[PATCH AUTOSEL 6.11 12/32] netfs: Downgrade i_rwsem for a buffered write

[Sasha Levin <sashal@kernel.org>]

From: David Howells <dhowells@redhat.com>

[ Upstream commit d6a77668a708f0b5ca6713b39c178c9d9563c35b ]

In the I/O locking code borrowed from NFS into netfslib, i_rwsem is held
locked across a buffered write - but this causes a performance regression
in cifs as it excludes buffered reads for the duration (cifs didn't use any
locking for buffered reads).

Mitigate this somewhat by downgrading the i_rwsem to a read lock across the
buffered write.  This at least allows parallel reads to occur whilst
excluding other writes, DIO, truncate and setattr.

Note that this shouldn't be a problem for a buffered write as a read
through an mmap can circumvent i_rwsem anyway.

Also note that we might want to make this change in NFS also.

Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/r/1317958.1729096113@warthog.procyon.org.uk
cc: Steve French <sfrench@samba.org>
cc: Paulo Alcantara <pc@manguebit.com>
cc: Trond Myklebust <trondmy@kernel.org>
cc: Jeff Layton <jlayton@kernel.org>
cc: netfs@lists.linux.dev
cc: linux-cifs@vger.kernel.org
cc: linux-nfs@vger.kernel.org
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/netfs/locking.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/netfs/locking.c b/fs/netfs/locking.c
index 75dc52a49b3a4..709a6aa101028 100644
--- a/fs/netfs/locking.c
+++ b/fs/netfs/locking.c
@@ -121,6 +121,7 @@ int netfs_start_io_write(struct inode *inode)
 		up_write(&inode->i_rwsem);
 		return -ERESTARTSYS;
 	}
+	downgrade_write(&inode->i_rwsem);
 	return 0;
 }
 EXPORT_SYMBOL(netfs_start_io_write);
@@ -135,7 +136,7 @@ EXPORT_SYMBOL(netfs_start_io_write);
 void netfs_end_io_write(struct inode *inode)
 	__releases(inode->i_rwsem)
 {
-	up_write(&inode->i_rwsem);
+	up_read(&inode->i_rwsem);
 }
 EXPORT_SYMBOL(netfs_end_io_write);
 
-- 
2.43.0