Messages in this thread | | | Date | Fri, 14 May 2021 11:59:02 +0100 | From | Catalin Marinas <> | Subject | Re: [PATCH] arm64/mm: Remove [PUD|PMD]_TABLE_BIT from [pud|pmd]_bad() |
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On Thu, May 13, 2021 at 10:44:04AM +0530, Anshuman Khandual wrote: > On 5/11/21 7:37 PM, Mark Rutland wrote: > > 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@arm.com> > >>>> Cc: Will Deacon <will@kernel.org> > >>>> Cc: Mark Rutland <mark.rutland@arm.com> > >>>> Cc: linux-arm-kernel@lists.infradead.org > >>>> Cc: linux-kernel@vger.kernel.org > >>>> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com> > >>> > >>> I have no strong feelings either way, so: > >>> > >>> Acked-by: Mark Rutland <mark.rutland@arm.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. > > Intermediate states like swap, migration or probably even splitting THP. > Though I am not really sure whether pxx_bad() only gets used for valid > table or leaf entries i.e things which are mapped. Hence checking just > for non table entry is better and even safer, than looking out for what > other states the entry could be in.
I had a quick look through some of the uses and it seems the expectation is that after a !pmd_bad(), the pmd is a table. The checks for migration, huge page etc. are prior to the pmd_bad() check.
For this patch:
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
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