[PATCH v3 4/4] mm/shmem: fix shmem_swapin() race with swapoff

[Miaohe Lin]

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
                                                percpu_ref_kill(&p->users)
                                                synchronize_rcu()
                                                wait_for_completion
                                                ..
                                                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 | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/mm/shmem.c b/mm/shmem.c
index 26c76b13ad23..936ba5595297 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1492,15 +1492,21 @@ static void shmem_pseudo_vma_destroy(struct vm_area_struct *vma)
 static struct page *shmem_swapin(swp_entry_t swap, gfp_t gfp,
 			struct shmem_inode_info *info, pgoff_t index)
 {
+	struct swap_info_struct *si;
 	struct vm_area_struct pvma;
 	struct page *page;
 	struct vm_fault vmf = {
 		.vma = &pvma,
 	};
 
+	/* Prevent swapoff from happening to us. */
+	si = get_swap_device(swap);
+	if (unlikely(!si))
+		return NULL;
 	shmem_pseudo_vma_init(&pvma, info, index);
 	page = swap_cluster_readahead(swap, gfp, &vmf);
 	shmem_pseudo_vma_destroy(&pvma);
+	put_swap_device(si);
 
 	return page;
 }
-- 
2.19.1