linux-kernel - Re: [PATCH v5 2/2] kvm: Use huge pages for DAX-backed files

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Date:   Thu, 12 Dec 2019 14:55:04 -0500
From:   Barret Rhoden <brho@...gle.com>
To:     Liran Alon <liran.alon@...cle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Dan Williams <dan.j.williams@...el.com>,
        David Hildenbrand <david@...hat.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        linux-nvdimm@...ts.01.org, x86@...nel.org, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, jason.zeng@...el.com
Subject: Re: [PATCH v5 2/2] kvm: Use huge pages for DAX-backed files

Hi -

On 12/12/19 1:49 PM, Liran Alon wrote:
> 
> 
>> On 12 Dec 2019, at 20:47, Liran Alon <liran.alon@...cle.com> wrote:
>>
>>
>>
>>> On 12 Dec 2019, at 20:22, 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.
>>
>> This change isn’t only relevant for TDP. It also affects when KVM use shadow-paging.
>> See how FNAME(page_fault)() calls transparent_hugepage_adjust().

Cool, I'll drop references to the EPT/TDP from the commit message.

>>> DAX pages are not PageTransCompound.  The existing check is trying to
>>> determine if the mapping for the pfn is a huge mapping or not.
>>
>> I would rephrase “The existing check is trying to determine if the pfn
>> is mapped as part of a transparent huge-page”.

Can do.

>>
>>> For
>>> non-DAX maps, e.g. hugetlbfs, that means checking PageTransCompound.
>>
>> This is not related to hugetlbfs but rather THP.

I thought that PageTransCompound also returned true for hugetlbfs (based 
off of comments in page-flags.h).  Though I do see the comment about the 
'level == PT_PAGE_TABLE_LEVEL' check excluding hugetlbfs pages.

Anyway, I'll remove the "e.g. hugetlbfs" from the commit message.

>>
>>> For DAX, we can check the page table itself.
>>>
>>> Note that KVM already faulted in the page (or huge page) in the host's
>>> page table, and we hold the KVM mmu spinlock.  We grabbed that lock in
>>> kvm_mmu_notifier_invalidate_range_end, before checking the mmu seq.
>>>
>>> Signed-off-by: Barret Rhoden <brho@...gle.com>
>>
>> I don’t think the right place to change for this functionality is transparent_hugepage_adjust()
>> which is meant to handle PFNs that are mapped as part of a transparent huge-page.
>>
>> For example, this would prevent mapping DAX-backed file page as 1GB.
>> As transparent_hugepage_adjust() only handles the case (level == PT_PAGE_TABLE_LEVEL).
>>
>> As you are parsing the page-tables to discover the page-size the PFN is mapped in,
>> I think you should instead modify kvm_host_page_size() to parse page-tables instead
>> of rely on vma_kernel_pagesize() (Which relies on vma->vm_ops->pagesize()) in case
>> of is_zone_device_page().
>> The main complication though of doing this is that at this point you don’t yet have the PFN
>> that is retrieved by try_async_pf(). So maybe you should consider modifying the order of calls
>> in tdp_page_fault() & FNAME(page_fault)().
>>
>> -Liran
> 
> Or alternatively when thinking about it more, maybe just rename transparent_hugepage_adjust()
> to not be specific to THP and better handle the case of parsing page-tables changing mapping-level to 1GB.
> That is probably easier and more elegant.

I can rename it to hugepage_adjust(), since it's not just THP anymore.

I was a little hesitant to change the this to handle 1 GB pages with 
this patchset at first.  I didn't want to break the non-DAX case stuff 
by doing so.

Specifically, can a THP page be 1 GB, and if so, how can you tell?  If 
you can't tell easily, I could walk the page table for all cases, 
instead of just zone_device().

I'd also have to drop the "level == PT_PAGE_TABLE_LEVEL" check, I think, 
which would open this up to hugetlbfs pages (based on the comments).  Is 
there any reason why that would be a bad idea?

Thanks,

Barret