linux-kernel - Re: [PATCH] arm64/mm: adds soft dirty page tracking

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Date: Tue, 12 Mar 2024 10:26:09 +0000
From: Joey Gouly <joey.gouly@....com>
To: David Hildenbrand <david@...hat.com>
Cc: Shivansh Vij <shivanshvij@...look.com>, shivanshvij@...pholelabs.io,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>, Ryan Roberts <ryan.roberts@....com>,
	Mark Rutland <mark.rutland@....com>, Marc Zyngier <maz@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kemeng Shi <shikemeng@...weicloud.com>,
	"Mike Rapoport (IBM)" <rppt@...nel.org>,
	"Matthew Wilcox (Oracle)" <willy@...radead.org>,
	Anshuman Khandual <anshuman.khandual@....com>,
	James Houghton <jthoughton@...gle.com>,
	Rick Edgecombe <rick.p.edgecombe@...el.com>,
	Peter Collingbourne <pcc@...gle.com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64/mm: adds soft dirty page tracking

On Tue, Mar 12, 2024 at 09:22:25AM +0100, David Hildenbrand wrote:
> On 12.03.24 02:16, Shivansh Vij wrote:
> 
> Hi,
> 
> > Checkpoint-Restore in Userspace (CRIU) needs to be able
> > to track a memory page's changes if we want to enable
> > pre-dumping, which is important for live migrations.
> > 
> > The PTE_DIRTY bit (defined in pgtable-prot.h) is already
> > used to track software dirty pages, and the PTE_WRITE and
> > PTE_READ bits are used to track hardware dirty pages.
> > 
> > This patch enables full soft dirty page tracking
> > (including swap PTE support) for arm64 systems, and is
> > based very closely on the x86 implementation.
> > 
> > It is based on an unfinished patch by
> > Bin Lu (bin.lu@....com) from 2017
> > (https://patchwork.kernel.org/project/linux-arm-kernel/patch/1512029649-61312-1-git-send-email-bin.lu@arm.com/),
> > but has been updated for newer 6.x kernels as well as
> > tested on various 5.x kernels.
> 
> There has also been more recently:
> 
> https://lore.kernel.org/lkml/20230703135526.930004-1-npache@redhat.com/#r
> 
> I recall that we are short on SW PTE bits:
> 
> "
> So if you need software dirty, it can only be done with another software
> PTE bit. The problem is that we are short of such bits (only one left if
> we move PTE_PROT_NONE to a different location). The userfaultfd people
> also want such bit.
> 
> Personally I'd reuse the four PBHA bits but I keep hearing that they may
> be used with some out of tree patches.
> "
> 
> https://lore.kernel.org/lkml/ZLQIaSMI74KpqsQQ@arm.com/
> 

I have some patches on the list (Permission Overlay) that also uses bit 60

	series: https://lore.kernel.org/linux-arm-kernel/20231124163510.1835740-1-joey.gouly@arm.com/
	commit: https://lore.kernel.org/linux-arm-kernel/20231124163510.1835740-9-joey.gouly@arm.com/

I will be sending out a v4 of that in several weeks.

Thanks,
Joey