On Mon, 26 Feb 2018 11:10:54 -0800 Mike Kravetz <mike.krav...@oracle.com> 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
> 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?