linux-kernel - Re: [PATCH] arm64/mm: Remove [PUD|PMD]_TABLE_BIT from [pud|pmd]

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Message-ID: <20210511140708.GC8933@C02TD0UTHF1T.local>
Date:   Tue, 11 May 2021 15:07:08 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     linux-arm-kernel@...ts.infradead.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64/mm: Remove [PUD|PMD]_TABLE_BIT from [pud|pmd]_bad()

On Tue, May 11, 2021 at 09:21:46AM +0530, Anshuman Khandual wrote:
> 
> On 5/10/21 8:13 PM, Mark Rutland wrote:
> > On Mon, May 10, 2021 at 04:37:51PM +0530, Anshuman Khandual wrote:
> >> Semantics wise, [pud|pmd]_bad() have always implied that a given [PUD|PMD]
> >> entry does not have a pointer to the next level page table. This had been
> >> made clear in the commit a1c76574f345 ("arm64: mm: use *_sect to check for
> >> section maps"). Hence explicitly check for a table entry rather than just
> >> testing a single bit. This basically redefines [pud|pmd]_bad() in terms of
> >> [pud|pmd]_table() making the semantics clear.
> >>
> >> Cc: Catalin Marinas <catalin.marinas@....com>
> >> Cc: Will Deacon <will@...nel.org>
> >> Cc: Mark Rutland <mark.rutland@....com>
> >> Cc: linux-arm-kernel@...ts.infradead.org
> >> Cc: linux-kernel@...r.kernel.org
> >> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
> > 
> > I have no strong feelings either way, so: 
> > 
> > Acked-by: Mark Rutland <mark.rutland@....com>
> > 
> > ... that said, I think that the "bad" naming is unclear and misleading,
> > and it'd be really nice if we could clean that up treewide with
> > something clearer than "bad".
> 
> Agreed, the name is misleading.
> 
> > It does seem that would roughly fit p??_leaf() if we had
> 
> But what if the platform does not support huge page aka leaf mapping
> at the given level ? Also a non table i.e bad entry might not always
> mean a leaf/section/huge page mapping, it could simply imply that the
> entry is not just pointing to next level and might be just in an bad
> intermediate or invalid state.

Ah, so that's also covering swap entries, too? It's not entirely clear
to me what "bad intermediate or invalid state" means, because I assume
it's not arbitrary junk or this wouldn't be sound genrally.

I had assumed it was only covering *valid* non-table entries.

> > p??_clear_leaf() and p??_none_or_clear_leaf() helpers.
> 
> Could you please elaborate how these new helpers might help define
> pxx_bad() ?

This was based on my (evidently wrong) prior understanding above.

Thanks,
Mark.