linux-kernel - Re: [PATCH v2 2/9] mm/mshare: pre-populate msharefs with information file

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Date:   Thu, 30 Jun 2022 14:37:45 -0700
From:   "Darrick J. Wong" <djwong@...nel.org>
To:     Khalid Aziz <khalid.aziz@...cle.com>
Cc:     akpm@...ux-foundation.org, willy@...radead.org,
        aneesh.kumar@...ux.ibm.com, arnd@...db.de, 21cnbao@...il.com,
        corbet@....net, dave.hansen@...ux.intel.com, david@...hat.com,
        ebiederm@...ssion.com, hagen@...u.net, jack@...e.cz,
        keescook@...omium.org, kirill@...temov.name, kucharsk@...il.com,
        linkinjeon@...nel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        longpeng2@...wei.com, luto@...nel.org, markhemm@...glemail.com,
        pcc@...gle.com, rppt@...nel.org, sieberf@...zon.com,
        sjpark@...zon.de, surenb@...gle.com, tst@...oebel-theuer.de,
        yzaikin@...gle.com
Subject: Re: [PATCH v2 2/9] mm/mshare: pre-populate msharefs with information
 file

On Wed, Jun 29, 2022 at 04:53:53PM -0600, Khalid Aziz wrote:
> Users of mshare feature to share page tables need to know the size
> and alignment requirement for shared regions. Pre-populate msharefs
> with a file, mshare_info, that provides this information.
> 
> Signed-off-by: Khalid Aziz <khalid.aziz@...cle.com>
> ---
>  mm/mshare.c | 62 +++++++++++++++++++++++++++++++++++++++++------------
>  1 file changed, 48 insertions(+), 14 deletions(-)
> 
> diff --git a/mm/mshare.c b/mm/mshare.c
> index c8fab3869bab..3e448e11c742 100644
> --- a/mm/mshare.c
> +++ b/mm/mshare.c
> @@ -25,8 +25,8 @@
>  static struct super_block *msharefs_sb;
>  
>  static const struct file_operations msharefs_file_operations = {
> -	.open	= simple_open,
> -	.llseek	= no_llseek,
> +	.open		= simple_open,
> +	.llseek		= no_llseek,

I feel like there's a lot of churn between the previous patch and this
one that could have been in the previous patch.

>  };
>  
>  static int
> @@ -42,23 +42,52 @@ msharefs_d_hash(const struct dentry *dentry, struct qstr *qstr)
>  	return 0;
>  }
>  
> +static void
> +mshare_evict_inode(struct inode *inode)
> +{
> +	clear_inode(inode);
> +}
> +
>  static const struct dentry_operations msharefs_d_ops = {
>  	.d_hash = msharefs_d_hash,
>  };
>  
> +static ssize_t
> +mshare_info_read(struct file *file, char __user *buf, size_t nbytes,
> +		loff_t *ppos)
> +{
> +	char s[80];
> +
> +	sprintf(s, "%ld", PGDIR_SIZE);

SO what is this "mshare_info" file supposed to reveal?  Hugepage size?
I wonder why this isn't exported in struct mshare_info?

> +	return simple_read_from_buffer(buf, nbytes, ppos, s, strlen(s));
> +}
> +
> +static const struct file_operations mshare_info_ops = {
> +	.read   = mshare_info_read,
> +	.llseek	= noop_llseek,
> +};
> +
> +static const struct super_operations mshare_s_ops = {
> +	.statfs	     = simple_statfs,
> +	.evict_inode = mshare_evict_inode,
> +};
> +
>  static int
>  msharefs_fill_super(struct super_block *sb, struct fs_context *fc)
>  {
> -	static const struct tree_descr empty_descr = {""};
> +	static const struct tree_descr mshare_files[] = {
> +		[2] = { "mshare_info", &mshare_info_ops, 0444},
> +		{""},
> +	};
>  	int err;
>  
> -	sb->s_d_op = &msharefs_d_ops;
> -	err = simple_fill_super(sb, MSHARE_MAGIC, &empty_descr);
> -	if (err)
> -		return err;
> -
> -	msharefs_sb = sb;
> -	return 0;
> +	err = simple_fill_super(sb, MSHARE_MAGIC, mshare_files);
> +	if (!err) {
> +		msharefs_sb = sb;
> +		sb->s_d_op = &msharefs_d_ops;
> +		sb->s_op = &mshare_s_ops;
> +	}
> +	return err;
>  }
>  
>  static int
> @@ -84,20 +113,25 @@ static struct file_system_type mshare_fs = {
>  	.kill_sb		= kill_litter_super,
>  };
>  
> -static int
> +static int __init
>  mshare_init(void)
>  {
>  	int ret = 0;
>  
>  	ret = sysfs_create_mount_point(fs_kobj, "mshare");
>  	if (ret)
> -		return ret;
> +		goto out;
>  
>  	ret = register_filesystem(&mshare_fs);
> -	if (ret)
> +	if (ret) {
>  		sysfs_remove_mount_point(fs_kobj, "mshare");
> +		goto out;
> +	}
> +
> +	return 0;
>  
> +out:
>  	return ret;
>  }
>  
> -fs_initcall(mshare_init);
> +core_initcall(mshare_init);
> -- 
> 2.32.0
>