linux-kernel - RE: [PATCH v2 repost 6/7] mm: add the related functions to get free page info

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Message-ID: <F2CBF3009FA73547804AE4C663CAB28E04213EBF@shsmsx102.ccr.corp.intel.com>
Date:	Thu, 28 Jul 2016 04:36:46 +0000
From:	"Li, Liang Z" <liang.z.li@...el.com>
To:	"Hansen, Dave" <dave.hansen@...el.com>,
	"Michael S. Tsirkin" <mst@...hat.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"virtualization@...ts.linux-foundation.org" 
	<virtualization@...ts.linux-foundation.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"virtio-dev@...ts.oasis-open.org" <virtio-dev@...ts.oasis-open.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"qemu-devel@...gnu.org" <qemu-devel@...gnu.org>,
	"dgilbert@...hat.com" <dgilbert@...hat.com>,
	"quintela@...hat.com" <quintela@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Vlastimil Babka" <vbabka@...e.cz>,
	Mel Gorman <mgorman@...hsingularity.net>,
	"Paolo Bonzini" <pbonzini@...hat.com>,
	Cornelia Huck <cornelia.huck@...ibm.com>,
	Amit Shah <amit.shah@...hat.com>
Subject: RE: [PATCH v2 repost 6/7] mm: add the related functions to get free
 page info

> On 07/27/2016 03:05 PM, Michael S. Tsirkin wrote:
> > On Wed, Jul 27, 2016 at 09:40:56AM -0700, Dave Hansen wrote:
> >> On 07/26/2016 06:23 PM, Liang Li wrote:
> >>> +	for_each_migratetype_order(order, t) {
> >>> +		list_for_each(curr, &zone->free_area[order].free_list[t]) {
> >>> +			pfn = page_to_pfn(list_entry(curr, struct page, lru));
> >>> +			if (pfn >= start_pfn && pfn <= end_pfn) {
> >>> +				page_num = 1UL << order;
> >>> +				if (pfn + page_num > end_pfn)
> >>> +					page_num = end_pfn - pfn;
> >>> +				bitmap_set(bitmap, pfn - start_pfn,
> page_num);
> >>> +			}
> >>> +		}
> >>> +	}
> >>
> >> Nit:  The 'page_num' nomenclature really confused me here.  It is the
> >> number of bits being set in the bitmap.  Seems like calling it
> >> nr_pages or num_pages would be more appropriate.
> >>
> >> Isn't this bitmap out of date by the time it's send up to the
> >> hypervisor?  Is there something that makes the inaccuracy OK here?
> >
> > Yes. Calling these free pages is unfortunate. It's likely to confuse
> > people thinking they can just discard these pages.
> >
> > Hypervisor sends a request. We respond with this list of pages, and
> > the guarantee hypervisor needs is that these were free sometime
> > between request and response, so they are safe to free if they are
> > unmodified since the request. hypervisor can detect modifications so
> > it can detect modifications itself and does not need guest help.
> 
> Ahh, that makes sense.
> 
> So the hypervisor is trying to figure out: "Which pages do I move?".  It wants
> to know which pages the guest thinks have good data and need to move.
> But, the list of free pages is (likely) smaller than the list of pages with good
> data, so it asks for that instead.
> 
> A write to a page means that it has valuable data, regardless of whether it
> was in the free list or not.
> 
> The hypervisor only skips moving pages that were free *and* were never
> written to.  So we never lose data, even if this "get free page info"
> stuff is totally out of date.
> 
> The patch description and code comments are, um, a _bit_ light for this level
> of subtlety. :)

I will add more description about this in v3.

Thanks!
Liang