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netfs: Downgrade i_rwsem for a buffered write

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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>
This commit is contained in:
David Howells 2024-10-16 17:28:33 +01:00 committed by Christian Brauner
parent 6ed469df0b
commit d6a77668a7
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GPG Key ID: 91C61BC06578DCA2
1 changed files with 2 additions and 1 deletions
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fs/netfs/locking.c
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@ -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);