2016-03-17 1:49 GMT+09:00 Lucas Stach <l.stach@xxxxxxxxxxxxxx>: > This fixes an annoying race in the CMA code leading to lots of "PFNs busy" > messages when CMA is used concurrently. This is harmless normally as CMA > will just retry the allocation at a different place, but it might lead to > increased fragmentation of the CMA area as well as failing allocations > when CMA is under memory pressure. > > The issue is that test_pages_isolated checks if the range is free by > checking that all pages in the range are buddy pages. For this to work > the start pfn needs to be aligned to the higher order buddy page > including the start pfn if there is any. > > This is not a problem for the memory hotplug code, as it always offlines > whole pageblocks, but CMA may want to isolate a smaller range. So for > the check to work correctly it down-aligns the start pfn to the higher > order buddy page. As the zone is not yet locked at that point a > concurrent page free might coalesce the pages to be checked into an > even bigger buddy page, causing the check to fail, while all pages are > in fact buddy pages. > > By moving the zone locking to the caller of the test function, it's > possible to do it before CMA tries to find the proper start page and stop > any concurrent page coalescing to happen until the check is finished. I think that this patch cannot prevent the same race on isolate_freepages_range(). If buddy merging happens after we passed test_pages_isolated(), isolate_freepages_range() cannot see buddy page and will fail. Thanks. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>