On 07/25/2013 08:40 AM, Wanpeng Li wrote: > On Tue, Jul 23, 2013 at 12:18:06AM +0530, Srivatsa S. Bhat wrote: >> The free-page stealing code in __rmqueue_fallback() is somewhat hard to >> follow, and has an incredible amount of subtlety hidden inside! >> >> First off, there is a minor bug in the reporting of change-of-ownership of >> pageblocks. Under some conditions, we try to move upto 'pageblock_nr_pages' >> no. of pages to the preferred allocation list. But we change the ownership >> of that pageblock to the preferred type only if we manage to successfully >> move atleast half of that pageblock (or if page_group_by_mobility_disabled >> is set). >> >> However, the current code ignores the latter part and sets the 'migratetype' >> variable to the preferred type, irrespective of whether we actually changed >> the pageblock migratetype of that block or not. So, the page_alloc_extfrag >> tracepoint can end up printing incorrect info (i.e., 'change_ownership' >> might be shown as 1 when it must have been 0). >> >> So fixing this involves moving the update of the 'migratetype' variable to >> the right place. But looking closer, we observe that the 'migratetype' variable >> is used subsequently for checks such as "is_migrate_cma()". Obviously the >> intent there is to check if the *fallback* type is MIGRATE_CMA, but since we >> already set the 'migratetype' variable to start_migratetype, we end up checking >> if the *preferred* type is MIGRATE_CMA!! >> >> To make things more interesting, this actually doesn't cause a bug in practice, >> because we never change *anything* if the fallback type is CMA. >> >> So, restructure the code in such a way that it is trivial to understand what >> is going on, and also fix the above mentioned bug. And while at it, also add a >> comment explaining the subtlety behind the migratetype used in the call to >> expand(). >> > > Greate catch! > Thank you :-) Regards, Srivatsa S. Bhat -- 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>