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

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Date:   Mon, 18 Jul 2022 11:00:19 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     David Hildenbrand <david@...hat.com>,
        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()

David Hildenbrand wrote:
> 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;

I tend to agree with David that this change makes the page_is_ram()
harder to read. I think the problem is that the "next" nature of
find_next_iomem_res() is meant to be handled by the caller. So it really
should be called find_iomem_res().

> >  }
> >  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

...a patch to rename find_next_iomem_res()

> 
> b) The actual change to page_is_ram()
> 
> ?