linux-kernel - Re: [PATCH 00/19] mm: Support huge pfnmaps

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Message-ID: <6313b27c-342f-4c7e-bba4-085f2ca2cd2b@redhat.com>
Date: Fri, 9 Aug 2024 20:12:48 +0200
From: David Hildenbrand <david@...hat.com>
To: Peter Xu <peterx@...hat.com>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Cc: Sean Christopherson <seanjc@...gle.com>,
 Oscar Salvador <osalvador@...e.de>, Jason Gunthorpe <jgg@...dia.com>,
 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 00/19] mm: Support huge pfnmaps

On 09.08.24 18:08, Peter Xu wrote:
> Overview
> ========
> 
> This series is based on mm-unstable, commit 98808d08fc0f of Aug 7th latest,
> plus dax 1g fix [1].  Note that this series should also apply if without
> the dax 1g fix series, but when without it, mprotect() will trigger similar
> errors otherwise on PUD mappings.
> 
> This series implements huge pfnmaps support for mm in general.  Huge pfnmap
> allows e.g. VM_PFNMAP vmas to map in either PMD or PUD levels, similar to
> what we do with dax / thp / hugetlb so far to benefit from TLB hits.  Now
> we extend that idea to PFN mappings, e.g. PCI MMIO bars where it can grow
> as large as 8GB or even bigger.
> 
> Currently, only x86_64 (1G+2M) and arm64 (2M) are supported.  The last
> patch (from Alex Williamson) will be the first user of huge pfnmap, so as
> to enable vfio-pci driver to fault in huge pfn mappings.
> 
> Implementation
> ==============
> 
> In reality, it's relatively simple to add such support comparing to many
> other types of mappings, because of PFNMAP's specialties when there's no
> vmemmap backing it, so that most of the kernel routines on huge mappings
> should simply already fail for them, like GUPs or old-school follow_page()
> (which is recently rewritten to be folio_walk* APIs by David).

Indeed, skimming most patches, there is very limit core-mm impact. I 
expected much more :)

I suspect primarily because DAX already paved the way. And as DAX likely 
supports fault-after-fork, which is why the fork() case wasn't relevant 
before.

-- 
Cheers,

David / dhildenb