linux-kernel - Re: [PATCH v9 10/21] mm: Add generic p?d

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Date:   Mon, 29 Jul 2019 12:38:44 +0100
From:   Steven Price <steven.price@....com>
To:     Anshuman Khandual <anshuman.khandual@....com>,
        Mark Rutland <mark.rutland@....com>
Cc:     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, linux-mm@...ck.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>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-arm-kernel@...ts.infradead.org,
        "Liang, Kan" <kan.liang@...ux.intel.com>
Subject: Re: [PATCH v9 10/21] mm: Add generic p?d_leaf() macros

On 28/07/2019 12:44, Anshuman Khandual wrote:
> 
> 
> On 07/23/2019 03:11 PM, Mark Rutland wrote:
>> On Mon, Jul 22, 2019 at 04:41:59PM +0100, Steven Price wrote:
>>> 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 that that level.
>>>
>>> Signed-off-by: Steven Price <steven.price@....com>
>>
>> Not a big deal, but it would probably make sense for this to be patch 1
>> in the series, given it defines the semantic of p?d_leaf(), and they're
>> not used until we provide all the architectural implemetnations anyway.
> 
> Agreed.
> 
>>
>> It might also be worth pointing out the reasons for this naming, e.g.
>> p?d_large() aren't currently generic, and this name minimizes potential
>> confusion between p?d_{large,huge}().
> 
> Agreed. But these fallback also need to first check non-availability of large
> pages. I am not sure whether CONFIG_HUGETLB_PAGE config being clear indicates
> that conclusively or not. Being a page table leaf entry has a broader meaning
> than a large page but that is really not the case today. All leaf entries here
> are large page entries from MMU perspective. This dependency can definitely be
> removed when there are other types of leaf entries but for now IMHO it feels
> bit problematic not to directly associate leaf entries with large pages in
> config restriction while doing exactly the same.

The intention here is that the page walkers are able to walk any type of
page table entry which the kernel may use. CONFIG_HUGETLB_PAGE only
controls whether "huge TLB pages" are used by user space processes. It's
quite possible that option to not be selected but the linear mapping to
have been mapped using "large pages" (i.e. leaf entries further up the
tree than normal).

One of the goals was to avoid tying the new functions to a configuration
option but instead match the hardware architecture. Of course this isn't
possible in the most general case (e.g. an architecture may have
multiple hardware page table formats). But to the extent that other
functions like p?d_none() work the desire is that p?d_leaf() should also
work.

Steve