On 02.08.2019 18:16, George Dunlap wrote: > Commits ec83f825627 "mm.h: add helper function to test-and-clear > _PGC_allocated" (and subsequent fix-up 44a887d021d "mm.h: fix BUG_ON() > condition in put_page_alloc_ref()") introduced a BUG_ON() to detect > unsafe behavior of callers. > > Unfortunately this condition still turns out to be too strict. > xenheap pages are somewhat "magic": calling free_domheap_pages() on > them will not cause free_heap_pages() to be called: whichever part of > Xen allocated them specially must call free_xenheap_pages() > specifically. (They'll also be handled appropriately at domain > destruction time.) > > Only crash Xen when put_page_alloc_ref() finds only a single refcount > if the page is not a xenheap page. > > Signed-off-by: George Dunlap <[email protected]>
Reviewed-by: Jan Beulich <[email protected]> albeit with a suggestion: > --- a/xen/include/xen/mm.h > +++ b/xen/include/xen/mm.h > @@ -666,15 +666,20 @@ static inline void > share_xen_page_with_privileged_guests( > static inline void put_page_alloc_ref(struct page_info *page) > { > /* > - * Whenever a page is assigned to a domain then the _PGC_allocated bit > - * is set and the reference count is set to at least 1. This function > - * clears that 'allocation reference' but it is unsafe to do so without > - * the caller holding an additional reference. I.e. the allocation > - * reference must never be the last reference held. > + * Whenever a page is assigned to a domain then the _PGC_allocated > + * bit is set and the reference count is set to at least 1. This > + * function clears that 'allocation reference' but it is unsafe to > + * do so to domheap pages without the caller holding an additional > + * reference. I.e. the allocation reference must never be the last > + * reference held. > + * > + * (It's safe for xenheap pages, because put_page() will not cause > + * them to be freed.) > */ > if ( test_and_clear_bit(_PGC_allocated, &page->count_info) ) > { > - BUG_ON((page->count_info & PGC_count_mask) <= 1); > + BUG_ON((page->count_info & PGC_count_mask) <= 1 && > + !(page->count_info & PGC_xen_heap)); This can be had with a single conditional: BUG_ON((page->count_info & (PGC_xen_heap | PGC_count_mask)) <= 1); Jan _______________________________________________ Xen-devel mailing list [email protected] https://lists.xenproject.org/mailman/listinfo/xen-devel
