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:

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On Wed, Jul 27, 2016 at 09:40:56AM -0700, Dave Hansen wrote:

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On 07/26/2016 06:23 PM, Liang Li wrote:

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+	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);

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+			}
+		}
+	}

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Isn't this bitmap out of date by the time it's send up to the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stuff is totally out of date.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