linux-kernel - Re: Tagged pointers in the XArray

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Date:   Tue, 28 Aug 2018 15:39:01 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Matthew Wilcox <willy@...radead.org>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org
Cc:     Gao Xiang <gaoxiang25@...wei.com>,
        zhong jiang <zhongjiang@...wei.com>,
        Chao Yu <yuchao0@...wei.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: Tagged pointers in the XArray

Just a question, please...

On 08/28/2018 03:27 PM, Matthew Wilcox wrote:
> 
> diff --git a/include/linux/xarray.h b/include/linux/xarray.h
> index c74556ea4258..d1b383f3063f 100644
> --- a/include/linux/xarray.h
> +++ b/include/linux/xarray.h
> @@ -150,6 +150,54 @@ static inline int xa_err(void *entry)
>  	return 0;
>  }
>  
> +/**
> + * xa_tag_pointer() - Create an XArray entry for a tagged pointer.
> + * @p: Plain pointer.
> + * @tag: Tag value (0, 1 or 3).
> + *

What's wrong with a tag value of 2?

and what happens when one is used?  [I don't see anything preventing that.]


> + * If the user of the XArray prefers, they can tag their pointers instead
> + * of storing value entries.  Three tags are available (0, 1 and 3).
> + * These are distinct from the xa_tag_t as they are not replicated up
> + * through the array and cannot be searched for.
> + *
> + * Context: Any context.
> + * Return: An XArray entry.
> + */
> +static inline void *xa_tag_pointer(void *p, unsigned long tag)
> +{
> +	return (void *)((unsigned long)p | tag);
> +}
> +
> +/**
> + * xa_untag_pointer() - Turn an XArray entry into a plain pointer.
> + * @entry: XArray entry.
> + *
> + * If you have stored a tagged pointer in the XArray, call this function
> + * to get the untagged version of the pointer.
> + *
> + * Context: Any context.
> + * Return: A pointer.
> + */
> +static inline void *xa_untag_pointer(void *entry)
> +{
> +	return (void *)((unsigned long)entry & ~3UL);
> +}
> +
> +/**
> + * xa_pointer_tag() - Get the tag stored in an XArray entry.
> + * @entry: XArray entry.
> + *
> + * If you have stored a tagged pointer in the XArray, call this function
> + * to get the tag of that pointer.
> + *
> + * Context: Any context.
> + * Return: A tag.
> + */
> +static inline unsigned int xa_pointer_tag(void *entry)
> +{
> +	return (unsigned long)entry & 3UL;
> +}

thanks,
-- 
~Randy