Re: [PATCH v4 2/2] kvm: Use huge pages for DAX-backed files

[Liran Alon <liran.alon@oracle.com>]

> On 12 Dec 2019, at 19:59, Dan Williams <dan.j.williams@intel.com> wrote:
> 
> On Thu, Dec 12, 2019 at 9:39 AM Liran Alon <liran.alon@oracle.com> wrote:
>> 
>> 
>> 
>>> On 12 Dec 2019, at 18:54, Dan Williams <dan.j.williams@intel.com> wrote:
>>> 
>>> On Thu, Dec 12, 2019 at 4:34 AM Liran Alon <liran.alon@oracle.com> wrote:
>>>> 
>>>> 
>>>> 
>>>>> On 11 Dec 2019, at 23:32, Barret Rhoden <brho@google.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.