For slab objects we memorize both alloc and free stacks. You'll never know in advance what information will be usefull
to fix an issue, so it usually better to provide more information. I don't think we should do anything different for pages.
Given that we already have the page_owner responsible for providing alloc/free stacks for pages, all that we should in KASAN do is to
enable the feature by default. Free stack saving should be decoupled from debug_pagealloc into separate option so that it can be enabled
by KASAN and/or debug_pagealloc.