Re: [RFC V2 0/1] mm/debug: Add tests for architecture exported page table helpers

From: Anshuman Khandual <anshuman.khandual@arm•com>
To: Matthew Wilcox <willy@infradead•org>
Cc: Mark Rutland <mark.rutland@arm•com>,
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	Michal Hocko <mhocko@kernel•org>,
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	"David S. Miller" <davem@davemloft•net>
Subject: Re: [RFC V2 0/1] mm/debug: Add tests for architecture exported page table helpers
Date: Mon, 26 Aug 2019 08:07:13 +0530	[thread overview]
Message-ID: <7a88f6bb-e8c7-3ac7-2f92-1de752a01f33@arm.com> (raw)
In-Reply-To: <20190809135202.GN5482@bombadil.infradead.org>

On 08/09/2019 07:22 PM, Matthew Wilcox wrote:
> On Fri, Aug 09, 2019 at 04:05:07PM +0530, Anshuman Khandual wrote:
>> On 08/09/2019 03:46 PM, Matthew Wilcox wrote:
>>> On Fri, Aug 09, 2019 at 01:03:17PM +0530, Anshuman Khandual wrote:
>>>> Should alloc_gigantic_page() be made available as an interface for general
>>>> use in the kernel. The test module here uses very similar implementation from
>>>> HugeTLB to allocate a PUD aligned memory block. Similar for mm_alloc() which
>>>> needs to be exported through a header.
>>>
>>> Why are you allocating memory at all instead of just using some
>>> known-to-exist PFNs like I suggested?
>>
>> We needed PFN to be PUD aligned for pfn_pud() and PMD aligned for mk_pmd().
>> Now walking the kernel page table for a known symbol like kernel_init()
> 
> I didn't say to walk the kernel page table.  I said to call virt_to_pfn()
> for a known symbol like kernel_init().
> 
>> as you had suggested earlier we might encounter page table page entries at PMD
>> and PUD which might not be PMD or PUD aligned respectively. It seemed to me
>> that alignment requirement is applicable only for mk_pmd() and pfn_pud()
>> which create large mappings at those levels but that requirement does not
>> exist for page table pages pointing to next level. Is not that correct ? Or
>> I am missing something here ?
> 
> Just clear the bottom bits off the PFN until you get a PMD or PUD aligned
> PFN.  It's really not hard.

As Mark pointed out earlier that might end up being just a synthetic PFN
which might not even exist on a given system.

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