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

From: Anshuman Khandual <anshuman.khandual@arm.com>
To: Steven Price <steven.price@arm.com>, linux-mm@kvack.org
Cc: "Andy Lutomirski" <luto@kernel.org>,
	"Ard Biesheuvel" <ard.biesheuvel@linaro.org>,
	"Arnd Bergmann" <arnd@arndb.de>, "Borislav Petkov" <bp@alien8.de>,
	"Catalin Marinas" <catalin.marinas@arm.com>,
	"Dave Hansen" <dave.hansen@linux.intel.com>,
	"Ingo Molnar" <mingo@redhat.com>,
	"James Morse" <james.morse@arm.com>,
	"Jérôme Glisse" <jglisse@redhat.com>,
	"Peter Zijlstra" <peterz@infradead.org>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Will Deacon" <will@kernel.org>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org,
	"Mark Rutland" <Mark.Rutland@arm.com>,
	"Liang, Kan" <kan.liang@linux.intel.com>,
	"Andrew Morton" <akpm@linux-foundation.org>
Subject: Re: [PATCH v9 12/21] mm: pagewalk: Allow walking without vma
Date: Sun, 28 Jul 2019 19:50:02 +0530	[thread overview]
Message-ID: <7fc50563-7d5d-7270-5a6a-63769e9c335a@arm.com> (raw)
In-Reply-To: <20190722154210.42799-13-steven.price@arm.com>

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@arm.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.