On 03/02/2018 04:56 PM, Andrew Morton wrote: > 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. > It works because all threads set migrate isolate on page blocks going from pfn low to pfn high. When they encounter a conflict, they know exactly which blocks they set and only undo those blocks. Perhaps, I am missing something, but it does not matter because ... >> 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. I'll look into adding an 'isolate' mutex to the zone structure and reworking this patch. -- Mike Kravetz -- 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>
