linux-kernel - Re: [PATCH RFC v2 0/4] mm: Introduce MAP_BELOW

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Message-ID: <e48d41f5-646c-413e-99e5-c935b5235576@lucifer.local>
Date: Fri, 30 Aug 2024 16:18:40 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Dave Hansen <dave.hansen@...el.com>
Cc: Charlie Jenkins <charlie@...osinc.com>, Arnd Bergmann <arnd@...db.de>,
        Richard Henderson <richard.henderson@...aro.org>,
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Subject: Re: [PATCH RFC v2 0/4] mm: Introduce MAP_BELOW_HINT

On Fri, Aug 30, 2024 at 08:03:25AM GMT, Dave Hansen wrote:
> On 8/29/24 01:42, Lorenzo Stoakes wrote:
> >> These applications work on x86 because x86 does an implicit 47-bit
> >> restriction of mmap() address that contain a hint address that is less
> >> than 48 bits.
> > You mean x86 _has_ to limit to physically available bits in a canonical
> > format 🙂 this will not be the case for 5-page table levels though...
>
> By "physically available bits" are you referring to the bits that can be
> used as a part of the virtual address?  "Physically" may not have been
> the best choice of words. ;)
>
> There's a canonical hole in 4-level paging and 5-level paging on x86.
> The 5-level canonical hole is just smaller.

Yeah sorry this is what I meant!

>
> Also, I should probably say that the >47-bit mmap() access hint was more
> of a crutch than something that we wanted to make ABI forever.  We knew
> that high addresses might break some apps and we hoped that the list of
> things it would break would go down over time so that we could
> eventually just let mmap() access the whole address space by default.
>
> That optimism may have been misplaced.

Interesting, thanks. This speaks again I think to it being unwise to rely
on these things.

I do think the only workable form of this series is a fixed
personality-based mapping limit.