_commit_transaction() to describe why
we don't need to clear the buffer_mapped bit for freeing file mapping
buffers whose page mapping is NULL.

Link: https://lore.kernel.org/r/20200217112706.20085-1-yi.zhang@huawei.com
Fixes: c96dceeabf76 ("jbd2: do not clear the BH_Mapped flag when forgetting a metadata buffer")
Suggested-by: Jan Kara <jack@suse.cz>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: zhangyi (F) <yi.zhang@huawei.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 fs/jbd2/commit.c |    7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

--- a/fs/jbd2/commit.c
+++ b/fs/jbd2/commit.c
@@ -992,9 +992,10 @@ restart_loop:
 			 * journalled data) we need to unmap buffer and clear
 			 * more bits. We also need to be careful about the check
 			 * because the data page mapping can get cleared under
-			 * out hands, which alse need not to clear more bits
-			 * because the page and buffers will be freed and can
-			 * never be reused once we are done with them.
+			 * our hands. Note that if mapping == NULL, we don't
+			 * need to make buffer unmapped because the page is
+			 * already detached from the mapping and buffers cannot
+			 * get reused.
 			 */
 			mapping = READ_ONCE(bh->b_page->mapping);
 			if (mapping && !sb_is_blkdev_sb(mapping->host->i_sb)) {