linux-kernel - Re: [PATCH v12 6/8] mm: support reporting free page blocks

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Message-ID: <20170725145333.GK26723@dhcp22.suse.cz>
Date:   Tue, 25 Jul 2017 16:53:34 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     "Wang, Wei W" <wei.w.wang@...el.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "qemu-devel@...gnu.org" <qemu-devel@...gnu.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "david@...hat.com" <david@...hat.com>,
        "cornelia.huck@...ibm.com" <cornelia.huck@...ibm.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "mgorman@...hsingularity.net" <mgorman@...hsingularity.net>,
        "aarcange@...hat.com" <aarcange@...hat.com>,
        "amit.shah@...hat.com" <amit.shah@...hat.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "liliang.opensource@...il.com" <liliang.opensource@...il.com>,
        "virtio-dev@...ts.oasis-open.org" <virtio-dev@...ts.oasis-open.org>,
        "yang.zhang.wz@...il.com" <yang.zhang.wz@...il.com>,
        "quan.xu@...yun.com" <quan.xu@...yun.com>
Subject: Re: [PATCH v12 6/8] mm: support reporting free page blocks

On Tue 25-07-17 14:47:16, Wang, Wei W wrote:
> On Tuesday, July 25, 2017 8:42 PM, hal Hocko wrote:
> > On Tue 25-07-17 19:56:24, Wei Wang wrote:
> > > On 07/25/2017 07:25 PM, Michal Hocko wrote:
> > > >On Tue 25-07-17 17:32:00, Wei Wang wrote:
> > > >>On 07/24/2017 05:00 PM, Michal Hocko wrote:
> > > >>>On Wed 19-07-17 20:01:18, Wei Wang wrote:
> > > >>>>On 07/19/2017 04:13 PM, Michal Hocko wrote:
> > > >>>[...
> > > We don't need to do the pfn walk in the guest kernel. When the API
> > > reports, for example, a 2MB free page block, the API caller offers to
> > > the hypervisor the base address of the page block, and size=2MB, to
> > > the hypervisor.
> > 
> > So you want to skip pfn walks by regularly calling into the page allocator to
> > update your bitmap. If that is the case then would an API that would allow you
> > to update your bitmap via a callback be s sufficient? Something like
> > 	void walk_free_mem(int node, int min_order,
> > 			void (*visit)(unsigned long pfn, unsigned long nr_pages))
> > 
> > The function will call the given callback for each free memory block on the given
> > node starting from the given min_order. The callback will be strictly an atomic
> > and very light context. You can update your bitmap from there.
> 
> I would need to introduce more about the background here:
> The hypervisor and the guest live in their own address space. The hypervisor's bitmap
> isn't seen by the guest. I think we also wouldn't be able to give a callback function 
> from the hypervisor to the guest in this case.

How did you plan to use your original API which export struct page array
then?

> > This would address my main concern that the allocator internals would get
> > outside of the allocator proper. 
> 
> What issue would it have to expose the internal, for_each_zone()?

zone is a MM internal concept. No code outside of the MM proper should
really care about zones. 
-- 
Michal Hocko
SUSE Labs