linux-kernel - Re: [PATCH 2/3] mm: hugetlb: make the hugetlb migration strategy consistent

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Date: Wed, 28 Feb 2024 09:41:02 +0100
From: Oscar Salvador <osalvador@...e.de>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: akpm@...ux-foundation.org, muchun.song@...ux.dev, david@...hat.com,
	linmiaohe@...wei.com, naoya.horiguchi@....com, mhocko@...nel.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] mm: hugetlb: make the hugetlb migration strategy
 consistent

On Wed, Feb 28, 2024 at 03:40:08PM +0800, Baolin Wang wrote:
> 
> 
> On 2024/2/27 23:17, Oscar Salvador wrote:
> > On Tue, Feb 27, 2024 at 09:52:26PM +0800, Baolin Wang wrote:
> > 
> > > --- a/mm/hugetlb.c
> > > +++ b/mm/hugetlb.c
> > > @@ -2567,13 +2567,38 @@ static struct folio *alloc_surplus_hugetlb_folio(struct hstate *h,
> > >   }
> > >   static struct folio *alloc_migrate_hugetlb_folio(struct hstate *h, gfp_t gfp_mask,
> > > -				     int nid, nodemask_t *nmask)
> > > +				     int nid, nodemask_t *nmask, int reason)
> > 
> > I still dislike taking the reason argument this far, and I'd rather have
> > this as a boolean specifing whether we allow fallback on other nodes.
> > That would mean parsing the reason in alloc_migration_target().
> > If we don't add a new helper e.g: gfp_allow_fallback(), we can just do
> > it right there an opencode it with a e.g: macro etc.
> > 
> > Although doing it in an inline helper might help hiding these details.
> > 
> > That's my take on this, but let's see what others have to say.
> 
> Sure. I also expressed my preference for hiding these details within the
> hugetlb core as much as possible.
> 
> Muchun, what do you think? Thanks.

JFTR: I'm talking about https://lore.kernel.org/linux-mm/ZdxXLTDZn8fD3pEn@localhost.localdomain/
or maybe something cleaner which doesn't need a new helper (we could if
we want though):

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index c1ee640d87b1..ddd794e861e6 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -73,6 +73,16 @@ struct resv_map {
 #endif
 };

+#define MIGRATE_MEMORY_HOTPLUG	1UL << MR_MEMORY_HOTPLUG
+#define MIGRATE_MEMORY_FAILURE	1UL << MR_MEMORY_FAILURE
+#define MIGRATE_SYSCALL		1UL << MR_SYSCALL
+#define MIGRATE_MBIND		1UL << MR_MEMPOLICY_MBIND
+#define HTLB_ALLOW_FALLBACK	(MIGRATE_MEMORY_HOTPLUG| \
+				 MIGRATE_MEMORY_FAILURE| \
+				 MIGRATE_SYSCALL| \
+				 MIGRATE_MBIND)
+
+
 /*
  * Region tracking -- allows tracking of reservations and instantiated pages
  *                    across the pages in a mapping.
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index ed1581b670d4..7e8d6b5885d6 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -2619,7 +2619,7 @@ struct folio *alloc_buddy_hugetlb_folio_with_mpol(struct hstate *h,

 /* folio migration callback function */
 struct folio *alloc_hugetlb_folio_nodemask(struct hstate *h, int preferred_nid,
-		nodemask_t *nmask, gfp_t gfp_mask)
+		nodemask_t *nmask, gfp_t gfp_mask, bool allow_fallback)
 {
 	spin_lock_irq(&hugetlb_lock);
 	if (available_huge_pages(h)) {
@@ -2634,6 +2634,12 @@ struct folio *alloc_hugetlb_folio_nodemask(struct hstate *h, int preferred_nid,
 	}
 	spin_unlock_irq(&hugetlb_lock);

+	/*
+	 * We cannot fallback to other nodes, as we could break the per-node pool
+	 */
+	if (!allow_fallback)
+		gfp_mask |= GFP_THISNODE;
+
 	return alloc_migrate_hugetlb_folio(h, gfp_mask, preferred_nid, nmask);
 }

diff --git a/mm/migrate.c b/mm/migrate.c
index cc9f2bcd73b4..c1f1d011629d 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -2016,10 +2016,15 @@ struct folio *alloc_migration_target(struct folio *src, unsigned long private)

 	if (folio_test_hugetlb(src)) {
 		struct hstate *h = folio_hstate(src);
+		bool allow_fallback = false;
+
+		if ((1UL << reason) & HTLB_ALLOW_FALLBACK)
+			allow_fallback = true;

 		gfp_mask = htlb_modify_alloc_mask(h, gfp_mask);
 		return alloc_hugetlb_folio_nodemask(h, nid,
-						mtc->nmask, gfp_mask);
+						mtc->nmask, gfp_mask,
+						allow_fallback);
 	}

 	if (folio_test_large(src)) {

-- 
Oscar Salvador
SUSE Labs