linux-kernel - [PATCH] nfs: remove redundant tests on reading new pages

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Date:	Mon, 1 Dec 2008 11:19:22 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-nfs@...r.kernel.org,
	Trond Myklebust <Trond.Myklebust@...app.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"stable@...nel.org" <stable@...nel.org>
Subject: [PATCH] nfs: remove redundant tests on reading new pages

aops->readpages() and its NFS helper readpage_async_filler() will only
be called to do readahead I/O for newly allocated pages. So it's not
necessary to test for the always 0 dirty/uptodate page flags.

The removal of nfs_wb_page() call also fixes a readahead bug: the NFS
readahead has been synchronous since 2.6.23, because that call will
clear PG_readahead, which is the reminder for asynchronous readahead.

More background: the PG_readahead page flag is shared with PG_reclaim,
one for read path and the other for write path. clear_page_dirty_for_io()
unconditionally clears PG_readahead to prevent possible readahead residuals,
assuming itself to be always called in the write path. However, NFS is one
and the only exception in that it _always_ calls clear_page_dirty_for_io()
in the read path, i.e. for readpages()/readpage().

Cc: Trond Myklebust <Trond.Myklebust@...app.com>
Signed-off-by: Wu Fengguang <wfg@...ux.intel.com>
---
 fs/nfs/read.c |    6 ------
 1 file changed, 6 deletions(-)

This bug was found when playing with my readahead tracing module posted at
http://lkml.org/lkml/2008/11/27/373 and confirmed to work.

The NFS readahead traces are now

[   59.972526] readahead-initial0(pid=3646(dd), dev=00:0f(0:f), ino=293189(1G), req=0+25, ra=0+60-30, async=0) = 60
[   59.992734] readahead-subsequent(pid=3646(dd), dev=00:0f(0:f), ino=293189(1G), req=30+20, ra=60+30-30, async=1) = 30
[   60.030900] readahead-subsequent(pid=3646(dd), dev=00:0f(0:f), ino=293189(1G), req=60+15, ra=90+30-30, async=1) = 30
[   60.031687] readahead-subsequent(pid=3646(dd), dev=00:0f(0:f), ino=293189(1G), req=90+10, ra=120+30-30, async=1) = 30

The async field was always 0 before this patch.

--- linux-2.6.orig/fs/nfs/read.c
+++ linux-2.6/fs/nfs/read.c
@@ -533,12 +533,6 @@ readpage_async_filler(void *data, struct
 	unsigned int len;
 	int error;

-	error = nfs_wb_page(inode, page);
-	if (error)
-		goto out_unlock;
-	if (PageUptodate(page))
-		goto out_unlock;
-
 	len = nfs_page_length(page);
 	if (len == 0)
 		return nfs_return_empty_page(page);
--
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