Re: [PATCH 04/22] mm: don't clear ->mapping in hmm_devmem_free

[Christoph Hellwig]

On Thu, Jun 13, 2019 at 07:05:07PM +0000, Jason Gunthorpe wrote:
> Hurm, is hmm following this comment from mm_types.h?
> 
>  * If you allocate the page using alloc_pages(), you can use some of the
>  * space in struct page for your own purposes.  The five words in the main
>  * union are available, except for bit 0 of the first word which must be
>  * kept clear.  Many users use this word to store a pointer to an object
>  * which is guaranteed to be aligned.  If you use the same storage as
>  * page->mapping, you must restore it to NULL before freeing the page.
> 
> Maybe the assumption was that a driver is using ->mapping ?

Maybe.  The union layou in struct page certainly doesn't help..