linux-kernel - Re: [PATCH v17 01/23] mm: Add generic p?d

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Date:   Fri, 20 Dec 2019 11:48:32 +0000
From:   Steven Price <steven.price@....com>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org
Cc:     Mark Rutland <mark.rutland@....com>, x86@...nel.org,
        Arnd Bergmann <arnd@...db.de>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        linux-kernel@...r.kernel.org,
        Jérôme Glisse <jglisse@...hat.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        James Morse <james.morse@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org,
        "Liang, Kan" <kan.liang@...ux.intel.com>
Subject: Re: [PATCH v17 01/23] mm: Add generic p?d_leaf() macros

On 19/12/2019 11:43, Michael Ellerman wrote:
> Steven Price <steven.price@....com> writes:
>> Exposing the pud/pgd levels of the page tables to walk_page_range() means
>> we may come across the exotic large mappings that come with large areas
>> of contiguous memory (such as the kernel's linear map).
>>
>> For architectures that don't provide all p?d_leaf() macros, provide
>> generic do nothing default that are suitable where there cannot be leaf
>> pages at that level. Futher patches will add implementations for
>> individual architectures.
>>
>> The name p?d_leaf() is chosen to minimize the confusion with existing
>> uses of "large" pages and "huge" pages which do not necessary mean that
>> the entry is a leaf (for example it may be a set of contiguous entries
>> that only take 1 TLB slot). For the purpose of walking the page tables
>> we don't need to know how it will be represented in the TLB, but we do
>> need to know for sure if it is a leaf of the tree.
>>
>> Signed-off-by: Steven Price <steven.price@....com>
>> Acked-by: Mark Rutland <mark.rutland@....com>
>> ---
>>  include/asm-generic/pgtable.h | 20 ++++++++++++++++++++
>>  1 file changed, 20 insertions(+)
>>
>> diff --git a/include/asm-generic/pgtable.h b/include/asm-generic/pgtable.h
>> index 798ea36a0549..e2e2bef07dd2 100644
>> --- a/include/asm-generic/pgtable.h
>> +++ b/include/asm-generic/pgtable.h
>> @@ -1238,4 +1238,24 @@ static inline bool arch_has_pfn_modify_check(void)
>>  #define mm_pmd_folded(mm)	__is_defined(__PAGETABLE_PMD_FOLDED)
>>  #endif
>>  
>> +/*
>> + * p?d_leaf() - true if this entry is a final mapping to a physical address.
>> + * This differs from p?d_huge() by the fact that they are always available (if
>> + * the architecture supports large pages at the appropriate level) even
>> + * if CONFIG_HUGETLB_PAGE is not defined.
>> + * Only meaningful when called on a valid entry.
>> + */
>> +#ifndef pgd_leaf
>> +#define pgd_leaf(x)	0
>> +#endif
>> +#ifndef p4d_leaf
>> +#define p4d_leaf(x)	0
>> +#endif
>> +#ifndef pud_leaf
>> +#define pud_leaf(x)	0
>> +#endif
>> +#ifndef pmd_leaf
>> +#define pmd_leaf(x)	0
>> +#endif
> 
> Any reason you made these #defines rather than static inlines?

No strong reason - but these have to be #defines in the arch overrides
so the #ifndef works, so I was being consistent here. I guess a static
inline might avoid warnings although I haven't seen any.

Steve