Re: [Xen-devel] [PATCH] mm: Safe to clear PGC_allocated on xenheap pages without an extra reference

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 02/08/2019 17:16, 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 <george.dunlap@xxxxxxxxxx>

This works for me, so Tested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Some suggestions however...

> ---
> NB this has been compile-tested only.
>
> CC: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> CC: Jan Beulich <jbeulich@xxxxxxxx>
> CC: Paul Durrant <paul.durrant@xxxxxxxxxx>
> ---
>  xen/include/xen/mm.h | 17 +++++++++++------
>  1 file changed, 11 insertions(+), 6 deletions(-)
>
> diff --git a/xen/include/xen/mm.h b/xen/include/xen/mm.h
> index 977e45aae7..297fc6ad7b 100644
> --- 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.
> +     * 

Trailing whitespace.

> +     * (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));

I know this is a matter of preference, but I think the logic would be
easier to follow as

BUG_ON(!(page->count_info & PGC_xen_heap) &&
       (page->count_info & PGC_count_mask) <= 1);

i.e. check for domheap pages before looking at the refcount.

~Andrew

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