linux-kernel - Re: [PATCH v5 10/19] mm: pagewalk: Add p4d_entry() and pgd

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Date:   Fri, 22 Mar 2019 10:37:14 +0000
From:   Steven Price <steven.price@....com>
To:     Mike Rapoport <rppt@...ux.ibm.com>
Cc:     Mark Rutland <Mark.Rutland@....com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Arnd Bergmann <arnd@...db.de>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Catalin Marinas <catalin.marinas@....com>, x86@...nel.org,
        Will Deacon <will.deacon@....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>,
        linux-arm-kernel@...ts.infradead.org,
        "Liang, Kan" <kan.liang@...ux.intel.com>
Subject: Re: [PATCH v5 10/19] mm: pagewalk: Add p4d_entry() and pgd_entry()

On 22/03/2019 10:29, Mike Rapoport wrote:
> On Fri, Mar 22, 2019 at 10:11:59AM +0000, Steven Price wrote:
>> On 21/03/2019 21:15, Mike Rapoport wrote:
>>> On Thu, Mar 21, 2019 at 02:19:44PM +0000, Steven Price wrote:
>>>> pgd_entry() and pud_entry() were removed by commit 0b1fbfe50006c410
>>>> ("mm/pagewalk: remove pgd_entry() and pud_entry()") because there were
>>>> no users. We're about to add users so reintroduce them, along with
>>>> p4d_entry() as we now have 5 levels of tables.
>>>>
>>>> Note that commit a00cc7d9dd93d66a ("mm, x86: add support for
>>>> PUD-sized transparent hugepages") already re-added pud_entry() but with
>>>> different semantics to the other callbacks. Since there have never
>>>> been upstream users of this, revert the semantics back to match the
>>>> other callbacks. This means pud_entry() is called for all entries, not
>>>> just transparent huge pages.
>>>>
>>>> Signed-off-by: Steven Price <steven.price@....com>
>>>> ---
>>>>  include/linux/mm.h |  9 ++++++---
>>>>  mm/pagewalk.c      | 27 ++++++++++++++++-----------
>>>>  2 files changed, 22 insertions(+), 14 deletions(-)
>>>>
>>>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>>>> index 76769749b5a5..2983f2396a72 100644
>>>> --- a/include/linux/mm.h
>>>> +++ b/include/linux/mm.h
>>>> @@ -1367,10 +1367,9 @@ void unmap_vmas(struct mmu_gather *tlb, struct vm_area_struct *start_vma,
>>>>
>>>>  /**
>>>>   * mm_walk - callbacks for walk_page_range
>>>> + * @pgd_entry: if set, called for each non-empty PGD (top-level) entry
>>>> + * @p4d_entry: if set, called for each non-empty P4D (1st-level) entry
>>>
>>> IMHO, p4d implies the 4th level :)
>>
>> You have a good point there... I was simply working back from the
>> existing definitions (below) of PTE:4th, PMD:3rd, PUD:2nd. But it's
>> already somewhat broken by PGD:0th and my cop-out was calling it "top".
>>
>>> I think it would make more sense to start counting from PTE rather than
>>> from PGD. Then it would be consistent across architectures with fewer
>>> levels.
>>
>> It would also be the opposite way round to architectures such as Arm
>> which number their levels, for example [1] refers to levels 0-3 (with 3
>> being PTE in Linux terms).
> 
> By consistent I meant that for architectures with fewer levels we won't be
> describing PTE as level 4 when the architecture only has 2 levels.

Ah I see, although we've apparently been doing that for over a decade
already[2] :)

[2]
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=e6473092bd9116583ce9ab8cf1b6570e1aa6fc83

>> [1]
>> https://developer.arm.com/docs/100940/latest/translation-tables-in-armv8-a
>>
>> Probably the least confusing thing is to drop the level numbers in
>> brackets since I don't believe they directly match any architecture, and
>> hopefully any user of the page walking code is already familiar with the
>> P?D terms used by the kernel.
> 
> That's a fair assumption :)
> Still, maybe we keep your (top-level) for PGD and use (lowest level) for
> PTE and drop those in the middle?

Yes that's a good compromise.

Thanks,

Steve