linux-kernel - Re: [PATCH 03/19] mm: Mark special bits for huge pfn mappings when inject

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Message-ID: <20240814155318.GQ2032816@nvidia.com>
Date: Wed, 14 Aug 2024 12:53:18 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Peter Xu <peterx@...hat.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Sean Christopherson <seanjc@...gle.com>,
	Oscar Salvador <osalvador@...e.de>,
	Axel Rasmussen <axelrasmussen@...gle.com>,
	linux-arm-kernel@...ts.infradead.org, x86@...nel.org,
	Will Deacon <will@...nel.org>, Gavin Shan <gshan@...hat.com>,
	Paolo Bonzini <pbonzini@...hat.com>, Zi Yan <ziy@...dia.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Ingo Molnar <mingo@...hat.com>,
	Alistair Popple <apopple@...dia.com>,
	Borislav Petkov <bp@...en8.de>,
	David Hildenbrand <david@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>, kvm@...r.kernel.org,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Alex Williamson <alex.williamson@...hat.com>,
	Yan Zhao <yan.y.zhao@...el.com>
Subject: Re: [PATCH 03/19] mm: Mark special bits for huge pfn mappings when
 inject

On Wed, Aug 14, 2024 at 11:23:41AM -0400, Peter Xu wrote:
> On Wed, Aug 14, 2024 at 09:40:00AM -0300, Jason Gunthorpe wrote:
> > On Fri, Aug 09, 2024 at 12:08:53PM -0400, Peter Xu wrote:
> > > We need these special bits to be around to enable gup-fast on pfnmaps.
> > 
> > It is not gup-fast you are after but follow_pfn/etc for KVM usage
> > right?
> 
> Gup-fast needs it to make sure we don't pmd_page() it and fail early.  So
> still needed in some sort..

Yes, but making gup-fast fail is not "enabling" it :)

> But yeah, this comment is ambiguous and not describing the whole picture,
> as multiple places will so far rely this bit, e.g. fork() to identify a
> private page or pfnmap. Similarly we'll do that in folio_walk_start(), and
> follow_pfnmap.  I plan to simplify that to:
> 
>   We need these special bits to be around on pfnmaps.  Mark properly for
>   !devmap case, reflecting that there's no page struct backing the entry.

Yes

Jason