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

[Alexandru Elisei <alexandru.elisei@arm.com>]

Hi Mike,

I really appreciate you having a look!

On Sat, Nov 25, 2023 at 12:03:22PM +0200, Mike Rapoport wrote:
> 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@arm.com>
> > ---
> >  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.

I did it this way so an arch won't be able to remove flags set by the MM code.
There's a similar pattern in do_mmap() -> calc_vm_flag_bits() ->
arch_calc_vm_flag_bits().

I'll change it to return the new mask if you think that's better.

Thanks,
Alex

> 
> >  		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.
>