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From: Barret Rhoden <brho@google.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Dan Williams" <dan.j.williams@intel.com>,
	"Dave Jiang" <dave.jiang@intel.com>,
	"Ross Zwisler" <zwisler@kernel.org>,
	"Vishal Verma" <vishal.l.verma@intel.com>,
	"Radim Krčmář" <rkrcmar@redhat.com>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Ingo Molnar" <mingo@redhat.com>,
	"Borislav Petkov" <bp@alien8.de>,
	linux-nvdimm@lists.01.org, linux-kernel@vger.kernel.org,
	"H. Peter Anvin" <hpa@zytor.com>,
	x86@kernel.org, kvm@vger.kernel.org, yu.c.zhang@intel.com,
	yi.z.zhang@intel.com
Subject: Re: [PATCH 2/2] kvm: Use huge pages for DAX-backed files
Date: Tue, 13 Nov 2018 11:21:48 -0500
Message-ID: <20181113112148.6205fc56@gnomeregan.cam.corp.google.com> (raw)
In-Reply-To: <861c4adb-e2f0-2caf-8f6e-9f09ecb0b624@redhat.com>

On 2018-11-12 at 20:31 Paolo Bonzini <pbonzini@redhat.com> wrote:
> Looks good.  What's the plan for removing PageReserved from DAX pages?

I hear that's going on in this thread:

https://lore.kernel.org/lkml/154145268025.30046.11742652345962594283.stgit@ahduyck-desk1.jf.intel.com/

Though it looks like it's speeding up page initialization, and not
explicitly making the PageReserved change, yet.

Alternatively, I could change kvm_is_reserved_pfn() to single out
zone_device pages if we don't want to wait or if there is a concern
that it won't happen.

On a related note, there are two places in KVM where we check
PageReserved outside of kvm_is_reserved_pfn().

For reference:

bool kvm_is_reserved_pfn(kvm_pfn_t pfn)
{
        if (pfn_valid(pfn))
                return PageReserved(pfn_to_page(pfn));

        return true;
}

One caller of PageReserved():

void kvm_set_pfn_dirty(kvm_pfn_t pfn)
{
        if (!kvm_is_reserved_pfn(pfn)) {
                struct page *page = pfn_to_page(pfn);

                if (!PageReserved(page))
                        SetPageDirty(page);
        }
}

In that one, the PageReserved() check looks redundant, since if the
page was PageReserved, then it would have been kvm_is_reserved.

The other is:

static bool kvm_is_mmio_pfn(kvm_pfn_t pfn)
{
        if (pfn_valid(pfn))
                return !is_zero_pfn(pfn) && PageReserved(pfn_to_page(pfn)) &&
                        /*
                         * Some reserved pages, such as those from NVDIMM
                         * DAX devices, are not for MMIO, and can be mapped
                         * with cached memory type for better performance.
                         * However, the above check misconceives those pages
                         * as MMIO, and results in KVM mapping them with UC
                         * memory type, which would hurt the performance.
                         * Therefore, we check the host memory type in addition
                         * and only treat UC/UC-/WC pages as MMIO.
                         */
                        (!pat_enabled() || pat_pfn_immune_to_uc_mtrr(pfn));

        return true;
}

Where the PAT stuff was motivated by DAX.  The PageReserved check here
looks like a broken-out version of kvm_is_reserved_pfn(), so that we can
make some extra checks around it.

Anyway, I can get rid of those two PageReserved checks and/or have
kvm_is_reserved_pfn() just check DAX pages, if everyone is OK with that.

Thanks,

Barret


  reply index

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-11-09 20:39 [PATCH 0/2] " Barret Rhoden
2018-11-09 20:39 ` [PATCH 1/2] mm: make dev_pagemap_mapping_shift() externally visible Barret Rhoden
2018-11-13  9:28   ` David Hildenbrand
2018-11-09 20:39 ` [PATCH 2/2] kvm: Use huge pages for DAX-backed files Barret Rhoden
2018-11-12 19:31   ` Paolo Bonzini
2018-11-13 16:21     ` Barret Rhoden [this message]
2018-11-13 18:22       ` Paolo Bonzini
2018-11-13  9:36   ` David Hildenbrand
2018-11-13 10:02     ` Pankaj Gupta
2018-11-13 12:41       ` Paolo Bonzini
2018-11-13 15:56       ` Barret Rhoden
2018-11-14  9:09         ` Pankaj Gupta
2018-11-13 15:50     ` Barret Rhoden
2018-11-14 21:51 ` [PATCH v2 0/3] " Barret Rhoden
2018-11-14 21:51   ` [PATCH v2 1/3] mm: make dev_pagemap_mapping_shift() externally visible Barret Rhoden
2018-11-26 16:50     ` Paolo Bonzini
2018-11-26 18:32     ` Dan Williams
2018-11-14 21:51   ` [PATCH v2 2/3] kvm: Use huge pages for DAX-backed files Barret Rhoden
2018-11-14 21:51   ` [PATCH v2 3/3] kvm: remove redundant PageReserved() check Barret Rhoden
2018-11-27 13:31     ` David Hildenbrand
2018-11-15  0:55   ` [PATCH v2 0/3] kvm: Use huge pages for DAX-backed files Dan Williams
2018-12-03 17:40     ` Barret Rhoden
2018-12-03 18:32       ` Alexander Duyck

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