linux-kernel - Re: [tip: x86/mm] x86/mm: Ensure input to pfn_to

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Date:   Thu, 23 Nov 2023 11:00:33 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     linux-tip-commits@...r.kernel.org,
        Dave Hansen <dave.hansen@...el.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Michael Roth <michael.roth@....com>,
        Ingo Molnar <mingo@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Rik van Riel <riel@...riel.com>, x86@...nel.org
Subject: Re: [tip: x86/mm] x86/mm: Ensure input to pfn_to_kaddr() is treated
 as a 64-bit type

On Thu, 23 Nov 2023 at 02:31, tip-bot2 for Michael Roth
<tip-bot2@...utronix.de> wrote:
>
> On 64-bit platforms, the pfn_to_kaddr() macro requires that the input
> value is 64 bits in order to ensure that valid address bits don't get
> lost when shifting that input by PAGE_SHIFT to calculate the physical
> address to provide a virtual address for.

Bah. The commit is obviously fine, but can we please just get rid of
that broken pfn_to_kaddr() thing entirely?

It's a bogus mis-spelling of pfn_to_virt(), and I don't know why that
horrid thing exists. In *all* other situations we talk about "virt"
for kernel virtual addresses, I don't know where that horrid "kaddr"
came from (ie "virt_to_page()" and friends).

Most notably, we have "virt_to_pfn()" being quite commonly used. We
don't even have that kaddr_to_pfn(), which just shows *how* bogus this
whole "pfn_to_kaddr()" crud is.

The good news is that there aren't a ton of users. Anybody willing to
just do a search-and-replace and get rid of this pointless and wrong
thing?

Using "pfn_to_virt()" has the added advantage that we have a generic
implementation of it that isn't duplicated pointlessly for N
architectures, and that didn't have this bug:

  static inline void *pfn_to_virt(unsigned long pfn)
  {
        return __va(pfn) << PAGE_SHIFT;
  }
  #define pfn_to_virt pfn_to_virt

Hmm?

Amusingly (or sadly), we have s390 holding up the flag of sanity, and having

    #define pfn_to_kaddr(pfn)  pfn_to_virt(pfn)

and then we'd only need to fix the hexagon version of that macro
(since Hexagon made its own version, with the old bug - but I guess
Hexagon is 32-bit only and hopefully never grows 64-bit (??) so maybe
nobody cares).

           Linus