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Subject[PATCH v5 4/4] mm/shmem: fix shmem_swapin() race with swapoff
Date
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@huawei.com>
---
mm/shmem.c | 12 ++++++++++++
1 file changed, 12 insertions(+)

diff --git a/mm/shmem.c b/mm/shmem.c
index 26c76b13ad23..2dafd65b0b42 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 (!si) {
+ error = EINVAL;
+ goto failed;
+ }
/* Look it up and read it in.. */
page = lookup_swap_cache(swap, NULL, 0);
if (!page) {
@@ -1765,6 +1772,8 @@ static int shmem_swapin_page(struct inode *inode, pgoff_t index,
swap_free(swap);

*pagep = page;
+ if (si)
+ put_swap_device(si);
return 0;
failed:
if (!shmem_confirm_swap(mapping, index, swap))
@@ -1775,6 +1784,9 @@ static int shmem_swapin_page(struct inode *inode, pgoff_t index,
put_page(page);
}

+ if (si)
+ put_swap_device(si);
+
return error;
}

--
2.23.0
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