Re: [PATCH RFC v2 04/27] mm: migrate/mempolicy: Add hook to modify migration target gfp

[Mike Rapoport]

On Sun, Nov 19, 2023 at 04:56:58PM +0000, Alexandru Elisei wrote:
> It might be desirable for an architecture to modify the gfp flags used to
> allocate the destination page for migration based on the page that it is
> being replaced. For example, if an architectures has metadata associated
> with a page (like arm64, when the memory tagging extension is implemented),
> it can request that the destination page similarly has storage for tags
> already allocated.
> 
> No functional change.
> 
> Signed-off-by: Alexandru Elisei <alexandru.elisei@xxxxxxx>
> ---
>  include/linux/migrate.h | 4 ++++
>  mm/mempolicy.c          | 2 ++
>  mm/migrate.c            | 3 +++
>  3 files changed, 9 insertions(+)
> 
> diff --git a/include/linux/migrate.h b/include/linux/migrate.h
> index 2ce13e8a309b..0acef592043c 100644
> --- a/include/linux/migrate.h
> +++ b/include/linux/migrate.h
> @@ -60,6 +60,10 @@ struct movable_operations {
>  /* Defined in mm/debug.c: */
>  extern const char *migrate_reason_names[MR_TYPES];
>  
> +#ifndef arch_migration_target_gfp
> +#define arch_migration_target_gfp(src, gfp) 0
> +#endif
> +
>  #ifdef CONFIG_MIGRATION
>  
>  void putback_movable_pages(struct list_head *l);
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 10a590ee1c89..50bc43ab50d6 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -1182,6 +1182,7 @@ static struct folio *alloc_migration_target_by_mpol(struct folio *src,
>  
>  		h = folio_hstate(src);
>  		gfp = htlb_alloc_mask(h);
> +		gfp |= arch_migration_target_gfp(src, gfp);

I think it'll be more robust to have arch_migration_target_gfp() to modify
the flags and return the new mask with added (or potentially removed)
flags.

>  		nodemask = policy_nodemask(gfp, pol, ilx, &nid);
>  		return alloc_hugetlb_folio_nodemask(h, nid, nodemask, gfp);
>  	}
> @@ -1190,6 +1191,7 @@ static struct folio *alloc_migration_target_by_mpol(struct folio *src,
>  		gfp = GFP_TRANSHUGE;
>  	else
>  		gfp = GFP_HIGHUSER_MOVABLE | __GFP_RETRY_MAYFAIL | __GFP_COMP;
> +	gfp |= arch_migration_target_gfp(src, gfp);
>  
>  	page = alloc_pages_mpol(gfp, order, pol, ilx, nid);
>  	return page_rmappable_folio(page);

-- 
Sincerely yours,
Mike.