Patch "netfs: Downgrade i_rwsem for a buffered write" has been added to the 6.11-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    netfs: Downgrade i_rwsem for a buffered write

to the 6.11-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     netfs-downgrade-i_rwsem-for-a-buffered-write.patch
and it can be found in the queue-6.11 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 0745bbe17bfe17f2696d4d6436dc344aede6a630
Author: David Howells <dhowells@xxxxxxxxxx>
Date:   Wed Oct 16 17:28:33 2024 +0100

    netfs: Downgrade i_rwsem for a buffered write
    
    [ 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@xxxxxxxxxx>
    Link: https://lore.kernel.org/r/1317958.1729096113@xxxxxxxxxxxxxxxxxxxxxx
    cc: Steve French <sfrench@xxxxxxxxx>
    cc: Paulo Alcantara <pc@xxxxxxxxxxxxx>
    cc: Trond Myklebust <trondmy@xxxxxxxxxx>
    cc: Jeff Layton <jlayton@xxxxxxxxxx>
    cc: netfs@xxxxxxxxxxxxxxx
    cc: linux-cifs@xxxxxxxxxxxxxxx
    cc: linux-nfs@xxxxxxxxxxxxxxx
    cc: linux-fsdevel@xxxxxxxxxxxxxxx
    Signed-off-by: Christian Brauner <brauner@xxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

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);