Re: [PATCH] vmap(): don't allow invalid pages

[Russell King (Oracle)]

On Wed, Jan 19, 2022 at 04:27:32PM +0000, Matthew Wilcox wrote:
> On Wed, Jan 19, 2022 at 01:28:14PM +0000, Robin Murphy wrote:
> > > +		if (WARN_ON(!pfn_valid(page_to_pfn(page))))
> > 
> > Is it page_to_pfn() guaranteed to work without blowing up if page is invalid
> > in the first place? Looking at the CONFIG_SPARSEMEM case I'm not sure that's
> > true...
> 
> Even if it does blow up, at least it's blowing up here where someone
> can start to debug it, rather than blowing up on first access, where
> we no longer have the invlid struct page pointer.
> 
> I don't think we have a 'page_valid' function which will tell us whether
> a random pointer is actually a struct page or not.

Isn't it supposed to be:

	if (!pfn_valid(pfn)) {
		handle invalid pfn;
	}

	page = pfn_to_page(pfn);

Anything else - even trying to convert an invalid page back to a pfn,
could well be unreliable (sparsemem or discontigmem). 

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