Re: [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions

[Jerome Glisse]

On Tue, Dec 04, 2018 at 05:15:19PM -0800, Matthew Wilcox wrote:
> On Tue, Dec 04, 2018 at 04:58:01PM -0800, John Hubbard wrote:
> > On 12/4/18 3:03 PM, Dan Williams wrote:
> > > Except the LRU fields are already in use for ZONE_DEVICE pages... how
> > > does this proposal interact with those?
> > 
> > Very badly: page->pgmap and page->hmm_data both get corrupted. Is there an entire
> > use case I'm missing: calling get_user_pages() on ZONE_DEVICE pages? Said another
> > way: is it reasonable to disallow calling get_user_pages() on ZONE_DEVICE pages?
> > 
> > If we have to support get_user_pages() on ZONE_DEVICE pages, then the whole 
> > LRU field approach is unusable.
> 
> We just need to rearrange ZONE_DEVICE pages.  Please excuse the whitespace
> damage:
> 
> +++ b/include/linux/mm_types.h
> @@ -151,10 +151,12 @@ struct page {
>  #endif
>                 };
>                 struct {        /* ZONE_DEVICE pages */
> +                       unsigned long _zd_pad_2;        /* LRU */
> +                       unsigned long _zd_pad_3;        /* LRU */
> +                       unsigned long _zd_pad_1;        /* uses mapping */
>                         /** @pgmap: Points to the hosting device page map. */
>                         struct dev_pagemap *pgmap;
>                         unsigned long hmm_data;
> -                       unsigned long _zd_pad_1;        /* uses mapping */
>                 };
>  
>                 /** @rcu_head: You can use this to free a page by RCU. */
> 
> You don't use page->private or page->index, do you Dan?

page->private and page->index are use by HMM DEVICE page.

Cheers,
Jérôme