On Mon, Jun 07, 2021 at 10:49:01AM +0200, David Hildenbrand wrote: > I'd like to point out that I think the seqlock is not in place to > synchronize with actual growing/shrinking but to get consistent zone ranges > -- like using atomics, but we have two inter-dependent values here. I guess so, at least that's what it should do. But the way it is placed right now is misleading. If we really want to get consistent zone ranges, we should start using zone's seqlock where it matters and that is pretty much all those places that use zone_spans_pfn(). Otherwise there is no way you can be sure the pfn you're checking is within the limits. Moreover, as Michal pointed out early, if we really want to go down that road the locking should be made in the caller evolving the operation, otheriwse things might change once the lock is dropped and you're working with a wrong assumption. I can see arguments for both riping it out and doing it right (but none for the way it is right now). For riping it out, one could say that those races might not be fatal, as usually the pfn you're working with (the one you want to check falls within a certain range) you know is valid, so the worst can happen is you get false positives/negatives and that might or might not be detected further down. How bad are false positive/negatives I guess it depends on the situation, but we already do that right now. The zone_spans_pfn() from page_outside_zone_boundaries() is the only one using locking right now, so well, if we survided this long without locks in other places using zone_spans_pfn() makes one wonder if it is that bad. On the other hand, one could argue that for correctness sake, we should be holding zone's seqlock whenever checking for zone_spans_pfn() to avoid any inconsistency. -- Oscar Salvador SUSE L3
