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

From: Yu Zhao <[email-masked]>
Date: Mon, 18 Feb 2019 22:32:05 -0700

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'm sure if we go back further, we can find similar stories: we
don't set PageTable on page tables other than pte; and we don't
account page tables other than pte. I don't have any objection if
you want change these two. But please make sure they are consistent
across all archs.

> We should not skip it for any page table page.

In fact, calling it on pmd/pud/p4d is peculiar, and may even be
considered wrong. AFAIK, no other arch does so.

> As stated before pgtable_pmd_page_ctor() is not a replacement for
> pgtable_page_ctor().

pgtable_pmd_page_ctor() must be used on user pmd. For kernel pmd,
it's okay to use pgtable_page_ctor() instead only because kernel
doesn't have thp.

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