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Subject[PATCH v5 2/4] swap: fix do_swap_page() race with swapoff
Date
When I was investigating the swap code, I found the below possible race
window:

CPU 1 CPU 2
----- -----
do_swap_page
if (data_race(si->flags & SWP_SYNCHRONOUS_IO)
swap_readpage
if (data_race(sis->flags & SWP_FS_OPS)) {
swapoff
..
p->swap_file = NULL;
..
struct file *swap_file = sis->swap_file;
struct address_space *mapping = swap_file->f_mapping;[oops!]

Note that for the pages that are swapped in through swap cache, this isn't
an issue. Because the page is locked, and the swap entry will be marked
with SWAP_HAS_CACHE, so swapoff() can not proceed until the page has been
unlocked.

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

Fixes: 0bcac06f27d7 ("mm,swap: skip swapcache for swapin of synchronous device")
Reported-by: kernel test robot <lkp@intel.com> (auto build test ERROR)
Reviewed-by: "Huang, Ying" <ying.huang@intel.com>
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
---
include/linux/swap.h | 9 +++++++++
mm/memory.c | 11 +++++++++--
2 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index c9e7fea10b83..46d51d058d05 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -527,6 +527,15 @@ static inline struct swap_info_struct *swp_swap_info(swp_entry_t entry)
return NULL;
}

+static inline struct swap_info_struct *get_swap_device(swp_entry_t entry)
+{
+ return NULL;
+}
+
+static inline void put_swap_device(struct swap_info_struct *si)
+{
+}
+
#define swap_address_space(entry) (NULL)
#define get_nr_swap_pages() 0L
#define total_swap_pages 0L
diff --git a/mm/memory.c b/mm/memory.c
index 27014c3bde9f..39c910678387 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3311,6 +3311,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
{
struct vm_area_struct *vma = vmf->vma;
struct page *page = NULL, *swapcache;
+ struct swap_info_struct *si = NULL;
swp_entry_t entry;
pte_t pte;
int locked;
@@ -3338,14 +3339,16 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
goto out;
}

+ /* Prevent swapoff from happening to us. */
+ si = get_swap_device(entry);
+ if (unlikely(!si))
+ goto out;

delayacct_set_flag(current, DELAYACCT_PF_SWAPIN);
page = lookup_swap_cache(entry, vma, vmf->address);
swapcache = page;

if (!page) {
- struct swap_info_struct *si = swp_swap_info(entry);
-
if (data_race(si->flags & SWP_SYNCHRONOUS_IO) &&
__swap_count(entry) == 1) {
/* skip swapcache */
@@ -3514,6 +3517,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
unlock:
pte_unmap_unlock(vmf->pte, vmf->ptl);
out:
+ if (si)
+ put_swap_device(si);
return ret;
out_nomap:
pte_unmap_unlock(vmf->pte, vmf->ptl);
@@ -3525,6 +3530,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
unlock_page(swapcache);
put_page(swapcache);
}
+ if (si)
+ put_swap_device(si);
return ret;
}

--
2.23.0
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