linux-kernel - [RFC PATCH 2/2] mm,page_alloc: Make alloc_contig

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Date:   Mon,  8 Feb 2021 11:38:12 +0100
From:   Oscar Salvador <osalvador@...e.de>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Muchun Song <songmuchun@...edance.com>,
        Michal Hocko <mhocko@...nel.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Oscar Salvador <osalvador@...e.de>
Subject: [RFC PATCH 2/2] mm,page_alloc: Make alloc_contig_range handle free hugetlb pages

Free hugetlb pages are trickier to handle as to in order to guarantee
no userspace appplication disruption, we need to replace the
current free hugepage with a new one.

In order to do that, a new function called alloc_and_dissolve_huge_page
in introduced.
This function will first try to get a new fresh hugetlb page, and if it
succeeds, it will dissolve the old one.

Signed-off-by: Oscar Salvador <osalvador@...e.de>
---
 include/linux/hugetlb.h |  6 ++++++
 mm/compaction.c         | 11 +++++++++++
 mm/hugetlb.c            | 35 +++++++++++++++++++++++++++++++++++
 3 files changed, 52 insertions(+)

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index ebca2ef02212..f81afcb86e89 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -505,6 +505,7 @@ struct huge_bootmem_page {
 	struct hstate *hstate;
 };
 
+bool alloc_and_dissolve_huge_page(struct page *page);
 struct page *alloc_huge_page(struct vm_area_struct *vma,
 				unsigned long addr, int avoid_reserve);
 struct page *alloc_huge_page_nodemask(struct hstate *h, int preferred_nid,
@@ -773,6 +774,11 @@ static inline void huge_ptep_modify_prot_commit(struct vm_area_struct *vma,
 #else	/* CONFIG_HUGETLB_PAGE */
 struct hstate {};
 
+static inline bool alloc_and_dissolve_huge_page(struct page *page)
+{
+	return false;
+}
+
 static inline struct page *alloc_huge_page(struct vm_area_struct *vma,
 					   unsigned long addr,
 					   int avoid_reserve)
diff --git a/mm/compaction.c b/mm/compaction.c
index 89cd2e60da29..7969ddc10856 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -952,6 +952,17 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
 					low_pfn += compound_nr(page) - 1;
 					goto isolate_success_no_list;
 				}
+			} else {
+				/*
+				 * Free hugetlb page. Allocate a new one and
+				 * dissolve this is if succeed.
+				 */
+				if (alloc_and_dissolve_huge_page(page)) {
+					unsigned long order = buddy_order_unsafe(page);
+
+					low_pfn += (1UL << order) - 1;
+					continue;
+				}
 			}
 			goto isolate_fail;
 		}
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 18f6ee317900..79ffbb64c4ee 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -2253,6 +2253,41 @@ static void restore_reserve_on_error(struct hstate *h,
 	}
 }
 
+bool alloc_and_dissolve_huge_page(struct page *page)
+{
+	NODEMASK_ALLOC(nodemask_t, nodes_allowed, GFP_KERNEL);
+	struct page *head;
+	struct hstate *h;
+	bool ret = false;
+	int nid;
+
+	if (!nodes_allowed)
+		return ret;
+
+	spin_lock(&hugetlb_lock);
+	head = compound_head(page);
+	h = page_hstate(head);
+	nid = page_to_nid(head);
+	spin_unlock(&hugetlb_lock);
+
+	init_nodemask_of_node(nodes_allowed, nid);
+
+	/*
+	 * Before dissolving the page, we need to allocate a new one,
+	 * so the pool remains stable.
+	 */
+	if (alloc_pool_huge_page(h, nodes_allowed, NULL)) {
+		/*
+		 * Ok, we have a free hugetlb-page to replace this
+		 * one. Dissolve the old page.
+		 */
+		if (!dissolve_free_huge_page(page))
+			ret = true;
+	}
+
+	return ret;
+}
+
 struct page *alloc_huge_page(struct vm_area_struct *vma,
 				    unsigned long addr, int avoid_reserve)
 {
-- 
2.16.3