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

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Message-ID: <Zr0_Y7tQ-fBMKxKH@linux.dev>
Date: Wed, 14 Aug 2024 16:36:03 -0700
From: Oliver Upton <oliver.upton@...ux.dev>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: Sean Christopherson <seanjc@...gle.com>, Peter Xu <peterx@...hat.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	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>, Marc Zyngier <maz@...nel.org>
Subject: Re: [PATCH 00/19] mm: Support huge pfnmaps

On Wed, Aug 14, 2024 at 07:10:31PM -0300, Jason Gunthorpe wrote:

[...]

> > Nope.  KVM ARM does (see get_vma_page_shift()) but I strongly suspect that's only
> > a win in very select use cases, and is overall a non-trivial loss.  
> 
> Ah that ARM behavior was probably what was being mentioned then! So
> take my original remark as applying to this :)
> 
> > > I don't quite understand your safety argument, if the VMA has 1G of
> > > contiguous physical memory described with 4K it is definitely safe for
> > > KVM to reassemble that same memory and represent it as 1G.
> >
> > That would require taking mmap_lock to get the VMA, which would be a net negative,
> > especially for workloads that are latency sensitive.
> 
> You can aggregate if the read and aggregating logic are protected by
> mmu notifiers, I think. A invalidation would still have enough
> information to clear the aggregate shadow entry. If you get a sequence
> number collision then you'd throw away the aggregation.
> 
> But yes, I also think it would be slow to have aggregation logic in
> KVM. Doing in the main mmu is much better.

+1.

For KVM/arm64 I'm quite hesitant to change the behavior to PTE mappings
in this situation (i.e. dump get_vma_page_shift()), as I'm quite certain
that'll have a performance regression on someone's workload. But once we
can derive huge PFNMAP from the primary MMU then we should just normalize
on that.

-- 
Thanks,
Oliver