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

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Message-ID: <CAPcyv4jW3HTU7mmctL_d+zTS8yT_NLxa5cx_w8nbqVSwBMdSLQ@mail.gmail.com>
Date:   Tue, 7 Jan 2020 17:39:28 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Barret Rhoden <brho@...gle.com>,
        Liran Alon <liran.alon@...cle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        X86 ML <x86@...nel.org>, KVM list <kvm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Zeng, Jason" <jason.zeng@...el.com>
Subject: Re: [PATCH v5 2/2] kvm: Use huge pages for DAX-backed files

On Tue, Jan 7, 2020 at 5:20 PM Sean Christopherson
<sean.j.christopherson@...el.com> wrote:
>
> 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().

PageCompound() returns false.

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

None of the head / tail page infrastructure is set up for dax pages.
They are just independent 'struct page' objects that are
opportunistically mapped by different pte sizes in the dax core.