The patch titled
Subject: zsmalloc: fix races between asynchronous zspage free and page migration
has been added to the -mm mm-unstable branch. Its filename is
zsmalloc-fix-races-between-asynchronous-zspage-free-and-page-migration.patch
This patch should soon appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Sultan Alsawaf <sultan@xxxxxxxxxxxxxxx>
Subject: zsmalloc: fix races between asynchronous zspage free and page migration
The asynchronous zspage free worker tries to lock a zspage's entire page
list without defending against page migration. Since pages which haven't
yet been locked can concurrently migrate off the zspage page list while
lock_zspage() churns away, lock_zspage() can suffer from a few different
lethal races.
It can lock a page which no longer belongs to the zspage and unsafely
dereference page_private(), it can unsafely dereference a torn pointer to
the next page (since there's a data race), and it can observe a spurious
NULL pointer to the next page and thus not lock all of the zspage's pages
(since a single page migration will reconstruct the entire page list, and
create_page_chain() unconditionally zeroes out each list pointer in the
process).
Fix the races by using migrate_read_lock() in lock_zspage() to synchronize
with page migration.
Link: https://lkml.kernel.org/r/20220509024703.243847-1-sultan@xxxxxxxxxxxxxxx
Fixes: 48b4800a1c6a ("zsmalloc: page migration support")
Signed-off-by: Sultan Alsawaf <sultan@xxxxxxxxxxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
Cc: Nitin Gupta <ngupta@xxxxxxxxxx>
Cc: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/zsmalloc.c | 37 +++++++++++++++++++++++++++++++++----
1 file changed, 33 insertions(+), 4 deletions(-)
--- a/mm/zsmalloc.c~zsmalloc-fix-races-between-asynchronous-zspage-free-and-page-migration
+++ a/mm/zsmalloc.c
@@ -1718,11 +1718,40 @@ static enum fullness_group putback_zspag
*/
static void lock_zspage(struct zspage *zspage)
{
- struct page *page = get_first_page(zspage);
+ struct page *curr_page, *page;
- do {
- lock_page(page);
- } while ((page = get_next_page(page)) != NULL);
+ /*
+ * Pages we haven't locked yet can be migrated off the list while we're
+ * trying to lock them, so we need to be careful and only attempt to
+ * lock each page under migrate_read_lock(). Otherwise, the page we lock
+ * may no longer belong to the zspage. This means that we may wait for
+ * the wrong page to unlock, so we must take a reference to the page
+ * prior to waiting for it to unlock outside migrate_read_lock().
+ */
+ while (1) {
+ migrate_read_lock(zspage);
+ page = get_first_page(zspage);
+ if (trylock_page(page))
+ break;
+ get_page(page);
+ migrate_read_unlock(zspage);
+ wait_on_page_locked(page);
+ put_page(page);
+ }
+
+ curr_page = page;
+ while ((page = get_next_page(curr_page))) {
+ if (trylock_page(page)) {
+ curr_page = page;
+ } else {
+ get_page(page);
+ migrate_read_unlock(zspage);
+ wait_on_page_locked(page);
+ put_page(page);
+ migrate_read_lock(zspage);
+ }
+ }
+ migrate_read_unlock(zspage);
}
static int zs_init_fs_context(struct fs_context *fc)
_
Patches currently in -mm which might be from sultan@xxxxxxxxxxxxxxx are
zsmalloc-fix-races-between-asynchronous-zspage-free-and-page-migration.patch
