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Subject: Re: [PATCH 4/6] mm/hmm: add output flag for compound page mapping
To:     Zi Yan <ziy@nvidia.com>
CC:     Christoph Hellwig <hch@lst.de>, <nouveau@lists.freedesktop.org>,
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From:   Ralph Campbell <rcampbell@nvidia.com>
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On 5/26/20 3:29 PM, Zi Yan wrote:
> On 8 May 2020, at 16:06, Ralph Campbell wrote:
>=20
>> On 5/8/20 12:51 PM, Christoph Hellwig wrote:
>>> On Fri, May 08, 2020 at 12:20:07PM -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 PF=
N
>>>> 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 l=
arger
>>>> than order 0 (PAGE_SIZE), there is no indication that the page is mapp=
ed
>>>> 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 be=
ing
>>>> mapped with 2MB PMD entries. However, the most common case is the mapp=
ing
>>>> size 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.
>>>
>>> Why do you need the flag?  The caller should be able to just use
>>> page_size() (or willys new thp_size helper).
>>>
>>
>> The question is whether or not a large page can be mapped with smaller
>> page table entries with different permissions. If one process has a 2MB
>> page mapped with 4K PTEs with different read/write permissions, I don't =
think
>> it would be OK for a device to map the whole 2MB with write access enabl=
ed.
>> The flag is supposed to indicate that the whole page can be mapped by th=
e
>> device with the indicated read/write permissions.
>=20
> If hmm_range_fault() only walks one VMA at a time, you would not have thi=
s permission
> issue, right? Since all pages from one VMA should have the same permissio=
n.
> But it seems that hmm_range_fault() deals with pages across multiple VMAs=
.
> Maybe we should make hmm_range_fault() bail out early when it encounters
> a VMA with a different permission than the existing ones?
>=20
>=20
> =E2=80=94
> Best Regards,
> Yan Zi

I don't think so. The VMA might have read/write permission but the page tab=
le might
have read-only permission in order to trigger a fault for copy-on-write. Or=
 the
PTE might be read-only or invalid to trigger faults for architectures that =
don't
have hardware updated accessed bits and are using the minor faults to updat=
e LRU.
The goal is that the MM core see the same faults whether the HMM device acc=
esses
memory or a CPU thread.