linux-ext4 - Re: [PATCH] fs: allow for fs-specific objects to be pruned as part of pruning inodes

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Date:	Wed, 23 Jan 2013 08:06:35 -0500
From:	Carlos Maiolino <cmaiolino@...hat.com>
To:	"Theodore Ts'o" <tytso@....edu>
Cc:	Ext4 Developers List <linux-ext4@...r.kernel.org>,
	gnehzuil.liu@...il.com, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] fs: allow for fs-specific objects to be pruned as part
 of pruning inodes

Looks good to me:

Reviewed-by: Carlos Maiolino <cmaiolino@...hat.com>

On Wed, Jan 23, 2013 at 01:06:08AM -0500, Theodore Ts'o wrote:
> The VFS's prune_super() function allows for the file system to prune
> file-system specific objects.  Ext4 would like to use this to prune
> parts of the inode's extent cache.  The object lifetime rules used by
> ext4 is somewhat different from the those of the dentry and inode in
> the VFS.  Ext4's extent cache objects can be pruned without removing
> the inode; however if an inode is pruned, all of the extent cache
> objects associated with the inode are immediately removed.
> 
> To accomodate this rule, we measure the number of fs-specific objects
> before the dentry and inodes are pruned, and then measure them again
> afterwards.  If the number of fs-specific objects have decreased, we
> credit that decrease as part of the shrink operation, so that we do
> not end up removing too many fs-specific objects.
> 
> In the case where fs-specific objects are not removed when inodes are
> removed, this will not change the behavior of prune_super() in any
> appreciable way.  (Currently the only other user of this facility is
> XFS, and this change should not affect XFS's usage of this facility
> for this reason.)
> 
> Signed-off-by: "Theodore Ts'o" <tytso@....edu>
> Cc: linux-fsdevel@...r.kernel.org
> ---
>  fs/super.c | 22 +++++++++++++++++++---
>  1 file changed, 19 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/super.c b/fs/super.c
> index 12f1237..fb57bd2 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -80,6 +80,7 @@ static int prune_super(struct shrinker *shrink, struct shrink_control *sc)
>  	if (sc->nr_to_scan) {
>  		int	dentries;
>  		int	inodes;
> +		int	fs_to_scan = 0;
>  
>  		/* proportion the scan between the caches */
>  		dentries = (sc->nr_to_scan * sb->s_nr_dentry_unused) /
> @@ -87,7 +88,7 @@ static int prune_super(struct shrinker *shrink, struct shrink_control *sc)
>  		inodes = (sc->nr_to_scan * sb->s_nr_inodes_unused) /
>  							total_objects;
>  		if (fs_objects)
> -			fs_objects = (sc->nr_to_scan * fs_objects) /
> +			fs_to_scan = (sc->nr_to_scan * fs_objects) /
>  							total_objects;
>  		/*
>  		 * prune the dcache first as the icache is pinned by it, then
> @@ -96,8 +97,23 @@ static int prune_super(struct shrinker *shrink, struct shrink_control *sc)
>  		prune_dcache_sb(sb, dentries);
>  		prune_icache_sb(sb, inodes);
>  
> -		if (fs_objects && sb->s_op->free_cached_objects) {
> -			sb->s_op->free_cached_objects(sb, fs_objects);
> +		/*
> +		 * If as a result of pruning the icache, we released some
> +		 * of the fs_objects, give credit to the fact and
> +		 * reduce the number of fs objects that we should try
> +		 * to release.
> +		 */
> +		if (fs_to_scan) {
> +			int fs_objects_now = sb->s_op->nr_cached_objects(sb);
> +
> +			if (fs_objects_now < fs_objects)
> +				fs_to_scan -= fs_objects - fs_objects_now;
> +			if (fs_to_scan < 0)
> +				fs_to_scan = 0;
> +		}
> +
> +		if (fs_to_scan && sb->s_op->free_cached_objects) {
> +			sb->s_op->free_cached_objects(sb, fs_to_scan);
>  			fs_objects = sb->s_op->nr_cached_objects(sb);
>  		}
>  		total_objects = sb->s_nr_dentry_unused +
> -- 
> 1.7.12.rc0.22.gcdd159b
> 
> --
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-- 
Carlos
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