[PATCH v1 07/23] mm/balloon_compaction: use a device-independent balloon (list) lock
From: David Hildenbrand <hidden>
Date: 2025-10-21 12:59:51
Also in:
linux-doc, linux-mm, lkml, virtualization
Subsystem:
memory management, the rest · Maintainers:
Andrew Morton, Linus Torvalds
In order to remove the dependency on the page lock for balloon pages, we need a lock that is independent of the page. It's crucial that we can handle the scenario where balloon deflation (clearing page->private) can race with page isolation (using page->private to obtain the balloon_dev_info where the lock currently resides). The current lock in balloon_dev_info is therefore not suitable. Fortunately, we never really have more than a single balloon device per VM, so we can just keep it simple and use a static lock to protect all balloon devices. Based on this change we will remove the dependency on the page lock next. Signed-off-by: David Hildenbrand <redacted> --- include/linux/balloon_compaction.h | 6 ++--- mm/balloon_compaction.c | 36 +++++++++++++++++------------- 2 files changed, 23 insertions(+), 19 deletions(-)
diff --git a/include/linux/balloon_compaction.h b/include/linux/balloon_compaction.h
index 3109d3c43d306..e2d9eb40e1fbb 100644
--- a/include/linux/balloon_compaction.h
+++ b/include/linux/balloon_compaction.h@@ -21,10 +21,10 @@ * i. Setting the PG_movable_ops flag and page->private with the following * lock order * +-page_lock(page); - * +--spin_lock_irq(&b_dev_info->pages_lock); + * +--spin_lock_irq(&balloon_pages_lock); * * ii. isolation or dequeueing procedure must remove the page from balloon - * device page list under b_dev_info->pages_lock. + * device page list under &balloon_pages_lock * * The functions provided by this interface are placed to help on coping with * the aforementioned balloon page corner case, as well as to ensure the simple
@@ -52,7 +52,6 @@ */ struct balloon_dev_info { unsigned long isolated_pages; /* # of isolated pages for migration */ - spinlock_t pages_lock; /* Protection to pages list */ struct list_head pages; /* Pages enqueued & handled to Host */ int (*migratepage)(struct balloon_dev_info *, struct page *newpage, struct page *page, enum migrate_mode mode);
@@ -71,7 +70,6 @@ extern size_t balloon_page_list_dequeue(struct balloon_dev_info *b_dev_info, static inline void balloon_devinfo_init(struct balloon_dev_info *balloon) { balloon->isolated_pages = 0; - spin_lock_init(&balloon->pages_lock); INIT_LIST_HEAD(&balloon->pages); balloon->migratepage = NULL; balloon->adjust_managed_page_count = false;
diff --git a/mm/balloon_compaction.c b/mm/balloon_compaction.c
index fd9ec47cf4670..97e838795354d 100644
--- a/mm/balloon_compaction.c
+++ b/mm/balloon_compaction.c@@ -11,6 +11,12 @@ #include <linux/export.h> #include <linux/balloon_compaction.h> +/* + * Lock protecting the balloon_dev_info of all devices. We don't really + * expect more than one device. + */ +static DEFINE_SPINLOCK(balloon_pages_lock); + static void balloon_page_enqueue_one(struct balloon_dev_info *b_dev_info, struct page *page) {
@@ -47,13 +53,13 @@ size_t balloon_page_list_enqueue(struct balloon_dev_info *b_dev_info, unsigned long flags; size_t n_pages = 0; - spin_lock_irqsave(&b_dev_info->pages_lock, flags); + spin_lock_irqsave(&balloon_pages_lock, flags); list_for_each_entry_safe(page, tmp, pages, lru) { list_del(&page->lru); balloon_page_enqueue_one(b_dev_info, page); n_pages++; } - spin_unlock_irqrestore(&b_dev_info->pages_lock, flags); + spin_unlock_irqrestore(&balloon_pages_lock, flags); return n_pages; } EXPORT_SYMBOL_GPL(balloon_page_list_enqueue);
@@ -83,7 +89,7 @@ size_t balloon_page_list_dequeue(struct balloon_dev_info *b_dev_info, unsigned long flags; size_t n_pages = 0; - spin_lock_irqsave(&b_dev_info->pages_lock, flags); + spin_lock_irqsave(&balloon_pages_lock, flags); list_for_each_entry_safe(page, tmp, &b_dev_info->pages, lru) { if (n_pages == n_req_pages) break;
@@ -106,7 +112,7 @@ size_t balloon_page_list_dequeue(struct balloon_dev_info *b_dev_info, dec_node_page_state(page, NR_BALLOON_PAGES); n_pages++; } - spin_unlock_irqrestore(&b_dev_info->pages_lock, flags); + spin_unlock_irqrestore(&balloon_pages_lock, flags); return n_pages; }
@@ -149,9 +155,9 @@ void balloon_page_enqueue(struct balloon_dev_info *b_dev_info, { unsigned long flags; - spin_lock_irqsave(&b_dev_info->pages_lock, flags); + spin_lock_irqsave(&balloon_pages_lock, flags); balloon_page_enqueue_one(b_dev_info, page); - spin_unlock_irqrestore(&b_dev_info->pages_lock, flags); + spin_unlock_irqrestore(&balloon_pages_lock, flags); } EXPORT_SYMBOL_GPL(balloon_page_enqueue);
@@ -191,11 +197,11 @@ struct page *balloon_page_dequeue(struct balloon_dev_info *b_dev_info) * BUG() here, otherwise the balloon driver may get stuck in * an infinite loop while attempting to release all its pages. */ - spin_lock_irqsave(&b_dev_info->pages_lock, flags); + spin_lock_irqsave(&balloon_pages_lock, flags); if (unlikely(list_empty(&b_dev_info->pages) && !b_dev_info->isolated_pages)) BUG(); - spin_unlock_irqrestore(&b_dev_info->pages_lock, flags); + spin_unlock_irqrestore(&balloon_pages_lock, flags); return NULL; } return list_first_entry(&pages, struct page, lru);
@@ -213,10 +219,10 @@ static bool balloon_page_isolate(struct page *page, isolate_mode_t mode) if (!b_dev_info) return false; - spin_lock_irqsave(&b_dev_info->pages_lock, flags); + spin_lock_irqsave(&balloon_pages_lock, flags); list_del(&page->lru); b_dev_info->isolated_pages++; - spin_unlock_irqrestore(&b_dev_info->pages_lock, flags); + spin_unlock_irqrestore(&balloon_pages_lock, flags); return true; }
@@ -230,10 +236,10 @@ static void balloon_page_putback(struct page *page) if (WARN_ON_ONCE(!b_dev_info)) return; - spin_lock_irqsave(&b_dev_info->pages_lock, flags); + spin_lock_irqsave(&balloon_pages_lock, flags); list_add(&page->lru, &b_dev_info->pages); b_dev_info->isolated_pages--; - spin_unlock_irqrestore(&b_dev_info->pages_lock, flags); + spin_unlock_irqrestore(&balloon_pages_lock, flags); } static int balloon_page_migrate(struct page *newpage, struct page *page,
@@ -253,7 +259,7 @@ static int balloon_page_migrate(struct page *newpage, struct page *page, rc = b_dev_info->migratepage(b_dev_info, newpage, page, mode); switch (rc) { case 0: - spin_lock_irqsave(&b_dev_info->pages_lock, flags); + spin_lock_irqsave(&balloon_pages_lock, flags); /* Insert the new page into the balloon list. */ get_page(newpage);
@@ -272,7 +278,7 @@ static int balloon_page_migrate(struct page *newpage, struct page *page, } break; case -ENOENT: - spin_lock_irqsave(&b_dev_info->pages_lock, flags); + spin_lock_irqsave(&balloon_pages_lock, flags); /* Old page was deflated but new page not inflated. */ __count_vm_event(BALLOON_DEFLATE);
@@ -285,7 +291,7 @@ static int balloon_page_migrate(struct page *newpage, struct page *page, } b_dev_info->isolated_pages--; - spin_unlock_irqrestore(&b_dev_info->pages_lock, flags); + spin_unlock_irqrestore(&balloon_pages_lock, flags); /* Free the now-deflated page we isolated in balloon_page_isolate(). */ balloon_page_finalize(page);
--
2.51.0