Re: [PATCH 1/2] hugetlb: fix update_and_free_page contig page struct assumption

[Mike Kravetz]

On 2/18/21 9:40 AM, Zi Yan wrote:
> On 18 Feb 2021, at 12:32, Jason Gunthorpe wrote:
> 
>> On Thu, Feb 18, 2021 at 12:27:58PM -0500, Zi Yan wrote:
>>> On 18 Feb 2021, at 12:25, Jason Gunthorpe wrote:
>>>
>>>> On Thu, Feb 18, 2021 at 02:45:54PM +0000, Matthew Wilcox wrote:
>>>>> On Wed, Feb 17, 2021 at 11:02:52AM -0800, Andrew Morton wrote:
>>>>>> On Wed, 17 Feb 2021 10:49:25 -0800 Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
>>>>>>> page structs are not guaranteed to be contiguous for gigantic pages.  The
>>>>>>
>>>>>> June 2014.  That's a long lurk time for a bug.  I wonder if some later
>>>>>> commit revealed it.
>>>>>
>>>>> I would suggest that gigantic pages have not seen much use.  Certainly
>>>>> performance with Intel CPUs on benchmarks that I've been involved with
>>>>> showed lower performance with 1GB pages than with 2MB pages until quite
>>>>> recently.
>>>>
>>>> I suggested in another thread that maybe it is time to consider
>>>> dropping this "feature"
>>>
>>> You mean dropping gigantic page support in hugetlb?
>>
>> No, I mean dropping support for arches that want to do:
>>
>>    tail_page != head_page + tail_page_nr
>>
>> because they can't allocate the required page array either virtually
>> or physically contiguously.
>>
>> It seems like quite a burden on the core mm for a very niche, and
>> maybe even non-existant, case.
>>
>> It was originally done for PPC, can these PPC systems use VMEMMAP now?
>>
>>>> The cost to fix GUP to be compatible with this will hurt normal
>>>> GUP performance - and again, that nobody has hit this bug in GUP
>>>> further suggests the feature isn't used..
>>>
>>> A easy fix might be to make gigantic hugetlb page depends on
>>> CONFIG_SPARSEMEM_VMEMMAP, which guarantee all struct pages are contiguous.
>>
>> Yes, exactly.
> 
> I actually have a question on CONFIG_SPARSEMEM_VMEMMAP. Can we assume
> PFN_A - PFN_B == struct_page_A - struct_page_B, meaning all struct pages
> are ordered based on physical addresses? I just wonder for two PFN ranges,
> e.g., [0 - 128MB], [128MB - 256MB], if it is possible to first online
> [128MB - 256MB] then [0 - 128MB] and the struct pages of [128MB - 256MB]
> are in front of [0 - 128MB] in the vmemmap due to online ordering.

I have not looked at the code which does the onlining and vmemmap setup.
But, these definitions make me believe it is true:

#elif defined(CONFIG_SPARSEMEM_VMEMMAP)

/* memmap is virtually contiguous.  */
#define __pfn_to_page(pfn)      (vmemmap + (pfn))
#define __page_to_pfn(page)     (unsigned long)((page) - vmemmap)

-- 
Mike Kravetz