On Fri, 2 Mar 2018 16:38:33 -0800 Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote: > On 03/02/2018 04:06 PM, Andrew Morton wrote: > > 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? > > I agree that it is a bit ugly. However, when a thread hits the above > condition it will only undo what it has done. Only one thread is able > to set migrate state to isolate (under the zone lock). So, a thread > will only undo what it has done. I don't get it. That would make sense if start_isolate_page_range() held zone->lock across the entire loop, but it doesn't do that. > The exact problem of one thread undoing what another thread has done > is possible with the code today and is what this patch is attempting > to address. > > > 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? > > I could not immediately come up with a good idea here. The zone lock > would be the obvious choice, but I don't think we want to hold it while > examining each of the page blocks. Perhaps a new lock or semaphore > associated with the zone? I'm open to suggestions. Yes, I think it would need a new lock. Hopefully a mutex. -- 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>
