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

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Date: Thu, 23 May 2024 19:07:06 -0400
From: Peter Xu <peterx@...hat.com>
To: Dave Hansen <dave.hansen@...el.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 Thu, May 23, 2024 at 03:48:22PM -0700, Dave Hansen wrote:
> On 5/23/24 15:37, Peter Xu wrote:
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index 317de2afd371..c4a2356b1a54 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -1135,7 +1135,7 @@ static void insert_pfn_pmd(struct vm_area_struct *vma, unsigned long addr,
> >  		goto out_unlock;
> >  	}
> >  
> > -	entry = pmd_mkhuge(pfn_t_pmd(pfn, prot));
> > +	entry = pmd_mkhuge(pfn_t_pmd(pfn, pgprot_4k_2_large(prot)));
> >  	if (pfn_t_devmap(pfn))
> >  		entry = pmd_mkdevmap(entry);
> >  	if (write) {
> 
> Does this even compile on non-x86 architectures?

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.

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

Thanks,

-- 
Peter Xu