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.
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.