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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