Re: [RFC] vfs/inode: For none-block-based filesystems default to sb->s_bdi

[Boaz Harrosh <bharrosh@panasas.com>]

On 10/04/2010 05:46 PM, Boaz Harrosh wrote:
> 
> In alloc_inode (inode_init_always) in the case that sb->s_bdev
> is NULL. Should we not use sb->s_bdi as the default
> mapping->backing_dev_info?
> 
> This fixes my none-block-based filesystem recent WARN_ON
> at fs/fs-writeback.c:87 inode_to_bdi()
> 
> If not done here I'll need to do this in 5 different cases
> in FS code. (OK the code could enjoy some re-factoring).
> 
> It does look logical the question is how many FSs will now
> get broken?
> 
> Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>
> ---
>  fs/inode.c |    2 ++
>  1 files changed, 2 insertions(+), 0 deletions(-)
> 
> diff --git a/fs/inode.c b/fs/inode.c
> index 8646433..200314f 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -181,6 +181,8 @@ int inode_init_always(struct super_block *sb, struct inode *inode)
>  
>  		bdi = sb->s_bdev->bd_inode->i_mapping->backing_dev_info;
>  		mapping->backing_dev_info = bdi;
> +	} else {
> +		mapping->backing_dev_info = sb->s_bdi;
>  	}
>  	inode->i_private = NULL;
>  	inode->i_mapping = mapping;

Sorry I've just seen Jan's patch:
From: Jan Kara <jack@suse.cz>
Date: Mon, 27 Sep 2010 23:56:48 +0200
Subject: [PATCH] bdi: Initialize inode->i_mapping.backing_dev_info to sb->s_bdi

Currently, we initialize inode->i_mapping.backing_dev_info to the bdi of device
sb->s_bdev points to. However there is quite a big number of filesystems that
do not set sb->s_bdev (because they do not have one) but do set sb->s_bdi.
These filesystems would generally benefit from setting
inode->i_mapping.backing_dev_info to their s_bdi because otherwise their inodes
would point to default_backing_dev_info and thus dirty inode tracking would
happen there. So change inode initialization code to use sb->s_bdi if it
is available.

Signed-off-by: Jan Kara <jack@suse.cz>
---
 fs/inode.c |   22 ++++++++++++++--------
 1 files changed, 14 insertions(+), 8 deletions(-)

diff --git a/fs/inode.c b/fs/inode.c
index 8646433..e415be4 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -172,15 +172,21 @@ int inode_init_always(struct super_block *sb, struct inode *inode)
 	mapping->writeback_index = 0;
 
 	/*
-	 * If the block_device provides a backing_dev_info for client
-	 * inodes then use that.  Otherwise the inode share the bdev's
-	 * backing_dev_info.
+	 * If the filesystem provides a backing_dev_info for client inodes
+	 * then use that. Otherwise inodes share default_backing_dev_info.
 	 */
-	if (sb->s_bdev) {
-		struct backing_dev_info *bdi;
-
-		bdi = sb->s_bdev->bd_inode->i_mapping->backing_dev_info;
-		mapping->backing_dev_info = bdi;
+	if (sb->s_bdi && sb->s_bdi != &noop_backing_dev_info) {
+		/*
+		 * Catch cases where filesystem might be bitten by using s_bdi
+		 * instead of sb->s_bdev. Can be removed in 2.6.38.
+		 */
+		if (sb->s_bdev) {
+			struct backing_dev_info *bdi =
+			  sb->s_bdev->bd_inode->i_mapping->backing_dev_info;
+			WARN(bdi != sb->s_bdi, "s_bdev bdi %s != s_bdi %s\n",
+			     bdi->name, sb->s_bdi->name);
+		}
+		mapping->backing_dev_info = sb->s_bdi;
 	}
 	inode->i_private = NULL;
 	inode->i_mapping = mapping;

That works for me as well. Was it decided how to solve this? Other wise
I'll need to patch exofs, ASAP for this -rc

Thanks
Boaz