The patch titled
Subject: mm, swap: fix race between swapoff and some swap operations
has been added to the -mm tree. Its filename is
mm-swap-fix-race-between-swapoff-and-some-swap-operations-v6.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/mm-swap-fix-race-between-swapoff-and-some-swap-operations-v6.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/mm-swap-fix-race-between-swapoff-and-some-swap-operations-v6.patch
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------------------------------------------------------
From: Huang Ying <ying.huang@xxxxxxxxx>
Subject: mm, swap: fix race between swapoff and some swap operations
- Add more comments to get_swap_device() to make it more clear about
possible swapoff or swapoff+swapon.
Link: http://lkml.kernel.org/r/20180223060010.954-1-ying.huang@xxxxxxxxx
Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/memory.c | 2 +-
mm/swapfile.c | 23 +++++++++++++++++++++++
2 files changed, 24 insertions(+), 1 deletion(-)
diff -puN mm/memory.c~mm-swap-fix-race-between-swapoff-and-some-swap-operations-v6 mm/memory.c
--- a/mm/memory.c~mm-swap-fix-race-between-swapoff-and-some-swap-operations-v6
+++ a/mm/memory.c
@@ -2925,7 +2925,7 @@ int do_swap_page(struct vm_fault *vmf)
struct swap_info_struct *si = swp_swap_info(entry);
if (si->flags & SWP_SYNCHRONOUS_IO &&
- __swap_count(entry) == 1) {
+ __swap_count(entry) == 1) {
/* skip swapcache */
page = alloc_page_vma(GFP_HIGHUSER_MOVABLE, vma,
vmf->address);
diff -puN mm/swapfile.c~mm-swap-fix-race-between-swapoff-and-some-swap-operations-v6 mm/swapfile.c
--- a/mm/swapfile.c~mm-swap-fix-race-between-swapoff-and-some-swap-operations-v6
+++ a/mm/swapfile.c
@@ -1113,6 +1113,29 @@ static struct swap_info_struct *swap_inf
* return pointer to swap_info_struct, and keep the swap entry valid
* via preventing the swap device from being swapoff, until
* put_swap_device() is called. Otherwise return NULL.
+ *
+ * Notice that swapoff or swapoff+swapon can still happen before the
+ * preempt_disable() in get_swap_device() or after the
+ * preempt_enable() in put_swap_device() if there isn't any other way
+ * to prevent swapoff, such as page lock, page table lock, etc. The
+ * caller must be prepared for that. For example, the following
+ * situation is possible.
+ *
+ * CPU1 CPU2
+ * do_swap_page()
+ * ... swapoff+swapon
+ * __read_swap_cache_async()
+ * swapcache_prepare()
+ * __swap_duplicate()
+ * // check swap_map
+ * // verify PTE not changed
+ *
+ * In __swap_duplicate(), the swap_map need to be checked before
+ * changing partly because the specified swap entry may be for another
+ * swap device which has been swapoff. And in do_swap_page(), after
+ * the page is read from the swap device, the PTE is verified not
+ * changed with the page table locked to check whether the swap device
+ * has been swapoff or swapoff+swapon.
*/
struct swap_info_struct *get_swap_device(swp_entry_t entry)
{
_
Patches currently in -mm which might be from ying.huang@xxxxxxxxx are
mm-fix-races-between-address_space-dereference-and-free-in-page_evicatable.patch
mm-fix-races-between-swapoff-and-flush-dcache.patch
mm-swap-fix-race-between-swapoff-and-some-swap-operations.patch
mm-swap-fix-race-between-swapoff-and-some-swap-operations-v6.patch
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