On 9/6/21 14:49, David Hildenbrand wrote: > On 06.09.21 14:45, Miaohe Lin wrote: >> On 2021/9/6 20:11, David Hildenbrand wrote: >>> On 06.09.21 14:02, David Hildenbrand wrote: >>>> On 04.09.21 11:18, Miaohe Lin wrote: >>>> >>>> Thanks! >>>> >>>> Reviewed-by: David Hildenbrand <david@xxxxxxxxxx> >>>> >>> >>> To make the confusion perfect (sorry) :D I tripple-checked: >>> >>> In unset_migratetype_isolate() we check that is_migrate_isolate_page(page) holds, otherwise we return. >>> >>> We call __isolate_free_page() only for such pages. >>> >>> __isolate_free_page() won't perform watermark checks on is_migrate_isolate(). >>> >>> Consequently, __isolate_free_page() should never fail when called from unset_migratetype_isolate() >>> >>> If that's correct then we could instead maybe add a VM_BUG_ON() and a comment why this can't fail. >>> >>> >>> Makes sense or am I missing something? >> >> I think you're right. __isolate_free_page() should never fail when called from unset_migratetype_isolate() >> as explained by you. But it might be too fragile to reply on the failure conditions of __isolate_free_page(). >> If that changes, VM_BUG_ON() here might trigger unexpectedly. Or am I just over-worried as failure conditions >> of __isolate_free_page() can hardly change? > > Maybe > > isolated_page = !!__isolate_free_page(page, order); > /* > * Isolating a free page in an isolated pageblock is expected to always > * work as watermarks don't apply here. > */ > VM_BUG_ON(isolated_page); > > > VM_BUG_ON() allows us to detect any issues when testing. Combined with > the comment it tells everybody messing with __isolate_free_page() what > we expect in this function. > > In production system, we would handle it gracefully. If this can be handled gracefully, then I'd rather go with VM_WARN_ON. Maybe even WARN_ON_ONCE?
