[PATCH 5/5] mm/swap_state: fix swap_cluster_readahead() race with swapoff

[Miaohe Lin <linmiaohe@huawei.com>]

swap_cluster_readahead() could race with swapoff and might dereference
si->swap_file after it's released by swapoff. Close this race window by
using get/put_swap_device() pair.

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
---
 mm/swap_state.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/mm/swap_state.c b/mm/swap_state.c
index 3bf0d0c297bc..eba6b0cf6cf9 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -626,12 +626,17 @@ struct page *swap_cluster_readahead(swp_entry_t entry, gfp_t gfp_mask,
 	unsigned long offset = entry_offset;
 	unsigned long start_offset, end_offset;
 	unsigned long mask;
-	struct swap_info_struct *si = swp_swap_info(entry);
+	struct swap_info_struct *si;
 	struct blk_plug plug;
 	bool do_poll = true, page_allocated;
 	struct vm_area_struct *vma = vmf->vma;
 	unsigned long addr = vmf->address;
 
+	si = get_swap_device(entry);
+	/* In case we raced with swapoff. */
+	if (!si)
+		return NULL;
+
 	mask = swapin_nr_pages(offset) - 1;
 	if (!mask)
 		goto skip;
@@ -673,7 +678,9 @@ struct page *swap_cluster_readahead(swp_entry_t entry, gfp_t gfp_mask,
 
 	lru_add_drain();	/* Push any new pages onto the LRU now */
 skip:
-	return read_swap_cache_async(entry, gfp_mask, vma, addr, do_poll);
+	page = read_swap_cache_async(entry, gfp_mask, vma, addr, do_poll);
+	put_swap_device(si);
+	return page;
 }
 
 int init_swap_address_space(unsigned int type, unsigned long nr_pages)
-- 
2.19.1