Re: [PATCH 02/15] mm: page_alloc: update migrate type of pages on pcp when isolating

[Mel Gorman <mel@csn.ul.ie>]

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

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