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

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Date:   Thu, 12 Dec 2019 20:49:52 +0200
From:   Liran Alon <liran.alon@...cle.com>
To:     Barret Rhoden <brho@...gle.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



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

-Liran