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

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Date:   Wed, 20 Feb 2019 04:24:18 -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 Wed, Feb 20, 2019 at 03:57:59PM +0530, Anshuman Khandual wrote:
> On 02/20/2019 03:58 AM, Yu Zhao wrote:
> > On Tue, Feb 19, 2019 at 11:47:12AM +0530, Anshuman Khandual wrote:
> >> 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")
> >>> 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")
> > 
> > Well, I was just trying to clarify how the ctor is meant to be used.
> > The rational behind it is probably another topic.
> > 
> > For starters, the number of pmd/pud/p4d/pgd is at least two orders
> > of magnitude less than the number of pte, which makes them almost
> > negligible. And some archs use kmem for them, so it's infeasible to
> > SetPageTable on or account them in the way the ctor does on those
> > archs.
> 
> I understand the kmem cases which are definitely problematic and should
> be fixed. IIRC there is a mechanism to custom init pages allocated for
> slab cache with a ctor function which in turn can call pgtable_page_ctor().
> But destructor helper support for slab has been dropped I guess.

You can't put a spinlock in the struct page if the page is allocated
through slab.  Slab uses basically all of struct page for its own
purposes.  I tried to make that clear with the new layout of struct
page where everything's in a union discriminated by what the page is
allocated for.