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    SubjectRe: [PATCH] vmap(): don't allow invalid pages
    On Wed, Jan 19, 2022 at 05:54:15PM +0000, Russell King (Oracle) wrote:
    > 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).

    This function is passed an array of pages. We have no way of doing
    what you propose.

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