On 8/5/19 10:42 AM, Jan Beulich wrote: > 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);
I'll commit this version. Thanks. -George _______________________________________________ Xen-devel mailing list [email protected] https://lists.xenproject.org/mailman/listinfo/xen-devel
