Re: [PATCH RFC 03/14] xen/grant-table: stop setting PG_private on pages for grant mapping

On Fri, 31 Jul 2026 22:13:26 -0400 Zi Yan <[email protected]> wrote:

> gnttab_alloc_pages() stores xen_page_foreign in allocated page->private.

in allocated page->private "for 32-bit only".

> On 32-bit, a pointer to xen_page_foreign is stored; on 64-bit,
> xen_page_foreign is stored inline. Checking page->private != NULL is enough
> to tell whether a xen_page_foreign needs to be freed on 32-bit and
> page->private is zeroed unconditionally on 64-bit.
> 
> It prepares for a future commit that remove PG_private.
> 
> No funtional change intended.
> 
> Assisted-by: Claude:claude-opus-4-8
> Assisted-by: Codex:gpt-5
> Signed-off-by: Zi Yan <[email protected]>
> To: Juergen Gross <[email protected]>
> To: Stefano Stabellini <[email protected]>
> Cc: Oleksandr Tyshchenko <[email protected]>
> Cc: [email protected]
> Cc: [email protected]
> ---
>  drivers/xen/balloon.c     | 5 +++++
>  drivers/xen/grant-table.c | 7 +++----
>  2 files changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
> index e7f1d4ca6d753..7f47b0ad05607 100644
> --- a/drivers/xen/balloon.c
> +++ b/drivers/xen/balloon.c
> @@ -182,6 +182,11 @@ static struct page *balloon_retrieve(bool require_lowmem)
>  
>  	__ClearPageOffline(page);
>  	dec_node_page_state(page, NR_BALLOON_PAGES);
> +	/*
> +	 * clear page->private before giving it out, since it might be used to
> +	 * store xen_page_foreign info.
> +	 */
> +	set_page_private(page, 0);
>  
>  	return page;
>  }
> diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c
> index 35f879dc5dfb8..cc348ba2e0786 100644
> --- a/drivers/xen/grant-table.c
> +++ b/drivers/xen/grant-table.c
> @@ -875,7 +875,7 @@ int gnttab_pages_set_private(int nr_pages, struct page **pages)
>  
>  		set_page_private(pages[i], (unsigned long)foreign);
>  #endif
> -		SetPagePrivate(pages[i]);
> +		/* Data is stored in page->private on 64-bit */

On 64-bit arch you just iterate an empty for loop. Cleaner to put the
whole for loop in ifdef?

>  	}
>  
>  	return 0;
> @@ -1031,12 +1031,11 @@ void gnttab_pages_clear_private(int nr_pages, struct page **pages)
>  	int i;
>  
>  	for (i = 0; i < nr_pages; i++) {
> -		if (PagePrivate(pages[i])) {
>  #if BITS_PER_LONG < 64
> +		if (page_private(pages[i]))
>  			kfree((void *)page_private(pages[i]));
>  #endif
> -			ClearPagePrivate(pages[i]);
> -		}
> +		set_page_private(pages[i], 0);
>  	}
>  }
>  EXPORT_SYMBOL_GPL(gnttab_pages_clear_private);
> 
> -- 
> 2.53.0
> 
>