cifs: Fix read-performance regression by dropping readahead expansion

cifs_expand_read() is causing a performance regression of around 30% by
causing extra pagecache to be allocated for an inode in the readahead path
before we begin actually dispatching RPC requests, thereby delaying the
actual I/O.  The expansion is sized according to the rsize parameter, which
seems to be 4MiB on my test system; this is a big step up from the first
requests made by the fio test program.

Simple repro (look at read bandwidth number):
     fio --name=writetest --filename=/xfstest.test/foo --time_based --runtime=60 --size=16M --numjobs=1 --rw=read

Fix this by removing cifs_expand_readahead().  Readahead expansion is
mostly useful for when we're using the local cache if the local cache has a
block size greater than PAGE_SIZE, so we can dispense with it when not
caching.

Fixes: 69c3c023 ("cifs: Implement netfslib hooks")
Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Paulo Alcantara (Red Hat) <pc@manguebit.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: Matthew Wilcox <willy@infradead.org>
cc: linux-cifs@vger.kernel.org
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
cc: linux-mm@kvack.org
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/smb/client/file.c

@@ -245,35 +245,6 @@ static int cifs_init_request(struct netfs_io_request *rreq, struct file *file)
 	return 0;
 }
 
-/*
- * Expand the size of a readahead to the size of the rsize, if at least as
- * large as a page, allowing for the possibility that rsize is not pow-2
- * aligned.
- */
-static void cifs_expand_readahead(struct netfs_io_request *rreq)
-{
-	unsigned int rsize = rreq->rsize;
-	loff_t misalignment, i_size = i_size_read(rreq->inode);
-
-	if (rsize < PAGE_SIZE)
-		return;
-
-	if (rsize < INT_MAX)
-		rsize = roundup_pow_of_two(rsize);
-	else
-		rsize = ((unsigned int)INT_MAX + 1) / 2;
-
-	misalignment = rreq->start & (rsize - 1);
-	if (misalignment) {
-		rreq->start -= misalignment;
-		rreq->len += misalignment;
-	}
-
-	rreq->len = round_up(rreq->len, rsize);
-	if (rreq->start < i_size && rreq->len > i_size - rreq->start)
-		rreq->len = i_size - rreq->start;
-}
-
 /*
  * Completion of a request operation.
  */
@@ -329,7 +300,6 @@ const struct netfs_request_ops cifs_req_ops = {
 	.init_request		= cifs_init_request,
 	.free_request		= cifs_free_request,
 	.free_subrequest	= cifs_free_subrequest,
-	.expand_readahead	= cifs_expand_readahead,
 	.clamp_length		= cifs_clamp_length,
 	.issue_read		= cifs_req_issue_read,
 	.done			= cifs_rreq_done,