On 11.06.22 00:35, Alex Williamson wrote:
> The commit referenced below subtly and inadvertently changed the logic
> to disallow pinning of zero pfns. This breaks device assignment with
> vfio and potentially various other users of gup. Exclude the zero page
> test from the negation.
I wonder which setups can reliably work with a long-term pin on a shared
zeropage. In a MAP_PRIVATE mapping, any write access via the page tables
will end up replacing the shared zeropage with an anonymous page.
Something similar should apply in MAP_SHARED mappings, when lazily
allocating disk blocks.
In the future, we might trigger unsharing when taking a R/O pin for the
shared zeropage, just like we do as of now upstream for shared anonymous
pages (!PageAnonExclusive). And something similar could then be done
when finding a !anon page in a MAP_SHARED mapping.
>
> Fixes: 1c563432588d ("mm: fix is_pinnable_page against a cma page")
Having that said, it indeed looks like that was an unintended change.
Acked-by: David Hildenbrand <david@xxxxxxxxxx>
> Signed-off-by: Alex Williamson <alex.williamson@xxxxxxxxxx>
> ---
>
> At least I assume this was inadvertent... If there's a better fix,
> please run with it as I'm out of the office the 1st half of next
> week and would like to see this fixed ASAP. Thanks!
>
> include/linux/mm.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index bc8f326be0ce..781fae17177d 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1600,7 +1600,7 @@ static inline bool is_pinnable_page(struct page *page)
> if (mt == MIGRATE_CMA || mt == MIGRATE_ISOLATE)
> return false;
> #endif
> - return !(is_zone_movable_page(page) || is_zero_pfn(page_to_pfn(page)));
> + return !is_zone_movable_page(page) || is_zero_pfn(page_to_pfn(page));
> }
> #else
> static inline bool is_pinnable_page(struct page *page)
>
>
--
Thanks,
David / dhildenb
