On Mon, 26 Feb 2018 11:10:54 -0800 Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote: > start_isolate_page_range() is used to set the migrate type of a > set of page blocks to MIGRATE_ISOLATE while attempting to start > a migration operation. It assumes that only one thread is > calling it for the specified range. This routine is used by > CMA, memory hotplug and gigantic huge pages. Each of these users > synchronize access to the range within their subsystem. However, > two subsystems (CMA and gigantic huge pages for example) could > attempt operations on the same range. If this happens, page > blocks may be incorrectly left marked as MIGRATE_ISOLATE and > therefore not available for page allocation. > > Without 'locking code' there is no easy way to synchronize access > to the range of page blocks passed to start_isolate_page_range. > However, if two threads are working on the same set of page blocks > one will stumble upon blocks set to MIGRATE_ISOLATE by the other. > In such conditions, make the thread noticing MIGRATE_ISOLATE > clean up as normal and return -EBUSY to the caller. > > This will allow start_isolate_page_range to serve as a > synchronization mechanism and will allow for more general use > of callers making use of these interfaces. So, update comments > in alloc_contig_range to reflect this new functionality. > > ... > > --- a/mm/page_isolation.c > +++ b/mm/page_isolation.c > @@ -28,6 +28,13 @@ static int set_migratetype_isolate(struct page *page, int migratetype, > > spin_lock_irqsave(&zone->lock, flags); > > + /* > + * We assume we are the only ones trying to isolate this block. > + * If MIGRATE_ISOLATE already set, return -EBUSY > + */ > + if (is_migrate_isolate_page(page)) > + goto out; > + > pfn = page_to_pfn(page); > arg.start_pfn = pfn; > arg.nr_pages = pageblock_nr_pages; Seems a bit ugly and I'm not sure that it's correct. If the loop in start_isolate_page_range() gets partway through a number of pages then we hit the race, start_isolate_page_range() will then go and "undo" the work being done by the thread which it is racing against? Even if that can't happen, blundering through a whole bunch of pages then saying whoops then undoing everything is unpleasing. Should we be looking at preventing these races at a higher level? -- 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>
