linux-kernel - Re: [PATCH RFC 2/2] mm/x86/pat: Do proper PAT bit shift for large mappings

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Message-ID: <7b6b6430-0237-4512-b99b-9eb815b3dc68@intel.com>
Date: Thu, 23 May 2024 20:30:19 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Peter Xu <peterx@...hat.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 Thomas Gleixner <tglx@...utronix.de>, Jason Gunthorpe <jgg@...dia.com>,
 Andrew Morton <akpm@...ux-foundation.org>, Al Viro
 <viro@...iv.linux.org.uk>, Dave Hansen <dave.hansen@...ux.intel.com>,
 Andy Lutomirski <luto@...nel.org>, Matthew Wilcox <willy@...radead.org>,
 Dan Williams <dan.j.williams@...el.com>,
 "Kirill A . Shutemov" <kirill@...temov.name>, Mike Rapoport
 <rppt@...nel.org>, Ingo Molnar <mingo@...hat.com>,
 Michal Hocko <mhocko@...e.com>, Alex Williamson
 <alex.williamson@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
 Suren Baghdasaryan <surenb@...gle.com>, Borislav Petkov <bp@...en8.de>,
 x86@...nel.org
Subject: Re: [PATCH RFC 2/2] mm/x86/pat: Do proper PAT bit shift for large
 mappings

On 5/23/24 16:07, Peter Xu wrote:
> Probably not..  I think I can define a pgprot_to_large() globally, pointing
> that to pgprot_4k_2_large() on x86 and make the fallback to be noop.  And
> if there's a new version I'll guarantee to run over my cross compilers.

I guess that would be functional, but it would be a bit mean to
everybody else.

> Any comments on the idea itself?  Do we have a problem, or maybe I
> overlooked something?

I think it's probably unnecessary to inflict this particular x86-ism on
generic code.  The arch-generic 'prot' should have PAT at its 4k
(_PAGE_BIT_PAT) position and then p*d_mkhuge() can shift it into the
_PAGE_BIT_PAT_LARGE spot.