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

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Message-ID: <90a9af31-304c-e8d5-b17c-0ddb4c98fddb@google.com>
Date:   Fri, 13 Dec 2019 09:13:05 -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

On 12/12/19 8:07 PM, Liran Alon wrote:
>> 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.
> 
> Why would it affect non-DAX case?
> Your patch should just make hugepage_adjust() to parse page-tables only in case is_zone_device_page(). Otherwise, page tables shouldn’t be parsed.
> i.e. THP merged pages should still be detected by PageTransCompoundMap().

That's what I already do.  But if I wanted to make the hugepage_adjust() 
function also handle the change to 1 GB, then that code would apply to 
THP too.  I didn't want to do that without knowing the implications for THP.

>> 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 prefer to walk page-tables only for is_zone_device_page().

Is there another way to tell if a THP page is 1 GB?  Anyway, this is the 
sort of stuff I didn't want to mess around with.

hugepage_adjust() seemed like a reasonable place to get a huge (2MB) 
page table entry out of a DAX mapping.  I didn't want to proliferate 
another special case for upgrading to a larger PTE size (i.e. how 
hugetlbfs and THP have separate mechanisms), so I hopped on to the "can 
we do a 2MB mapping even though host_mapping_level() didn't say so" case 
- which is my interpretation of what huge_adjust() is for.

Barret