linux-kernel - Re: [PATCH v2 1/3] arm64: mm: use appropriate ctors for page tables

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Date:   Tue, 19 Feb 2019 17:34:07 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     Yu Zhao <yuzhao@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Nick Piggin <npiggin@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Mark Rutland <mark.rutland@....com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Chintan Pandya <cpandya@...eaurora.org>,
        Jun Yao <yaojun8558363@...il.com>,
        Laura Abbott <labbott@...hat.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2 1/3] arm64: mm: use appropriate ctors for page tables

On Tue, Feb 19, 2019 at 11:47:12AM +0530, Anshuman Khandual wrote:
> + Matthew Wilcox
> On 02/19/2019 11:02 AM, Yu Zhao wrote:
> > On Tue, Feb 19, 2019 at 09:51:01AM +0530, Anshuman Khandual wrote:
> >>
> >>
> >> On 02/19/2019 04:43 AM, Yu Zhao wrote:
> >>> For pte page, use pgtable_page_ctor(); for pmd page, use
> >>> pgtable_pmd_page_ctor() if not folded; and for the rest (pud,
> >>> p4d and pgd), don't use any.
> >> pgtable_page_ctor()/dtor() is not optional for any level page table page
> >> as it determines the struct page state and zone statistics.
> > 
> > This is not true. pgtable_page_ctor() is only meant for user pte
> > page. The name isn't perfect (we named it this way before we had
> > split pmd page table lock, and never bothered to change it).
> > 
> > The commit cccd843f54be ("mm: mark pages in use for page tables")

Where did you get that commit ID from?  In Linus' tree, it's
1d40a5ea01d53251c23c7be541d3f4a656cfc537

> > clearly states so:
> >   Note that only pages currently accounted as NR_PAGETABLES are
> >   tracked as PageTable; this does not include pgd/p4d/pud/pmd pages.
> 
> I think the commit is the following one and it does say so. But what is
> the rationale of tagging only PTE page as PageTable and updating the zone
> stat but not doing so for higher level page table pages ? Are not they
> used as page table pages ? Should not they count towards NR_PAGETABLE ?
> 
> 1d40a5ea01d53251c ("mm: mark pages in use for page tables")

I think they should all be accounted towards NR_PAGETABLE and marked
as being PageTable.  Somebody needs to make the case for that and
send the patches.  That patch even says that there should be follow-up
patches to do that.  I've been a little busy and haven't got back to it.
I thought you said you were going to do it.

> pgtable_page_ctor/dtor() use across arch is not consistent and there is a need
> for generalization which has been already acknowledged earlier. But for now we
> can atleast fix this on arm64.
> 
> https://lore.kernel.org/lkml/1547619692-7946-1-git-send-email-anshuman.khandual@arm.com/

... were you not listening when you were told that was completely
inadequate?