page_ext and memdescs

[Matthew Wilcox <willy@infradead.org>]

On Mon, Jun 16, 2025 at 07:09:30PM +0530, Bharata B Rao wrote:
> diff --git a/include/linux/page_ext.h b/include/linux/page_ext.h
> index 76c817162d2f..4300c9dbafec 100644
> --- a/include/linux/page_ext.h
> +++ b/include/linux/page_ext.h
> @@ -40,8 +40,25 @@ enum page_ext_flags {
>  	PAGE_EXT_YOUNG,
>  	PAGE_EXT_IDLE,
>  #endif
> +	/*
> +	 * 32 bits following this are used by the migrator.
> +	 * The next available bit position is 33.
> +	 */
> +	PAGE_EXT_MIGRATE_READY,
>  };
>  
> +#define PAGE_EXT_MIG_NID_WIDTH	10
> +#define PAGE_EXT_MIG_FREQ_WIDTH	3
> +#define PAGE_EXT_MIG_TIME_WIDTH	18
> +
> +#define PAGE_EXT_MIG_NID_SHIFT	(PAGE_EXT_MIGRATE_READY + 1)
> +#define PAGE_EXT_MIG_FREQ_SHIFT	(PAGE_EXT_MIG_NID_SHIFT + PAGE_EXT_MIG_NID_WIDTH)
> +#define PAGE_EXT_MIG_TIME_SHIFT	(PAGE_EXT_MIG_FREQ_SHIFT + PAGE_EXT_MIG_FREQ_WIDTH)
> +
> +#define PAGE_EXT_MIG_NID_MASK	((1UL << PAGE_EXT_MIG_NID_SHIFT) - 1)
> +#define PAGE_EXT_MIG_FREQ_MASK	((1UL << PAGE_EXT_MIG_FREQ_SHIFT) - 1)
> +#define PAGE_EXT_MIG_TIME_MASK	((1UL << PAGE_EXT_MIG_TIME_SHIFT) - 1)

OK, so we need to have a conversation about page_ext.  Sorry this is
happening to you.  I've kind of skipped over page_ext when talking
about folios and memdescs up to now, so it's not that you've missed
anything.

As the comment says,

 * Page Extension can be considered as an extended mem_map.

and we need to do this because we don't want to grow struct page beyond
64 bytes.  But memdescs are dynamically allocated, so we don't need
page_ext any more, and all that code can go away.

lib/alloc_tag.c:struct page_ext_operations page_alloc_tagging_ops = {
mm/page_ext.c:static struct page_ext_operations page_idle_ops __initdata = {
mm/page_ext.c:static struct page_ext_operations *page_ext_ops[] __initdata = {
mm/page_owner.c:struct page_ext_operations page_owner_ops = {
mm/page_table_check.c:struct page_ext_operations page_table_check_ops = {

I think all of these are actually per-memdesc thangs and not per-page
things, so we can get rid of them all.  That means I don't want to see
new per-page data being added to page_ext.

So, what's this really used for?  It seems like it's really
per-allocation, not per-page.  Does it need to be preserved across
alloc/free or can it be reset at free time?