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

[Mike Rapoport]

On Mon, Nov 27, 2023 at 11:52:56AM +0000, Alexandru Elisei wrote:
> 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 at 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().

Ok, just add a sentence about it to the commit message.
 
> 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.
> > 

-- 
Sincerely yours,
Mike.