[PATCH v4 4/4] mm/shmem: fix shmem_swapin() race with swapoff

[Miaohe Lin <linmiaohe@xxxxxxxxxx>]

When I was investigating the swap code, I found the below possible race
window:

CPU 1                                         CPU 2
-----                                         -----
shmem_swapin
  swap_cluster_readahead
    if (likely(si->flags & (SWP_BLKDEV | SWP_FS_OPS))) {
                                              swapoff
                                                ..
                                                si->swap_file = NULL;
                                                ..
    struct inode *inode = si->swap_file->f_mapping->host;[oops!]

Close this race window by using get/put_swap_device() to guard against
concurrent swapoff.

Fixes: 8fd2e0b505d1 ("mm: swap: check if swap backing device is congested or not")
Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
---
 mm/shmem.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/mm/shmem.c b/mm/shmem.c
index 26c76b13ad23..be388d0cf8b5 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1696,6 +1696,7 @@ static int shmem_swapin_page(struct inode *inode, pgoff_t index,
 	struct address_space *mapping = inode->i_mapping;
 	struct shmem_inode_info *info = SHMEM_I(inode);
 	struct mm_struct *charge_mm = vma ? vma->vm_mm : current->mm;
+	struct swap_info_struct *si;
 	struct page *page;
 	swp_entry_t swap;
 	int error;
@@ -1704,6 +1705,12 @@ static int shmem_swapin_page(struct inode *inode, pgoff_t index,
 	swap = radix_to_swp_entry(*pagep);
 	*pagep = NULL;
 
+	/* Prevent swapoff from happening to us. */
+	si = get_swap_device(swap);
+	if (unlikely(!si)) {
+		error = EINVAL;
+		goto failed;
+	}
 	/* Look it up and read it in.. */
 	page = lookup_swap_cache(swap, NULL, 0);
 	if (!page) {
@@ -1720,6 +1727,7 @@ static int shmem_swapin_page(struct inode *inode, pgoff_t index,
 			goto failed;
 		}
 	}
+	put_swap_device(si);
 
 	/* We have to do this with page locked to prevent races */
 	lock_page(page);
@@ -1775,6 +1783,9 @@ static int shmem_swapin_page(struct inode *inode, pgoff_t index,
 		put_page(page);
 	}
 
+	if (si)
+		put_swap_device(si);
+
 	return error;
 }
 
-- 
2.19.1