mm/swapfile: use percpu_ref to serialize against concurrent swapoff

Patch series "close various race windows for swap", v6.

When I was investigating the swap code, I found some possible race
windows.  This series aims to fix all these races.  But using current
get/put_swap_device() to guard against concurrent swapoff for
swap_readpage() looks terrible because swap_readpage() may take really
long time.  And to reduce the performance overhead on the hot-path as much
as possible, it appears we can use the percpu_ref to close this race
window(as suggested by Huang, Ying).  The patch 1 adds percpu_ref support
for swap and most of the remaining patches try to use this to close
various race windows.  More details can be found in the respective
changelogs.

This patch (of 4):

Using current get/put_swap_device() to guard against concurrent swapoff
for some swap ops, e.g.  swap_readpage(), looks terrible because they
might take really long time.  This patch adds the percpu_ref support to
serialize against concurrent swapoff(as suggested by Huang, Ying).  Also
we remove the SWP_VALID flag because it's used together with RCU solution.

Link: https://lkml.kernel.org/r/20210426123316.806267-1-linmiaohe@huawei.com
Link: https://lkml.kernel.org/r/20210426123316.806267-2-linmiaohe@huawei.com
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: "Huang, Ying" <ying.huang@intel.com>
Cc: Alex Shi <alexs@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Dennis Zhou <dennis@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Wei Yang <richard.weiyang@gmail.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Yu Zhao <yuzhao@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 3 insertions, 2 deletions

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 144727041e78..c9e7fea10b83 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -177,7 +177,6 @@ enum {
 	SWP_PAGE_DISCARD = (1 << 10),	/* freed swap page-cluster discards */
 	SWP_STABLE_WRITES = (1 << 11),	/* no overwrite PG_writeback pages */
 	SWP_SYNCHRONOUS_IO = (1 << 12),	/* synchronous IO is efficient */
-	SWP_VALID	= (1 << 13),	/* swap is valid to be operated on? */
 					/* add others here before... */
 	SWP_SCANNING	= (1 << 14),	/* refcount in scan_swap_map */
 };
@@ -240,6 +239,7 @@ struct swap_cluster_list {
  * The in-memory structure used to track swap areas.
  */
 struct swap_info_struct {
+	struct percpu_ref users;	/* indicate and keep swap device valid. */
 	unsigned long	flags;		/* SWP_USED etc: see above */
 	signed short	prio;		/* swap priority of this type */
 	struct plist_node list;		/* entry in swap_active_head */
@@ -260,6 +260,7 @@ struct swap_info_struct {
 	struct block_device *bdev;	/* swap device or bdev of swap file */
 	struct file *swap_file;		/* seldom referenced */
 	unsigned int old_block_size;	/* seldom referenced */
+	struct completion comp;		/* seldom referenced */
 #ifdef CONFIG_FRONTSWAP
 	unsigned long *frontswap_map;	/* frontswap in-use, one bit per page */
 	atomic_t frontswap_pages;	/* frontswap pages in-use counter */
@@ -511,7 +512,7 @@ sector_t swap_page_sector(struct page *page);
 
 static inline void put_swap_device(struct swap_info_struct *si)
 {
-	rcu_read_unlock();
+	percpu_ref_put(&si->users);
 }
 
 #else /* CONFIG_SWAP */