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

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Date:   Tue, 7 Jan 2020 17:20:15 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Barret Rhoden <brho@...gle.com>
Cc:     Liran Alon <liran.alon@...cle.com>,
        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>,
        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 Tue, Jan 07, 2020 at 02:19:06PM -0500, Barret Rhoden wrote:
> On 1/7/20 2:05 PM, Sean Christopherson wrote:
> >Hopefully you haven't put too much effort into the rework, because I want
> >to commandeer the proposed changes and use them as the basis for a more
> >aggressive overhaul of KVM's hugepage handling.  Ironically, there's a bug
> >in KVM's THP handling that I _think_ can be avoided by using the DAX
> >approach of walking the host PTEs.
> >
> >I'm in the process of testing, hopefully I'll get a series sent out later
> >today.  If not, I should at least be able to provide an update.
> 
> Nice timing.  I was just about to get back to this, so I haven't put any
> time in yet.  =)
> 
> Please CC me, and I'll try your patches out on my end.

Will do.  Barring last minute hiccups, the code is ready, just need to
finish off a few changelogs.  Should get it out early tomorrow.

One question that may help avoid some churn: are huge DAX pages not
tracked as compound pages?  The comment from your/this patch is pretty
unequivocal, but I wanted to double check that they will really return
false for PageCompound(), as opposed to only returning false for
PageTransCompoundMap().

	/*
	 * DAX pages do not use compound pages.  ...
	 */

Thanks!