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Date:	Tue, 8 Feb 2011 09:39:25 +0300
From:	"Alexander V. Lukyanov" <lav@...is.ru>
To:	linux-ext4@...r.kernel.org
Cc:	"Theodore Ts'o" <tytso@....edu>, Andreas Dilger <adilger@....com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH] ext4: allow inode_readahead_blks=0 (linux-2.6.37)

Hello!

I cannot disable inode-read-ahead feature of ext4 (on 2.6.37):

# echo 0 > /sys/fs/ext4/sda2/inode_readahead_blks 
bash: echo: write error: Invalid argument

On a server with lots of small files and random access this read-ahead makes
performance worse, and I'd like to disable it. I work around this problem
by using value of 1, but it still reads an extra block.

This patch fixes the problem by checking for zero explicitly.

--- a/fs/ext4/super.c.0	2010-11-16 10:48:33.418629215 +0300
+++ b/fs/ext4/super.c	2010-11-16 10:46:07.739753246 +0300
@@ -1657,7 +1657,7 @@ set_qf_format:
 				return 0;
 			if (option < 0 || option > (1 << 30))
 				return 0;
-			if (!is_power_of_2(option)) {
+			if (option && !is_power_of_2(option)) {
 				ext4_msg(sb, KERN_ERR,
 					 "EXT4-fs: inode_readahead_blks"
 					 " must be a power of 2");
@@ -2274,7 +2274,7 @@ static ssize_t inode_readahead_blks_stor
 	if (parse_strtoul(buf, 0x40000000, &t))
 		return -EINVAL;
 
-	if (!is_power_of_2(t))
+	if (t && !is_power_of_2(t))
 		return -EINVAL;
 
 	sbi->s_inode_readahead_blks = t;

Signed-off-by: Alexander V. Lukyanov <lav@...is.ru>

-- 
   Alexander.
--
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