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mm/balloon_compaction: remove dependency on page lock

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Let's stop using the page lock in balloon code and instead use only the
balloon_device_lock.

As soon as we set the PG_movable_ops flag, we might now get isolation
callbacks for that page as we are no longer holding the page lock.  In
there, we'll simply synchronize using the balloon_device_lock.

So in balloon_page_isolate() lookup the balloon_dev_info through
page->private under balloon_device_lock.

It's crucial that we update page->private under the balloon_device_lock,
so the isolation callback can properly deal with concurrent deflation.

Consequently, make sure that balloon_page_finalize() is called under
balloon_device_lock as we remove a page from the list and clear
page->private.  balloon_page_insert() is already called with the
balloon_device_lock held.

Note that the core will still lock the pages, for example in
isolate_movable_ops_page().  The lock is there still relevant for handling
the PageMovableOpsIsolated flag, but that can be later changed to use an
atomic test-and-set instead, or moved into the movable_ops backends.

Link: https://lkml.kernel.org/r/20260119230133.3551867-10-david@kernel.org
Signed-off-by: David Hildenbrand (Red Hat) <david@kernel.org>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Eugenio Pérez <eperezma@redhat.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Jerrin Shaji George <jerrin.shaji-george@broadcom.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: SeongJae Park <sj@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
David Hildenbrand (Red Hat) 2026-01-20 00:01:17 +01:00 committed by Andrew Morton
parent 8202313e3d
commit a3fafdd389
2 changed files with 25 additions and 38 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

25
include/linux/balloon_compaction.h
View File

@ -12,25 +12,27 @@
* is derived from the page type (PageOffline()) combined with the
* PG_movable_ops flag (PageMovableOps()).
*
* Once the page type and the PG_movable_ops are set, migration code
* can initiate page isolation by invoking the
* movable_operations()->isolate_page() callback
*
* As long as page->private is set, the page is either on the balloon list
* or isolated for migration. If page->private is not set, the page is
* either still getting inflated, or was deflated to be freed by the balloon
* driver soon. Isolation is impossible in both cases.
*
* As the page isolation scanning step a compaction thread does is a lockless
* procedure (from a page standpoint), it might bring some racy situations while
* performing balloon page compaction. In order to sort out these racy scenarios
* and safely perform balloon's page compaction and migration we must, always,
* ensure following these simple rules:
*
* i. Setting the PG_movable_ops flag and page->private with the following
* lock order
* +-page_lock(page);
* +--spin_lock_irq(&balloon_pages_lock);
* i. Inflation/deflation must set/clear page->private under the
* balloon_pages_lock
*
* ii. isolation or dequeueing procedure must remove the page from balloon
* 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
* set of exposed rules are satisfied while we are dealing with balloon pages
* compaction / migration.
*
* Copyright (C) 2012, Red Hat, Inc. Rafael Aquini <aquini@redhat.com>
*/
#ifndef _LINUX_BALLOON_COMPACTION_H
@ -93,8 +95,7 @@ static inline struct balloon_dev_info *balloon_page_device(struct page *page)
* @balloon : pointer to balloon device
* @page : page to be assigned as a 'balloon page'
*
* Caller must ensure the page is locked and the spin_lock protecting balloon
* pages list is held before inserting a page into the balloon device.
* Caller must ensure the balloon_pages_lock is held.
*/
static inline void balloon_page_insert(struct balloon_dev_info *balloon,
struct page *page)
@ -119,7 +120,7 @@ static inline gfp_t balloon_mapping_gfp_mask(void)
* balloon list for release to the page allocator
* @page: page to be released to the page allocator
*
* Caller must ensure that the page is locked.
* Caller must ensure the balloon_pages_lock is held.
*/
static inline void balloon_page_finalize(struct page *page)
{

38
mm/balloon_compaction.c
View File

@ -20,15 +20,7 @@ static DEFINE_SPINLOCK(balloon_pages_lock);
static void balloon_page_enqueue_one(struct balloon_dev_info *b_dev_info,
struct page *page)
{
/*
* Block others from accessing the 'page' when we get around to
* establishing additional references. We should be the only one
* holding a reference to the 'page' at this point. If we are not, then
* memory corruption is possible and we should stop execution.
*/
BUG_ON(!trylock_page(page));
balloon_page_insert(b_dev_info, page);
unlock_page(page);
if (b_dev_info->adjust_managed_page_count)
adjust_managed_page_count(page, -1);
__count_vm_event(BALLOON_INFLATE);
@ -93,22 +85,12 @@ size_t balloon_page_list_dequeue(struct balloon_dev_info *b_dev_info,
list_for_each_entry_safe(page, tmp, &b_dev_info->pages, lru) {
if (n_pages == n_req_pages)
break;
/*
* Block others from accessing the 'page' while we get around to
* establishing additional references and preparing the 'page'
* to be released by the balloon driver.
*/
if (!trylock_page(page))
continue;
list_del(&page->lru);
if (b_dev_info->adjust_managed_page_count)
adjust_managed_page_count(page, 1);
balloon_page_finalize(page);
__count_vm_event(BALLOON_DEFLATE);
list_add(&page->lru, pages);
unlock_page(page);
dec_node_page_state(page, NR_BALLOON_PAGES);
n_pages++;
}
@ -213,13 +195,19 @@ EXPORT_SYMBOL_GPL(balloon_page_dequeue);
static bool balloon_page_isolate(struct page *page, isolate_mode_t mode)
{
struct balloon_dev_info *b_dev_info = balloon_page_device(page);
struct balloon_dev_info *b_dev_info;
unsigned long flags;
if (!b_dev_info)
return false;
spin_lock_irqsave(&balloon_pages_lock, flags);
b_dev_info = balloon_page_device(page);
if (!b_dev_info) {
/*
* The page already got deflated and removed from the
* balloon list.
*/
spin_unlock_irqrestore(&balloon_pages_lock, flags);
return false;
}
list_del(&page->lru);
b_dev_info->isolated_pages++;
spin_unlock_irqrestore(&balloon_pages_lock, flags);
@ -253,9 +241,6 @@ static int balloon_page_migrate(struct page *newpage, struct page *page,
unsigned long flags;
int rc;
VM_BUG_ON_PAGE(!PageLocked(page), page);
VM_BUG_ON_PAGE(!PageLocked(newpage), newpage);
/*
* When we isolated the page, the page was still inflated in a balloon
* device. As isolated balloon pages cannot get deflated, we still have
@ -293,10 +278,11 @@ static int balloon_page_migrate(struct page *newpage, struct page *page,
}
b_dev_info->isolated_pages--;
spin_unlock_irqrestore(&balloon_pages_lock, flags);
/* Free the now-deflated page we isolated in balloon_page_isolate(). */
balloon_page_finalize(page);
spin_unlock_irqrestore(&balloon_pages_lock, flags);
put_page(page);
return 0;