linux-kernel - Re: [PATCH v3 4/4] parse options in the vfs level

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Date:	Mon, 15 Aug 2011 17:09:32 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Glauber Costa <glommer@...allels.com>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	containers@...ts.linux-foundation.org,
	Pavel Emelyanov <xemul@...allels.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Hugh Dickins <hughd@...gle.com>,
	Nick Piggin <npiggin@...nel.dk>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	James Bottomley <JBottomley@...allels.com>,
	Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH v3 4/4] parse options in the vfs level

On Sun, Aug 14, 2011 at 07:13:52PM +0400, Glauber Costa wrote:
> This patch introduces a simple generic vfs option parser.
> Right now, the only option we have is to limit the size of the dcache.
> 
> So any user that wants to have a dcache entries limit, can specify:
> 
>   mount -o whatever_options,vfs_dcache_size=XXX <dev> <mntpoint>
> 
> It is supposed to work well with remounts, allowing it to change
> multiple over the course of the filesystem's lifecycle.
> 
> I find mount a natural interface for handling filesystem options,
> so that's what I've choosen. Feel free to yell at it at will if
> you disagree.

It's already been noted by both myself and Al that there is not a
1:1 mapping between mount point and superblocks....

> @@ -2350,6 +2449,12 @@ long do_mount(char *dev_name, char *dir_name, char *type_page,
>  	else
>  		retval = do_new_mount(&path, type_page, flags, mnt_flags,
>  				      dev_name, data_page);
> +
> +	/* bind mounts get to respect their parents decision */
> +	if (!retval && !(flags & MS_BIND))
> +		vfs_set_dcache_size(path.mnt->mnt_sb,
> +				    vfs_options.vfs_dcache_size);
> +			

And I'm not sure that silently ignoring it in certain cases
is the best way around that problem.

>  dput_out:
>  	path_put(&path);
>  	return retval;
> diff --git a/include/linux/dcache.h b/include/linux/dcache.h
> index d37d2a7..1a309f3 100644
> --- a/include/linux/dcache.h
> +++ b/include/linux/dcache.h
> @@ -251,6 +251,10 @@ extern int d_invalidate(struct dentry *);
>  
>  /* only used at mount-time */
>  extern struct dentry * d_alloc_root(struct inode *);
> +extern int vfs_set_dcache_size(struct super_block *sb, int size);
> +
> +#define DCACHE_MIN_SIZE 1024
> +extern int vfs_set_dcache_size(struct super_block *sb, int size);

Where did this minimum size come from and why was that figure
chosen? Needs documenting.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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