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

[Zi Yan]

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.


—
Best Regards,
Yan Zi

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