Subject: [PATCH 02/13] NFS: do not take i_rwsem for swap IO
From:   NeilBrown <neilb@suse.de>
To:     Trond Myklebust <trond.myklebust@hammerspace.com>,
        Anna Schumaker <anna.schumaker@netapp.com>,
        Chuck Lever <chuck.lever@oracle.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mel Gorman <mgorman@suse.de>
Cc:     linux-nfs@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Date:   Tue, 16 Nov 2021 13:44:04 +1100
Message-ID: <163703064452.25805.5738767545414940042.stgit@noble.brown>
In-Reply-To: <163702956672.25805.16457749992977493579.stgit@noble.brown>
References: <163702956672.25805.16457749992977493579.stgit@noble.brown>
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Taking the i_rwsem for swap IO triggers lockdep warnings regarding
possible deadlocks with "fs_reclaim".  These deadlocks could, I believe,
eventuate if a buffered read on the swapfile was attempted.

We don't need coherence with the page cache for a swap file, and
buffered writes are forbidden anyway.  There is no other need for
i_rwsem during direct IO.

So don't take the rwsem or set the NFS_INO_ODIRECT flag during IO to the
swap file.

Signed-off-by: NeilBrown <neilb@suse.de>
---
 fs/nfs/io.c |    9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/fs/nfs/io.c b/fs/nfs/io.c
index b5551ed8f648..83b4dfbb826d 100644
--- a/fs/nfs/io.c
+++ b/fs/nfs/io.c
@@ -118,11 +118,18 @@ static void nfs_block_buffered(struct nfs_inode *nfsi, struct inode *inode)
  * NFS_INO_ODIRECT.
  * Note that buffered writes and truncates both take a write lock on
  * inode->i_rwsem, meaning that those are serialised w.r.t. O_DIRECT.
+ *
+ * When inode IS_SWAPFILE we ignore the flag and don't take the rwsem
+ * as it triggers lockdep warnings and possible deadlocks.
+ * bufferred writes are forbidden anyway, and buffered reads will not
+ * be coherent.
  */
 void
 nfs_start_io_direct(struct inode *inode)
 {
 	struct nfs_inode *nfsi = NFS_I(inode);
+	if (IS_SWAPFILE(inode))
+		return;
 	/* Be an optimist! */
 	down_read(&inode->i_rwsem);
 	if (test_bit(NFS_INO_ODIRECT, &nfsi->flags) != 0)
@@ -144,5 +151,7 @@ nfs_start_io_direct(struct inode *inode)
 void
 nfs_end_io_direct(struct inode *inode)
 {
+	if (IS_SWAPFILE(inode))
+		return;
 	up_read(&inode->i_rwsem);
 }