linux-kernel - Re: [PATCH v9 12/21] mm: pagewalk: Allow walking without vma

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Date:   Sun, 28 Jul 2019 19:50:02 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Steven Price <steven.price@....com>, linux-mm@...ck.org
Cc:     Andy Lutomirski <luto@...nel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        James Morse <james.morse@....com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Mark Rutland <Mark.Rutland@....com>,
        "Liang, Kan" <kan.liang@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v9 12/21] mm: pagewalk: Allow walking without vma



On 07/22/2019 09:12 PM, Steven Price wrote:
> Since 48684a65b4e3: "mm: pagewalk: fix misbehavior of walk_page_range
> for vma(VM_PFNMAP)", page_table_walk() will report any kernel area as
> a hole, because it lacks a vma.
> 
> This means each arch has re-implemented page table walking when needed,
> for example in the per-arch ptdump walker.
> 
> Remove the requirement to have a vma except when trying to split huge
> pages.
> 
> Signed-off-by: Steven Price <steven.price@....com>
> ---
>  mm/pagewalk.c | 25 +++++++++++++++++--------
>  1 file changed, 17 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/pagewalk.c b/mm/pagewalk.c
> index 98373a9f88b8..1cbef99e9258 100644
> --- a/mm/pagewalk.c
> +++ b/mm/pagewalk.c
> @@ -36,7 +36,7 @@ static int walk_pmd_range(pud_t *pud, unsigned long addr, unsigned long end,
>  	do {
>  again:
>  		next = pmd_addr_end(addr, end);
> -		if (pmd_none(*pmd) || !walk->vma) {
> +		if (pmd_none(*pmd)) {
>  			if (walk->pte_hole)
>  				err = walk->pte_hole(addr, next, walk);
>  			if (err)
> @@ -59,9 +59,14 @@ static int walk_pmd_range(pud_t *pud, unsigned long addr, unsigned long end,
>  		if (!walk->pte_entry)
>  			continue;
>  
> -		split_huge_pmd(walk->vma, pmd, addr);
> -		if (pmd_trans_unstable(pmd))
> -			goto again;
> +		if (walk->vma) {
> +			split_huge_pmd(walk->vma, pmd, addr);

Check for a PMD THP entry before attempting to split it ?

> +			if (pmd_trans_unstable(pmd))
> +				goto again;
> +		} else if (pmd_leaf(*pmd)) {
> +			continue;
> +		}
> +
>  		err = walk_pte_range(pmd, addr, next, walk);
>  		if (err)
>  			break;
> @@ -81,7 +86,7 @@ static int walk_pud_range(p4d_t *p4d, unsigned long addr, unsigned long end,
>  	do {
>   again:
>  		next = pud_addr_end(addr, end);
> -		if (pud_none(*pud) || !walk->vma) {
> +		if (pud_none(*pud)) {
>  			if (walk->pte_hole)
>  				err = walk->pte_hole(addr, next, walk);
>  			if (err)
> @@ -95,9 +100,13 @@ static int walk_pud_range(p4d_t *p4d, unsigned long addr, unsigned long end,
>  				break;
>  		}
>  
> -		split_huge_pud(walk->vma, pud, addr);
> -		if (pud_none(*pud))
> -			goto again;
> +		if (walk->vma) {
> +			split_huge_pud(walk->vma, pud, addr);

Check for a PUD THP entry before attempting to split it ?

> +			if (pud_none(*pud))
> +				goto again;
> +		} else if (pud_leaf(*pud)) {
> +			continue;
> +		}

This is bit cryptic. walk->vma check should be inside a helper is_user_page_table()
or similar to make things clear. p4d_leaf() check missing in walk_p4d_range() for
kernel page table walk ? Wondering if p?d_leaf() test should be moved earlier while
calling p?d_entry() for kernel page table walk.