linux-kernel - Re: [PATCH v2 3/4] arm64/mm: Elide tlbi in contpte

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Message-ID: <20250303115244.GB74129@e133081.arm.com>
Date: Mon, 3 Mar 2025 11:52:44 +0000
From: Mikołaj Lenczewski <miko.lenczewski@....com>
To: David Hildenbrand <david@...hat.com>
Cc: ryan.roberts@....com, suzuki.poulose@....com,
	yang@...amperecomputing.com, catalin.marinas@....com,
	will@...nel.org, joro@...tes.org, jean-philippe@...aro.org,
	mark.rutland@....com, joey.gouly@....com, oliver.upton@...ux.dev,
	james.morse@....com, broonie@...nel.org, maz@...nel.org,
	akpm@...ux-foundation.org, jgg@...pe.ca, nicolinc@...dia.com,
	mshavit@...gle.com, jsnitsel@...hat.com, smostafa@...gle.com,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	iommu@...ts.linux.dev
Subject: Re: [PATCH v2 3/4] arm64/mm: Elide tlbi in contpte_convert() under
 BBML2

> > I think I understand what you mean. I agree that it is possible for a
> > concurrent walk to see an invalid TLBI state, if it is on the same TLB
> > that the repaint is happening on. For other TLBs, the flush has not yet
> > propagated our invalidated PTEs (from `__ptep_get_and_clear()`) though?
> 
> What I am saying is: if there is no TLB entry yet, HW will walk the page
> table to find no present PTE and trigger a fault.

Yes, that is 100% correct. I believe that this is unavoidable.

> > That invalidation will only be seen by other TLBs after the
> > `__flush_tlb_range()`, so we should save a few faults because only
> > "local" threads will ever see the invalid entry, as opposed to all
> > threads that try to read our modified range?
> 
> So what you say is, that deferring the flush means that if there is already
> a TLB entry, flushing deferred reduces the likelihood that a page table walk
> is triggered that could find no present PTE: consequently, reducing the
> likelihood that a page fault is triggered.
> 
> (I use the word likelihood, because I assume other action could result in a
> TLB entry getting flushed in the meantime, such as TLB entry reuse)
> 
> Correct?

Yes, and your language here is clearer than the original commit message
(and cover letter). Will amend it to be closer to your wording.

-- 
Kind regards,
Mikołaj Lenczewski