Re: [PATCH v6 2/6] mm/memplicy: add page allocation function for MPOL_PREFERRED_MANY policy

[Feng Tang <feng.tang@intel.com>]

On Wed, Jul 28, 2021 at 02:42:26PM +0200, Michal Hocko wrote:
> On Mon 12-07-21 16:09:30, Feng Tang wrote:
> > The semantics of MPOL_PREFERRED_MANY is similar to MPOL_PREFERRED,
> > that it will first try to allocate memory from the preferred node(s),
> > and fallback to all nodes in system when first try fails.
> > 
> > Add a dedicated function for it just like 'interleave' policy.
> > 
> > Link: https://lore.kernel.org/r/20200630212517.308045-9-ben.widawsky@intel.com
> > Suggested-by: Michal Hocko <mhocko@suse.com>
> > Co-developed-by: Ben Widawsky <ben.widawsky@intel.com>
> > Signed-off-by: Ben Widawsky <ben.widawsky@intel.com>
> > Signed-off-by: Feng Tang <feng.tang@intel.com>
> 
> It would be better to squash this together with the actual user of the
> function added by the next patch.
 
Ok, will do

> > ---
> >  mm/mempolicy.c | 19 +++++++++++++++++++
> >  1 file changed, 19 insertions(+)
> > 
> > diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> > index 17b5800b7dcc..d17bf018efcc 100644
> > --- a/mm/mempolicy.c
> > +++ b/mm/mempolicy.c
> > @@ -2153,6 +2153,25 @@ static struct page *alloc_page_interleave(gfp_t gfp, unsigned order,
> >  	return page;
> >  }
> >  
> > +static struct page *alloc_page_preferred_many(gfp_t gfp, unsigned int order,
> > +						struct mempolicy *pol)
> 
> We likely want a node parameter to know which one we want to start with
> for locality. Callers should use policy_node for that.
 
Yes, locality should be considered, something like this?

	int pnid, lnid = numa_node_id();

	if (is_nodeset(lnid, &pol->nodes))
		pnid = local_nid;
	else
		pnid = first_node(pol->nodes);

	page = __alloc_pages(((gfp | __GFP_NOWARN) & ~__GFP_DIRECT_RECLAIM),
				order, pnid, &pol->nodes);
	if (!page)
		page = __alloc_pages(gfp, order, lnid, NULL);
	return page;


> > +{
> > +	struct page *page;
> > +
> > +	/*
> > +	 * This is a two pass approach. The first pass will only try the
> > +	 * preferred nodes but skip the direct reclaim and allow the
> > +	 * allocation to fail, while the second pass will try all the
> > +	 * nodes in system.
> > +	 */
> > +	page = __alloc_pages(((gfp | __GFP_NOWARN) & ~__GFP_DIRECT_RECLAIM),
> > +				order, first_node(pol->nodes), &pol->nodes);
> 
> Although most users will likely have some form of GFP_*USER* here and
> clearing __GFP_DIRECT_RECLAIM will put all other reclaim modifiers out
> of game I think it would be better to explicitly disable some of them to
> prevent from surprises. E.g. any potential __GFP_NOFAIL would be more
> than surprising here. We do not have any (hopefully) but this should be
> pretty cheap to exclude as we already have to modify already.
> 
> 	preferred_gfp = gfp | __GFP_NOWARN;
> 	preferred_gfp &= ~(__GFP_DIRECT_RECLAIM | __GFP_NOFAIL)

OK, will add.

Thanks,
Feng

> 
> > +	if (!page)
> > +		page = __alloc_pages(gfp, order, numa_node_id(), NULL);
> > +
> > +	return page;
> > +}
> > +
> >  /**
> >   * alloc_pages_vma - Allocate a page for a VMA.
> >   * @gfp: GFP flags.
> > -- 
> > 2.7.4
> 
> -- 
> Michal Hocko
> SUSE Labs