Re: [PATCH v9 13/21] mm: pagewalk: Add test_p?d callbacks

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 13/21] mm: pagewalk: Add test_p?d callbacks
Date: Sun, 28 Jul 2019 19:11:00 +0530	[thread overview]
Message-ID: <b74e545f-cbe0-9dd0-004c-5919e5cabb6f@arm.com> (raw)
In-Reply-To: <20190722154210.42799-14-steven.price@arm.com>

On 07/22/2019 09:12 PM, Steven Price wrote:
> It is useful to be able to skip parts of the page table tree even when
> walking without VMAs. Add test_p?d callbacks similar to test_walk but
> which are called just before a table at that level is walked. If the
> callback returns non-zero then the entire table is skipped.
> 
> Signed-off-by: Steven Price <steven.price@arm.com>
> ---
>  include/linux/mm.h | 11 +++++++++++
>  mm/pagewalk.c      | 24 ++++++++++++++++++++++++
>  2 files changed, 35 insertions(+)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index b22799129128..325a1ca6f820 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1447,6 +1447,11 @@ void unmap_vmas(struct mmu_gather *tlb, struct vm_area_struct *start_vma,
>   *             value means "do page table walk over the current vma,"
>   *             and a negative one means "abort current page table walk
>   *             right now." 1 means "skip the current vma."
> + * @test_pmd:  similar to test_walk(), but called for every pmd.
> + * @test_pud:  similar to test_walk(), but called for every pud.
> + * @test_p4d:  similar to test_walk(), but called for every p4d.
> + *             Returning 0 means walk this part of the page tables,
> + *             returning 1 means to skip this range.
>   * @mm:        mm_struct representing the target process of page table walk
>   * @vma:       vma currently walked (NULL if walking outside vmas)
>   * @private:   private data for callbacks' usage
> @@ -1471,6 +1476,12 @@ struct mm_walk {
>  			     struct mm_walk *walk);
>  	int (*test_walk)(unsigned long addr, unsigned long next,
>  			struct mm_walk *walk);
> +	int (*test_pmd)(unsigned long addr, unsigned long next,
> +			pmd_t *pmd_start, struct mm_walk *walk);
> +	int (*test_pud)(unsigned long addr, unsigned long next,
> +			pud_t *pud_start, struct mm_walk *walk);
> +	int (*test_p4d)(unsigned long addr, unsigned long next,
> +			p4d_t *p4d_start, struct mm_walk *walk);
>  	struct mm_struct *mm;
>  	struct vm_area_struct *vma;
>  	void *private;
> diff --git a/mm/pagewalk.c b/mm/pagewalk.c
> index 1cbef99e9258..6bea79b95be3 100644
> --- a/mm/pagewalk.c
> +++ b/mm/pagewalk.c
> @@ -32,6 +32,14 @@ static int walk_pmd_range(pud_t *pud, unsigned long addr, unsigned long end,
>  	unsigned long next;
>  	int err = 0;
>  
> +	if (walk->test_pmd) {
> +		err = walk->test_pmd(addr, end, pmd_offset(pud, 0UL), walk);
> +		if (err < 0)
> +			return err;
> +		if (err > 0)
> +			return 0;
> +	}

Though this attempts to match semantics with test_walk() and be comprehensive
just wondering what are the real world situations when page walking need to be
aborted based on error condition at a given page table level.