On Tue, Dec 4, 2018 at 5:15 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> 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?
I don't use page->private, but page->index is used by the
memory-failure path to do an rmap.
