linux-kernel - [PATCH] netfs: Downgrade i

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [day] [month] [year] [list]

Message-ID: <1317958.1729096113@warthog.procyon.org.uk>
Date: Wed, 16 Oct 2024 17:28:33 +0100
From: David Howells <dhowells@...hat.com>
To: Christian Brauner <brauner@...nel.org>,
    Steve French <sfrench@...ba.org>
cc: dhowells@...hat.com, Paulo Alcantara <pc@...guebit.com>,
    Trond Myklebust <trondmy@...nel.org>,
    Jeff Layton <jlayton@...nel.org>, netfs@...ts.linux.dev,
    linux-cifs@...r.kernel.org, linux-nfs@...r.kernel.org,
    linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] netfs: Downgrade i_rwsem for a buffered write

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@...hat.com>
cc: Steve French <sfrench@...ba.org>
cc: Paulo Alcantara <pc@...guebit.com>
cc: Trond Myklebust <trondmy@...nel.org>
cc: Jeff Layton <jlayton@...nel.org>
cc: netfs@...ts.linux.dev
cc: linux-cifs@...r.kernel.org
cc: linux-nfs@...r.kernel.org
cc: linux-fsdevel@...r.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 21eab56ee2f9..2249ecd09d0a 100644
--- a/fs/netfs/locking.c
+++ b/fs/netfs/locking.c
@@ -109,6 +109,7 @@ 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);
@@ -123,7 +124,7 @@ 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);