On Tue, Jan 31, 2012 at 05:23:59PM +0100, Marek Szyprowski wrote:

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+		page = pfn_to_page(pfn);
+		if (PageBuddy(page)) {
+			pfn += 1 << page_order(page);
+		} else if (page_count(page) == 0) {
+			set_page_private(page, MIGRATE_ISOLATE);
+			++pfn;

This is dangerous for two reasons. If the page_count is 0, it could
be because the page is in the process of being freed and is not
necessarily on the per-cpu lists yet and you cannot be sure if the
contents of page->private are important. Second, there is nothing to
prevent another CPU allocating this page from its per-cpu list while
the private field is getting updated from here which might lead to
some interesting races.

I recognise that what you are trying to do is respond to Gilad's
request that you really check if an IPI here is necessary. I think what
you need to do is check if a page with a count of 0 is encountered
and if it is, then a draining of the per-cpu lists is necessary. To
address Gilad's concerns, be sure to only this this once per attempt at
CMA rather than for every page encountered with a count of 0 to avoid a
storm of IPIs.

It's actually more then that.

This is the same issue that I first fixed with a change to free_pcppages_bulk()
function[1].  At the time of positing, you said you'd like me to try and find
a different solution which would not involve paying the price of calling
get_pageblock_migratetype().  Later I also realised that this solution is
not enough.

[1] http://article.gmane.org/gmane.linux.kernel.mm/70314

Yes. I had forgotten the history but looking at that patch again,
I would reach the conclusion that this was adding a new call to
get_pageblock_migratetype() in the bulk free path. That would affect
everybody whether they were using CMA or not.

This will be a bit ugly, but we can also use that code and compile it conditionally
when CMA has been enabled.

That would also be very unfortunate because it means enabling CMA incurs
a performance cost to everyone whether they use CMA or not. For ARM,
this may not be a problem but it would be for other arches if they
wanted to use CMA or if it ever became part of a distro contig.

Pages, which have incorrect migrate type on free finally
causes pageblock migration type change from MIGRATE_CMA to MIGRATE_MOVABLE.

I'm not quite seeing this. In free_hot_cold_page(), the pageblock
type is checked so the page private should be set to MIGRATE_CMA or
MIGRATE_ISOLATE for the CMA area. It's not clear how this can change a
pageblock to MIGRATE_MOVABLE in error. If it turns out that you
absolutely have to call get_pageblock_migratetype() from
free_pcppages_bulk() and my alternative suggestion did not work out then
document all these issues in a comment when putting the call under
CONFIG_CMA so that it is not forgotten.

-- 
Mel Gorman
SUSE Labs