linux-kernel - Re: [PATCH v11 14/22] mm: pagewalk: Add 'depth' parameter to pte_hole

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Message-ID: <7baa4861-eb68-5dcb-137b-482a3b5f52a6@arm.com>
Date:   Wed, 9 Oct 2019 12:00:00 +0100
From:   Steven Price <steven.price@....com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Mark Rutland <Mark.Rutland@....com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Arnd Bergmann <arnd@...db.de>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Catalin Marinas <catalin.marinas@....com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.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 v11 14/22] mm: pagewalk: Add 'depth' parameter to pte_hole

On 07/10/2019 19:11, Jason Gunthorpe wrote:
> On Mon, Oct 07, 2019 at 05:20:30PM +0100, Steven Price wrote:
>> On 07/10/2019 17:10, Jason Gunthorpe wrote:
>>> On Mon, Oct 07, 2019 at 04:38:14PM +0100, Steven Price wrote:
>>>> diff --git a/mm/hmm.c b/mm/hmm.c
>>>> index 902f5fa6bf93..34fe904dd417 100644
>>>> +++ b/mm/hmm.c
>>>> @@ -376,7 +376,7 @@ static void hmm_range_need_fault(const struct hmm_vma_walk *hmm_vma_walk,
>>>>  }
>>>>  
>>>>  static int hmm_vma_walk_hole(unsigned long addr, unsigned long end,
>>>> -			     struct mm_walk *walk)
>>>> +			     __always_unused int depth, struct mm_walk *walk)
>>>
>>> It this __always_unused on function arguments something we are doing
>>> now?
>>
>> $ git grep __always_unused | wc -l
>> 191
>>
>> It's elsewhere in the kernel tree. It seems like a good way of both
>> documenting and silencing compiler warnings. But I'm open to other
>> suggestions.
> 
> The normal kernel build doesn't generate warnings for unused function
> parameters because there are alot of false positives, IIRC. So, seems
> weird to see things like this.
> 
>>> Can we have negative depth? Should it be unsigned?
>>
>> As per the documentation added in this patch:
>>
>>  * @pte_hole:	if set, called for each hole at all levels,
>>  *		depth is -1 if not known, 0:PGD, 1:P4D, 2:PUD, 3:PMD
>>  *		4:PTE. Any folded depths (where PTRS_PER_P?D is equal
>>  *		to 1) are skipped.
>>
>> So it's signed to allow "-1" in the cases where pte_hole is called
>> without knowing the actual depth. This is used in the function
>> walk_page_test() because it don't walk the actual page tables, but is
>> called on a VMA instead. This means that there may not be a single depth
>> for the range provided.
> 
> So are the depth values below OK? I would have expected -1 by this
> definition

Good spot - that indeed was very sloppy of me. In these cases the value
is ignored, but -1 would indeed have been a better value to use. I'll
fix that up.

Thanks,

Steve

>>>>  {
>>>>  	struct hmm_vma_walk *hmm_vma_walk = walk->private;
>>>>  	struct hmm_range *range = hmm_vma_walk->range;
>>>> @@ -564,7 +564,7 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
>>>>  again:
>>>>  	pmd = READ_ONCE(*pmdp);
>>>>  	if (pmd_none(pmd))
>>>> -		return hmm_vma_walk_hole(start, end, walk);
>>>> +		return hmm_vma_walk_hole(start, end, 0, walk);
>>>>  
>>>>  	if (thp_migration_supported() && is_pmd_migration_entry(pmd)) {
>>>>  		bool fault, write_fault;
>>>> @@ -666,7 +666,7 @@ static int hmm_vma_walk_pud(pud_t *pudp, unsigned long start, unsigned long end,
>>>>  again:
>>>>  	pud = READ_ONCE(*pudp);
>>>>  	if (pud_none(pud))
>>>> -		return hmm_vma_walk_hole(start, end, walk);
>>>> +		return hmm_vma_walk_hole(start, end, 0, walk);
>>>>  
>>>>  	if (pud_huge(pud) && pud_devmap(pud)) {
>>>>  		unsigned long i, npages, pfn;
>>>> @@ -674,7 +674,7 @@ static int hmm_vma_walk_pud(pud_t *pudp, unsigned long start, unsigned long end,
>>>>  		bool fault, write_fault;
>>>>  
>>>>  		if (!pud_present(pud))
>>>> -			return hmm_vma_walk_hole(start, end, walk);
>>>> +			return hmm_vma_walk_hole(start, end, 0, walk);
>>>>  
>>>>  		i = (addr - range->start) >> PAGE_SHIFT;
>>>>  		npages = (end - addr) >> PAGE_SHIFT;
> 
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