On 02/09/2018 07:28 AM, Matthew Wilcox wrote:
> union {
> + /*
> + * If the page is neither PageSlab nor PageAnon, the value
> + * stored here may help distinguish it from page cache pages.
> + * See page-flags.h for a list of page types which are
> + * currently stored here.
> + */
> + unsigned int page_type;
> +
> _slub_counter_t counters;
> unsigned int active; /* SLAB */
> struct { /* SLUB */
> @@ -107,11 +115,6 @@ struct page {
> /*
> * Count of ptes mapped in mms, to show when
> * page is mapped & limit reverse map searches.
> - *
> - * Extra information about page type may be
> - * stored here for pages that are never mapped,
> - * in which case the value MUST BE <= -2.
> - * See page-flags.h for more details.
> */
> atomic_t _mapcount;
Are there any straightforward rules that we can enforce here? For
instance, if you are using "page_type", you can never have PG_lru set.
Not that we have done this at all for 'struct page' historically, it
would be really convenient to have a clear definition for when
"page_type" is valid vs. "_mapcount".
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