linux-kernel - Re: [PATCH v15 4/5] mm: support reporting free page blocks

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Date:   Mon, 28 Aug 2017 16:09:58 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Wei Wang <wei.w.wang@...el.com>
Cc:     virtio-dev@...ts.oasis-open.org, linux-kernel@...r.kernel.org,
        qemu-devel@...gnu.org, virtualization@...ts.linux-foundation.org,
        kvm@...r.kernel.org, linux-mm@...ck.org, mst@...hat.com,
        akpm@...ux-foundation.org, mawilcox@...rosoft.com,
        david@...hat.com, cornelia.huck@...ibm.com,
        mgorman@...hsingularity.net, aarcange@...hat.com,
        amit.shah@...hat.com, pbonzini@...hat.com, willy@...radead.org,
        liliang.opensource@...il.com, yang.zhang.wz@...il.com,
        quan.xu@...yun.com
Subject: Re: [PATCH v15 4/5] mm: support reporting free page blocks

On Mon 28-08-17 15:33:26, Michal Hocko wrote:
> On Mon 28-08-17 18:08:32, Wei Wang wrote:
> > This patch adds support to walk through the free page blocks in the
> > system and report them via a callback function. Some page blocks may
> > leave the free list after zone->lock is released, so it is the caller's
> > responsibility to either detect or prevent the use of such pages.
> > 
> > One use example of this patch is to accelerate live migration by skipping
> > the transfer of free pages reported from the guest. A popular method used
> > by the hypervisor to track which part of memory is written during live
> > migration is to write-protect all the guest memory. So, those pages that
> > are reported as free pages but are written after the report function
> > returns will be captured by the hypervisor, and they will be added to the
> > next round of memory transfer.
> 
> OK, looks much better. I still have few nits.
> 
> > +extern void walk_free_mem_block(void *opaque,
> > +				int min_order,
> > +				bool (*report_page_block)(void *, unsigned long,
> > +							  unsigned long));
> > +
> 
> please add names to arguments of the prototype

And one more thing. Your callback returns bool and true usually means a
success while you are using it to break out from the loop. This is
rather confusing. I would expect iterating until false is returned so
the opposite than what you have. You could also change this to int and
return 0 on success and < 0 to break out. 

-- 
Michal Hocko
SUSE Labs