linux-kernel - Re: [PATCH 05/19] mm/gup: Detect huge pfnmap entries in gup-fast

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Message-ID: <20240814124228.GG2032816@nvidia.com>
Date: Wed, 14 Aug 2024 09:42:28 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Peter Xu <peterx@...hat.com>
Cc: David Hildenbrand <david@...hat.com>, 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>,
	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 05/19] mm/gup: Detect huge pfnmap entries in gup-fast

On Fri, Aug 09, 2024 at 12:59:40PM -0400, Peter Xu wrote:
> > In gup_fast_pte_range() we check after checking pte_devmap(). Do we want to
> > do it in a similar fashion here, or is there a reason to do it differently?
> 
> IIUC they should behave the same, as the two should be mutual exclusive so
> far.  E.g. see insert_pfn():

Yes, agree no functional difference, but David has a point to try to
keep the logic structurally the same in all pte/pmd/pud copies.

> 	if (pfn_t_devmap(pfn))
> 		entry = pte_mkdevmap(pfn_t_pte(pfn, prot));
> 	else
> 		entry = pte_mkspecial(pfn_t_pte(pfn, prot));
> 
> It might change for sure if Alistair move on with the devmap work, though..
> these two always are processed together now, so I hope that won't add much
> burden which series will land first, then we may need some care on merging
> them.  I don't expect anything too tricky in merge if that was about
> removal of the devmap bits.

Removing pte_mkdevmap can only make things simpler :)

Jason