linux-kernel - Re: [PATCH v1 4/7] arm64/pgtable: support __HAVE_ARCH_PTE_SWP

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Message-ID: <YjN2qsXkmlEUTg4u@arm.com>
Date:   Thu, 17 Mar 2022 17:58:02 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        John Hubbard <jhubbard@...dia.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Yang Shi <shy828301@...il.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
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        x86@...nel.org, linux-arm-kernel@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org
Subject: Re: [PATCH v1 4/7] arm64/pgtable: support
 __HAVE_ARCH_PTE_SWP_EXCLUSIVE

On Thu, Mar 17, 2022 at 11:04:18AM +0100, David Hildenbrand wrote:
> On 16.03.22 19:27, Catalin Marinas wrote:
> > On Tue, Mar 15, 2022 at 03:18:34PM +0100, David Hildenbrand wrote:
> >> @@ -909,12 +925,13 @@ static inline pmd_t pmdp_establish(struct vm_area_struct *vma,
> >>  /*
> >>   * Encode and decode a swap entry:
> >>   *	bits 0-1:	present (must be zero)
> >> - *	bits 2-7:	swap type
> >> + *	bits 2:		remember PG_anon_exclusive
> >> + *	bits 3-7:	swap type
> >>   *	bits 8-57:	swap offset
> >>   *	bit  58:	PTE_PROT_NONE (must be zero)
> > 
> > I don't remember exactly why we reserved bits 0 and 1 when, from the
> > hardware perspective, it's sufficient for bit 0 to be 0 and the whole
> > pte becomes invalid. We use bit 1 as the 'table' bit (when 0 at pmd
> > level, it's a huge page) but we shouldn't check for this on a swap
> > entry.
> 
> You mean
> 
> arch/arm64/include/asm/pgtable-hwdef.h:#define PTE_TABLE_BIT            (_AT(pteval_t, 1) << 1)
> 
> right?

Yes.

> I wonder why it even exists, for arm64 I only spot:
> 
> arch/arm64/include/asm/pgtable.h:#define pte_mkhuge(pte)                (__pte(pte_val(pte) & ~PTE_TABLE_BIT))
> 
> I don't really see code that sets PTE_TABLE_BIT.
> 
> Similarly, I don't see code that sets PMD_TABLE_BIT/PUD_TABLE_BIT/P4D_TABLE_BIT.
> Most probably setting code is not using the defines,  that's why I'm not finding it.

It gets set as part of P*D_TYPE_TABLE via p*d_populate(). We use the
P*D_TABLE_BIT mostly for checking whether it's a huge page or not (the
arm64 hugetlbpage.c code).

-- 
Catalin