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

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Date:   Sat, 21 Dec 2019 21:35:58 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Steven Price <steven.price@....com>,
        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

Steven Price <steven.price@....com> writes:
> 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.

We handle that usually just with eg:

static inline bool pgd_leaf(pgd_t pgd)
{
	...
}
#define pgd_leaf pgd_leaf

> I guess a static inline might avoid warnings although I haven't seen
> any.

If anything I'd expect it to cause warnings, for example if someone is
doing pgd_leaf(pmd), but that would be good to catch.

cheers