linux-kernel - Re: [RFC 0/4] arm64/mm: Clean up pte

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Date:   Mon, 10 Jul 2023 07:50:30 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     David Hildenbrand <david@...hat.com>,
        linux-arm-kernel@...ts.infradead.org
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Ryan Roberts <ryan.roberts@....com>,
        Mark Rutland <mark.rutland@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org
Subject: Re: [RFC 0/4] arm64/mm: Clean up pte_dirty() state management



On 7/7/23 17:41, David Hildenbrand wrote:
> On 07.07.23 07:33, Anshuman Khandual wrote:
>> These pte_dirty() changes make things explicitly clear, while improving the
>> code readability. This optimizes HW dirty state transfer into SW dirty bit.
>> This also adds a new arm64 documentation explaining overall pte dirty state
>> management in detail. This series applies on the latest mainline kernel.
>>
>>
> 
> I skimmed over most of the series, and I am not convinced that this is actually a cleanup. If we cannot really always differentiate between sw/hw clearing, why have separate primitives that give one the illusion that it could be done and that they are two different concepts?

These are indeed two different concepts working together, the current code just
obscures that. Without these primitives it's even hard to follow how the SW and
HW dirty parts are intertwined in implementing the generic pte_dirty() state.

The current code acknowledges these two different concepts in identifying them
i.e via pte_hw_dirty() and pte_sw_dirty().

#define pte_hw_dirty(pte)       (pte_write(pte) && !(pte_val(pte) & PTE_RDONLY))
#define pte_sw_dirty(pte)       (!!(pte_val(pte) & PTE_DIRTY))

But then falls short in demonstrating how these two states are being managed i.e
created and cleared. The first patch tries to clear the situation to some extent.

> 
> Maybe there are other opinions, but at least for me this made the code harder to grasp.
> 
> I do appreciate a doc update, though :)
These two changes also streamline HW dirty bit saving in SW dirty bit efficiently,
skipping the re-doing of HW dirty bits setting which might be cleared off. That is
the primary reason for saving it off in SW dirty bit in the first place.

  arm64/mm: Call pte_sw_mkdirty() while preserving the HW dirty state
  arm64/mm: Add pte_preserve_hw_dirty(

Regardless, I am still trying to understand how the first patch does not improve
the clarity in modifying the PTE dirty state.