Re: [PATCH v17 4/9] mm: hugetlb: alloc the vmemmap pages associated with each HugeTLB page

When we free a HugeTLB page to the buddy allocator, we should allocate
the vmemmap pages associated with it. But we may cannot allocate vmemmap
pages when the system is under memory pressure, in this case, we just
refuse to free the HugeTLB page instead of looping forever trying to
allocate the pages. This changes some behavior (list below) on some
corner cases.

 1) Failing to free a huge page triggered by the user (decrease nr_pages).

    Need try again later by the user.

 2) Failing to free a surplus huge page when freed by the application.

    Try again later when freeing a huge page next time.

 3) Failing to dissolve a free huge page on ZONE_MOVABLE via
    offline_pages().

    This is a bit unfortunate if we have plenty of ZONE_MOVABLE memory
    but are low on kernel memory. For example, migration of huge pages
    would still work, however, dissolving the free page does not work.
    This is a corner cases. When the system is that much under memory
    pressure, offlining/unplug can be expected to fail. This is
    unfortunate because it prevents from the memory offlining which
    shouldn't happen for movable zones. People depending on the memory
    hotplug and movable zone should carefuly consider whether savings
    on unmovable memory are worth losing their hotplug functionality
    in some situations.

 4) Failing to dissolve a huge page on CMA/ZONE_MOVABLE via
    alloc_contig_range() - once we have that handling in place. Mainly
    affects CMA and virtio-mem.

    Similar to 3). virito-mem will handle migration errors gracefully.
    CMA might be able to fallback on other free areas within the CMA
    region.

Vmemmap pages are allocated from the page freeing context. In order for
those allocations to be not disruptive (e.g. trigger oom killer)
__GFP_NORETRY is used. hugetlb_lock is dropped for the allocation
because a non sleeping allocation would be too fragile and it could fail
too easily under memory pressure. GFP_ATOMIC or other modes to access
memory reserves is not used because we want to prevent consuming
reserves under heavy hugetlb freeing.

Signed-off-by: Muchun Song <redacted>
---
 Documentation/admin-guide/mm/hugetlbpage.rst |  8 +++
 include/linux/mm.h                           |  2 +
 mm/hugetlb.c                                 | 92 +++++++++++++++++++++-------
 mm/hugetlb_vmemmap.c                         | 32 ++++++----
 mm/hugetlb_vmemmap.h                         | 23 +++++++
 mm/sparse-vmemmap.c                          | 75 ++++++++++++++++++++++-
 6 files changed, 197 insertions(+), 35 deletions(-)

diff --git a/Documentation/admin-guide/mm/hugetlbpage.rst b/Documentation/admin-guide/mm/hugetlbpage.rst
index f7b1c7462991..6988895d09a8 100644
--- a/Documentation/admin-guide/mm/hugetlbpage.rst
+++ b/Documentation/admin-guide/mm/hugetlbpage.rst

@@ -60,6 +60,10 @@ HugePages_Surp
         the pool above the value in ``/proc/sys/vm/nr_hugepages``. The
         maximum number of surplus huge pages is controlled by
         ``/proc/sys/vm/nr_overcommit_hugepages``.
+	Note: When the feature of freeing unused vmemmap pages associated
+	with each hugetlb page is enabled, the number of surplus huge pages
+	may be temporarily larger than the maximum number of surplus huge
+	pages when the system is under memory pressure.
 Hugepagesize
 	is the default hugepage size (in Kb).
 Hugetlb

@@ -80,6 +84,10 @@ returned to the huge page pool when freed by a task.  A user with root
 privileges can dynamically allocate more or free some persistent huge pages
 by increasing or decreasing the value of ``nr_hugepages``.
 
+Note: When the feature of freeing unused vmemmap pages associated with each
+hugetlb page is enabled, we can fail to free the huge pages triggered by
+the user when ths system is under memory pressure.  Please try again later.
+
 Pages that are used as huge pages are reserved inside the kernel and cannot
 be used for other purposes.  Huge pages cannot be swapped out under
 memory pressure.

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 4ddfc31f21c6..77693c944a36 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h

@@ -2973,6 +2973,8 @@ static inline void print_vma_addr(char *prefix, unsigned long rip)
 
 void vmemmap_remap_free(unsigned long start, unsigned long end,
 			unsigned long reuse);
+int vmemmap_remap_alloc(unsigned long start, unsigned long end,
+			unsigned long reuse, gfp_t gfp_mask);
 
 void *sparse_buffer_alloc(unsigned long size);
 struct page * __populate_section_memmap(unsigned long pfn,

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 43fed6785322..b6e4e3f31ad2 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c

@@ -1304,16 +1304,59 @@ static inline void destroy_compound_gigantic_page(struct page *page,
 						unsigned int order) { }
 #endif
 
-static void update_and_free_page(struct hstate *h, struct page *page)
+static int update_and_free_page(struct hstate *h, struct page *page)
+	__releases(&hugetlb_lock) __acquires(&hugetlb_lock)
 {
 	int i;
 	struct page *subpage = page;
+	int nid = page_to_nid(page);
 
 	if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported())
-		return;
+		return 0;
 
 	h->nr_huge_pages--;
-	h->nr_huge_pages_node[page_to_nid(page)]--;
+	h->nr_huge_pages_node[nid]--;
+	VM_BUG_ON_PAGE(hugetlb_cgroup_from_page(page), page);
+	VM_BUG_ON_PAGE(hugetlb_cgroup_from_page_rsvd(page), page);
+	set_page_refcounted(page);
+	set_compound_page_dtor(page, NULL_COMPOUND_DTOR);
+
+	/*
+	 * If the vmemmap pages associated with the HugeTLB page can be
+	 * optimized or the page is gigantic, we might block in
+	 * alloc_huge_page_vmemmap() or free_gigantic_page(). In both
+	 * cases, drop the hugetlb_lock.
+	 */
+	if (free_vmemmap_pages_per_hpage(h) || hstate_is_gigantic(h))
+		spin_unlock(&hugetlb_lock);
+
+	if (alloc_huge_page_vmemmap(h, page)) {
+		spin_lock(&hugetlb_lock);
+		INIT_LIST_HEAD(&page->lru);
+		set_compound_page_dtor(page, HUGETLB_PAGE_DTOR);
+		h->nr_huge_pages++;
+		h->nr_huge_pages_node[nid]++;
+
+		/*
+		 * If we cannot allocate vmemmap pages, just refuse to free the
+		 * page and put the page back on the hugetlb free list and treat
+		 * as a surplus page.
+		 */
+		h->surplus_huge_pages++;
+		h->surplus_huge_pages_node[nid]++;
+
+		/*
+		 * The refcount can be perfectly increased by memory-failure or
+		 * soft_offline handlers.
+		 */
+		if (likely(put_page_testzero(page))) {
+			arch_clear_hugepage_flags(page);
+			enqueue_huge_page(h, page);
+		}
+
+		return -ENOMEM;
+	}
+
 	for (i = 0; i < pages_per_huge_page(h);
 	     i++, subpage = mem_map_next(subpage, page, i)) {
 		subpage->flags &= ~(1 << PG_locked | 1 << PG_error |

@@ -1321,22 +1364,18 @@ static void update_and_free_page(struct hstate *h, struct page *page)
 				1 << PG_active | 1 << PG_private |
 				1 << PG_writeback);
 	}
-	VM_BUG_ON_PAGE(hugetlb_cgroup_from_page(page), page);
-	VM_BUG_ON_PAGE(hugetlb_cgroup_from_page_rsvd(page), page);
-	set_compound_page_dtor(page, NULL_COMPOUND_DTOR);
-	set_page_refcounted(page);
+
 	if (hstate_is_gigantic(h)) {
-		/*
-		 * Temporarily drop the hugetlb_lock, because
-		 * we might block in free_gigantic_page().
-		 */
-		spin_unlock(&hugetlb_lock);
 		destroy_compound_gigantic_page(page, huge_page_order(h));
 		free_gigantic_page(page, huge_page_order(h));
-		spin_lock(&hugetlb_lock);
 	} else {
 		__free_pages(page, huge_page_order(h));
 	}
+
+	if (free_vmemmap_pages_per_hpage(h) || hstate_is_gigantic(h))
+		spin_lock(&hugetlb_lock);
+
+	return 0;
 }
 
 struct hstate *size_to_hstate(unsigned long size)

@@ -1404,9 +1443,9 @@ static void __free_huge_page(struct page *page)
 	} else if (h->surplus_huge_pages_node[nid]) {
 		/* remove the page from active list */
 		list_del(&page->lru);
-		update_and_free_page(h, page);
 		h->surplus_huge_pages--;
 		h->surplus_huge_pages_node[nid]--;
+		update_and_free_page(h, page);
 	} else {
 		arch_clear_hugepage_flags(page);
 		enqueue_huge_page(h, page);

@@ -1447,7 +1486,7 @@ void free_huge_page(struct page *page)
 	/*
 	 * Defer freeing if in non-task context to avoid hugetlb_lock deadlock.
 	 */
-	if (!in_task()) {
+	if (!in_atomic()) {
 		/*
 		 * Only call schedule_work() if hpage_freelist is previously
 		 * empty. Otherwise, schedule_work() had been called but the

@@ -1699,8 +1738,7 @@ static int free_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed,
 				h->surplus_huge_pages--;
 				h->surplus_huge_pages_node[node]--;
 			}
-			update_and_free_page(h, page);
-			ret = 1;
+			ret = !update_and_free_page(h, page);
 			break;
 		}
 	}

@@ -1713,10 +1751,14 @@ static int free_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed,
  * nothing for in-use hugepages and non-hugepages.
  * This function returns values like below:
  *
- *  -EBUSY: failed to dissolved free hugepages or the hugepage is in-use
- *          (allocated or reserved.)
- *       0: successfully dissolved free hugepages or the page is not a
- *          hugepage (considered as already dissolved)
+ *  -ENOMEM: failed to allocate vmemmap pages to free the freed hugepages
+ *           when the system is under memory pressure and the feature of
+ *           freeing unused vmemmap pages associated with each hugetlb page
+ *           is enabled.
+ *  -EBUSY:  failed to dissolved free hugepages or the hugepage is in-use
+ *           (allocated or reserved.)
+ *       0:  successfully dissolved free hugepages or the page is not a
+ *           hugepage (considered as already dissolved)
  */
 int dissolve_free_huge_page(struct page *page)
 {

@@ -1771,8 +1813,12 @@ int dissolve_free_huge_page(struct page *page)
 		h->free_huge_pages--;
 		h->free_huge_pages_node[nid]--;
 		h->max_huge_pages--;
-		update_and_free_page(h, head);
-		rc = 0;
+		rc = update_and_free_page(h, head);
+		if (rc) {
+			h->surplus_huge_pages--;
+			h->surplus_huge_pages_node[nid]--;
+			h->max_huge_pages++;
+		}
 	}
 out:
 	spin_unlock(&hugetlb_lock);

diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index 0209b736e0b4..f7ab3d99250a 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c

@@ -181,21 +181,31 @@
 #define RESERVE_VMEMMAP_NR		2U
 #define RESERVE_VMEMMAP_SIZE		(RESERVE_VMEMMAP_NR << PAGE_SHIFT)
 
-/*
- * How many vmemmap pages associated with a HugeTLB page that can be freed
- * to the buddy allocator.
- *
- * Todo: Returns zero for now, which means the feature is disabled. We will
- * enable it once all the infrastructure is there.
- */
-static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
+static inline unsigned long free_vmemmap_pages_size_per_hpage(struct hstate *h)
 {
-	return 0;
+	return (unsigned long)free_vmemmap_pages_per_hpage(h) << PAGE_SHIFT;
 }
 
-static inline unsigned long free_vmemmap_pages_size_per_hpage(struct hstate *h)
+int alloc_huge_page_vmemmap(struct hstate *h, struct page *head)
 {
-	return (unsigned long)free_vmemmap_pages_per_hpage(h) << PAGE_SHIFT;
+	unsigned long vmemmap_addr = (unsigned long)head;
+	unsigned long vmemmap_end, vmemmap_reuse;
+
+	if (!free_vmemmap_pages_per_hpage(h))
+		return 0;
+
+	vmemmap_addr += RESERVE_VMEMMAP_SIZE;
+	vmemmap_end = vmemmap_addr + free_vmemmap_pages_size_per_hpage(h);
+	vmemmap_reuse = vmemmap_addr - PAGE_SIZE;