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Subject: Re: [PATCH 09/16] mm/hmm: add output flag for compound page mapping
To:     Jason Gunthorpe <jgg@mellanox.com>
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From:   Ralph Campbell <rcampbell@nvidia.com>
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On 6/22/20 4:18 PM, Jason Gunthorpe wrote:
> On Mon, Jun 22, 2020 at 11:10:05AM -0700, Ralph Campbell wrote:
>>
>> On 6/22/20 10:25 AM, Jason Gunthorpe wrote:
>>> On Fri, Jun 19, 2020 at 02:56:42PM -0700, Ralph Campbell wrote:
>>>> hmm_range_fault() returns an array of page frame numbers and flags for
>>>> how the pages are mapped in the requested process' page tables. The PFN
>>>> can be used to get the struct page with hmm_pfn_to_page() and the page size
>>>> order can be determined with compound_order(page) but if the page is larger
>>>> than order 0 (PAGE_SIZE), there is no indication that the page is mapped
>>>> using a larger page size. To be fully general, hmm_range_fault() would need
>>>> to return the mapping size to handle cases like a 1GB compound page being
>>>> mapped with 2MB PMD entries. However, the most common case is the mapping
>>>> size is the same as the underlying compound page size.
>>>> Add a new output flag to indicate this so that callers know it is safe to
>>>> use a large device page table mapping if one is available.
>>>
>>> But what size should the caller use?
>>>
>>> You already explained that the caller cannot use compound_ordet() to
>>> get the size, so what should it be?
>>>
>>> Probably this needs to be two flags, PUD and PMD, and the caller should
>>> use the PUD and PMD sizes to figure out how big it is?
>>>
>>> Jason
>>>
>>
>> I guess I didn't explain it as clearly as I thought. :-)
>>
>> The page size *can* be determined with compound_order(page) but without the
>> flag, the caller doesn't know how much of that page is being mapped by the
>> CPU. The flag says the CPU is mapping the whole compound page (based on compound_order)
>> and that the caller can use device mappings up to the size of compound_order(page).
> 
> No, I got it, I just don't like the assumption that just because a PMD
> or PUD points to a page that the only possible value for
> compound_page() is PMD or PUD respectively. Partial mapping should be
> possible in both cases, if not today, then maybe down the road with
> some of the large page work that has been floating about
> 
> It seems much safer to just directly encode the PUD/PMD size in the
> flags
> 
> Jason

That is fine with me. I'll make that change for v2.
I was just trying to minimize the number of flags being added.