tree b5b4fad518ac511dce279f05b9ec74ca630b4e31
parent aeba8c49262cf9fe3e6d5f858b9df6455292a8bf
author Benjamin Coddington <bcodding@redhat.com> 1489183666 -0500
committer Greg Kroah-Hartman <gregkh@linuxfoundation.org> 1581715867 -0500

NFS: switch back to to ->iterate()

[ Upstream commit b044f64513843e960f4b8d8e2e042abca1b7c029 ]

NFS has some optimizations for readdir to choose between using READDIR or
READDIRPLUS based on workload, and which NFS operation to use is determined
by subsequent interactions with lookup, d_revalidate, and getattr.

Concurrent use of nfs_readdir() via ->iterate_shared() can cause those
optimizations to repeatedly invalidate the pagecache used to store
directory entries during readdir(), which causes some very bad performance
for directories with many entries (more than about 10000).

There's a couple ways to fix this in NFS, but no fix would be as simple as
going back to ->iterate() to serialize nfs_readdir(), and neither fix I
tested performed as well as going back to ->iterate().

The first required taking the directory's i_lock for each entry, with the
result of terrible contention.

The second way adds another flag to the nfs_inode, and so keeps the
optimizations working for large directories.  The difference from using
->iterate() here is that much more memory is consumed for a given workload
without any performance gain.

The workings of nfs_readdir() are such that concurrent users are serialized
within read_cache_page() waiting to retrieve pages of entries from the
server.  By serializing this work in iterate_dir() instead, contention for
cache pages is reduced.  Waiting processes can have an uncontended pass at
the entirety of the directory's pagecache once previous processes have
completed filling it.

v2 - Keep the bits needed for parallel lookup

Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>