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

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Date:   Wed, 26 Jul 2017 12:47:47 +0000
From:   "Wang, Wei W" <wei.w.wang@...el.com>
To:     Michal Hocko <mhocko@...nel.org>
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 Wednesday, July 26, 2017 7:55 PM, Michal Hocko wrote:
> On Wed 26-07-17 19:44:23, Wei Wang wrote:
> [...]
> > I thought about it more. Probably we can use the callback function
> > with a little change like this:
> >
> > void walk_free_mem(void *opaque1, void (*visit)(void *opaque2,
> > unsigned long pfn,
> >            unsigned long nr_pages))
> > {
> >     ...
> >     for_each_populated_zone(zone) {
> >                    for_each_migratetype_order(order, type) {
> >                         report_unused_page_block(zone, order, type,
> > &page); // from patch 6
> >                         pfn = page_to_pfn(page);
> >                         visit(opaque1, pfn, 1 << order);
> >                     }
> >     }
> > }
> >
> > The above function scans all the free list and directly sends each
> > free page block to the hypervisor via the virtio_balloon callback
> > below. No need to implement a bitmap.
> >
> > In virtio-balloon, we have the callback:
> > void *virtio_balloon_report_unused_pages(void *opaque,  unsigned long
> > pfn, unsigned long nr_pages) {
> >     struct virtio_balloon *vb = (struct virtio_balloon *)opaque;
> >     ...put the free page block to the the ring of vb; }
> >
> >
> > What do you think?
> 
> I do not mind conveying a context to the callback. I would still prefer
> to keep the original min_order to check semantic though. Why? Well,
> it doesn't make much sense to scan low order free blocks all the time
> because they are simply too volatile. Larger blocks tend to surivive for
> longer. So I assume you would only care about larger free blocks. This
> will also make the call cheaper.
> --

OK, I will keep min order there in the next version.

Best,
Wei