linux-kernel - Re: [PATCH v9 13/21] mm: pagewalk: Add test

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Date:   Mon, 29 Jul 2019 13:34:48 +0100
From:   Steven Price <steven.price@....com>
To:     Anshuman Khandual <anshuman.khandual@....com>, linux-mm@...ck.org
Cc:     Mark Rutland <Mark.Rutland@....com>, x86@...nel.org,
        Arnd Bergmann <arnd@...db.de>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        linux-kernel@...r.kernel.org,
        Jérôme Glisse <jglisse@...hat.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        James Morse <james.morse@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-arm-kernel@...ts.infradead.org,
        "Liang, Kan" <kan.liang@...ux.intel.com>
Subject: Re: [PATCH v9 13/21] mm: pagewalk: Add test_p?d callbacks

On 28/07/2019 14:41, Anshuman Khandual wrote:
> 
> 
> 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@....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.

I'm not aware of a situation yet where aborting early is necessary - but
as you say this matches the semantics of test_walk() and was easy to
implement.

Steve