On Fri, Jun 04, 2021 at 09:41:45AM +0200, Oscar Salvador wrote: > On Thu, Jun 03, 2021 at 02:45:13PM +0200, Michal Hocko wrote: > > I believe we need to define the purpose of the locking first. The > > If you ask me, this locking would be meant to make sure zone's zone_start_pfn > or spanned_pages do not change under us, in case we __need__ the value to be > stable. > > > existing locking doesn't serve much purpose, does it? The state might > > Well, half-way. Currently, the locking is taken in write mode whenever > the zone is expanded or shrinked, and in read mode when called from > bad_range()->page_outside_zone_boundaries() (only on VM_DEBUG). > > But as you pointed out, such state might change right after the locking is > released and all the work would be for nothing. > So indeed, the __whole__ operation should be envolved by the lock in the caller > The way that stands right now is not optimal. > > > change right after the lock is released and the caller cannot really > > rely on the result. So aside of the current implementation, I would > > argue that any locking has to be be done on the caller layer. > > > > But the primary question is whether anybody actually cares about > > potential races in the first place. > > I have been checking move_freepages_block() and alloc_contig_pages(), which > are two of the functions that call zone_spans_pfn(). > > move_freepages_block() uses it in a way to align the given pfn to pageblock > top and bottom, and then check that aligned pfns are still within the same zone. > From a memory-hotplug perspective that's ok as we know that we are offlining > PAGES_PER_SECTION (which implies whole pageblocks). > > alloc_contig_pages() (used by the hugetlb gigantic allocator) runs through a > node's zonelist and checks whether zone->zone_start_pfn + nr_pages stays within > the same zone. > IMHO, the race with zone_spans_last_pfn() vs mem-hotplug would not be that bad, > as it will be caught afters by e.g: __alloc_contig_pages when pages cannot be > isolated because they are offline etc. > > So, I would say we do not really need the lock, but I might be missing something. > But if we chose to care about this, then the locking should be done right, not > half-way as it is right now. Any thoughts on this? :-) -- Oscar Salvador SUSE L3
