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Date: Thu, 12 Dec 2019 20:32:17 +0200
From: Liran Alon <liran.alon@...cle.com>
To: Dan Williams <dan.j.williams@...el.com>
Cc: Barret Rhoden <brho@...gle.com>,
Paolo Bonzini <pbonzini@...hat.com>,
David Hildenbrand <david@...hat.com>,
Dave Jiang <dave.jiang@...el.com>,
Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
linux-nvdimm <linux-nvdimm@...ts.01.org>,
X86 ML <x86@...nel.org>, KVM list <kvm@...r.kernel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
"Zeng, Jason" <jason.zeng@...el.com>
Subject: Re: [PATCH v4 2/2] kvm: Use huge pages for DAX-backed files
> On 12 Dec 2019, at 19:59, Dan Williams <dan.j.williams@...el.com> wrote:
>
> On Thu, Dec 12, 2019 at 9:39 AM Liran Alon <liran.alon@...cle.com> wrote:
>>
>>
>>
>>> On 12 Dec 2019, at 18:54, Dan Williams <dan.j.williams@...el.com> wrote:
>>>
>>> On Thu, Dec 12, 2019 at 4:34 AM Liran Alon <liran.alon@...cle.com> wrote:
>>>>
>>>>
>>>>
>>>>> On 11 Dec 2019, at 23:32, Barret Rhoden <brho@...gle.com> wrote:
>>>>>
>>>>> This change allows KVM to map DAX-backed files made of huge pages with
>>>>> huge mappings in the EPT/TDP.
>>>>>
>>>>> DAX pages are not PageTransCompound. The existing check is trying to
>>>>> determine if the mapping for the pfn is a huge mapping or not. For
>>>>> non-DAX maps, e.g. hugetlbfs, that means checking PageTransCompound.
>>>>> For DAX, we can check the page table itself.
>>>>
>>>> For hugetlbfs pages, tdp_page_fault() -> mapping_level() -> host_mapping_level() -> kvm_host_page_size() -> vma_kernel_pagesize()
>>>> will return the page-size of the hugetlbfs without the need to parse the page-tables.
>>>> See vma->vm_ops->pagesize() callback implementation at hugetlb_vm_ops->pagesize()==hugetlb_vm_op_pagesize().
>>>>
>>>> Only for pages that were originally mapped as small-pages and later merged to larger pages by THP, there is a need to check for PageTransCompound(). Again, instead of parsing page-tables.
>>>>
>>>> Therefore, it seems more logical to me that:
>>>> (a) If DAX-backed files are mapped as large-pages to userspace, it should be reflected in vma->vm_ops->page_size() of that mapping. Causing kvm_host_page_size() to return the right size without the need to parse the page-tables.
>>>
>>> A given dax-mapped vma may have mixed page sizes so ->page_size()
>>> can't be used reliably to enumerating the mapping size.
>>
>> Naive question: Why don’t split the VMA in this case to multiple VMAs with different results for ->page_size()?
>
> Filesystems traditionally have not populated ->pagesize() in their
> vm_operations, there was no compelling reason to go add it and the
> complexity seems prohibitive.
I understand. Though this is technical debt that breaks ->page_size() semantics which might cause a complex bug some day...
>
>> What you are describing sounds like DAX is breaking this callback semantics in an unpredictable manner.
>
> It's not unpredictable. vma_kernel_pagesize() returns PAGE_SIZE.
Of course. :) I meant it may be unexpected by the caller.
> Huge
> pages in the page cache has a similar issue.
Ok. I haven’t known that. Thanks for the explanation.
>
>>>> (b) If DAX-backed files small-pages can be later merged to large-pages by THP, then the “struct page” of these pages should be modified as usual to make PageTransCompound() return true for them. I’m not highly familiar with this mechanism, but I would expect THP to be able to merge DAX-backed files small-pages to large-pages in case DAX provides “struct page” for the DAX pages.
>>>
>>> DAX pages do not participate in THP and do not have the
>>> PageTransCompound accounting. The only mechanism that records the
>>> mapping size for dax is the page tables themselves.
>>
>> What is the rational behind this? Given that DAX pages can be described with “struct page” (i.e. ZONE_DEVICE), what prevents THP from manipulating page-tables to merge multiple DAX PFNs to a larger page?
>
> THP accounting is a function of the page allocator. ZONE_DEVICE pages
> are excluded from the page allocator. ZONE_DEVICE is just enough
> infrastructure to support pfn_to_page(), page_address(), and
> get_user_pages(). Other page allocator services beyond that are not
> present.
Ok.
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