linux-kernel - Re: [PATCH] resource: re-factor page_is

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Date:   Mon, 18 Jul 2022 12:48:08 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Vaibhav Jain <vaibhav@...ux.ibm.com>, linux-kernel@...r.kernel.org
Cc:     "Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Muchun Song <songmuchun@...edance.com>
Subject: Re: [PATCH] resource: re-factor page_is_ram()

On 01.06.22 18:32, Vaibhav Jain wrote:
> Presently page_is_ram() relies on walk_system_ram_range() that performs a walk
> on kernel iomem resources hierarchy with a dummy callback __is_ram(). Before
> calling find_next_iomem_res(), walk_system_ram_range() does some book-keeping
> which can be avoided for page_is_ram() use-case.
> 
> Hence this patch proposes to update page_is_ram() to directly call
> find_next_iomem_res() with minimal book-keeping needed.
> 
> To avoid allocating a 'struct resource' the patch also updates
> find_next_iomem_res() to not return -EINVAL in case 'res == NULL'. Instead
> out 'struct resource *res' is only populated when its not NULL.
> 
> Signed-off-by: Vaibhav Jain <vaibhav@...ux.ibm.com>
> ---
>  kernel/resource.c | 19 ++++++++-----------
>  1 file changed, 8 insertions(+), 11 deletions(-)
> 
> diff --git a/kernel/resource.c b/kernel/resource.c
> index 34eaee179689..ecf6b9a50adc 100644
> --- a/kernel/resource.c
> +++ b/kernel/resource.c
> @@ -311,7 +311,7 @@ EXPORT_SYMBOL(release_resource);
>   *
>   * If a resource is found, returns 0 and @*res is overwritten with the part
>   * of the resource that's within [@start..@end]; if none is found, returns
> - * -ENODEV.  Returns -EINVAL for invalid parameters.
> + * -ENODEV.
>   *

There is still another -EINVAL in that function ...

>   * @start:	start address of the resource searched for
>   * @end:	end address of same resource
> @@ -328,9 +328,6 @@ static int find_next_iomem_res(resource_size_t start, resource_size_t end,
>  {
>  	struct resource *p;
>  
> -	if (!res)
> -		return -EINVAL;
> -
>  	if (start >= end)
>  		return -EINVAL;

As all callers guarantee that, we might just remove it.

>  
> @@ -356,7 +353,7 @@ static int find_next_iomem_res(resource_size_t start, resource_size_t end,
>  		break;
>  	}
>  
> -	if (p) {
> +	if (p && res) {
>  		/* copy data */
>  		*res = (struct resource) {
>  			.start = max(start, p->start),
> @@ -474,18 +471,18 @@ int walk_system_ram_range(unsigned long start_pfn, unsigned long nr_pages,
>  	return ret;
>  }
>  
> -static int __is_ram(unsigned long pfn, unsigned long nr_pages, void *arg)
> -{
> -	return 1;
> -}
> -
>  /*
>   * This generic page_is_ram() returns true if specified address is
>   * registered as System RAM in iomem_resource list.
>   */
>  int __weak page_is_ram(unsigned long pfn)
>  {
> -	return walk_system_ram_range(pfn, 1, NULL, __is_ram) == 1;
> +	const resource_size_t pfn_res = PFN_PHYS(pfn);
> +
> +	return find_next_iomem_res(pfn_res,
> +				   pfn_res + 1,
> +				   IORESOURCE_SYSTEM_RAM | IORESOURCE_BUSY,
> +				   IORES_DESC_NONE, NULL) == 0;
>  }
>  EXPORT_SYMBOL_GPL(page_is_ram);
>  

What about

a) A cleanup patch upfront that removes both -EINVAL cases from
find_next_iomem_res() followed by

b) The actual change to page_is_ram()

?

-- 
Thanks,

David / dhildenb