The patch titled
Subject: mm: swap: fix race between free_swap_and_cache() and swapoff()
has been added to the -mm mm-unstable branch. Its filename is
mm-swap-fix-race-between-free_swap_and_cache-and-swapoff.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-swap-fix-race-between-free_swap_and_cache-and-swapoff.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Ryan Roberts <ryan.roberts@xxxxxxx>
Subject: mm: swap: fix race between free_swap_and_cache() and swapoff()
Date: Wed, 6 Mar 2024 14:03:56 +0000
There was previously a theoretical window where swapoff() could run and
teardown a swap_info_struct while a call to free_swap_and_cache() was
running in another thread. This could cause, amongst other bad
possibilities, swap_page_trans_huge_swapped() (called by
free_swap_and_cache()) to access the freed memory for swap_map.
This is a theoretical problem and I haven't been able to provoke it from a
test case. But there has been agreement based on code review that this is
possible (see link below).
Fix it by using get_swap_device()/put_swap_device(), which will stall
swapoff(). There was an extra check in _swap_info_get() to confirm that
the swap entry was not free. This isn't present in get_swap_device()
because it doesn't make sense in general due to the race between getting
the reference and swapoff. So I've added an equivalent check directly in
free_swap_and_cache().
Details of how to provoke one possible issue (thanks to David Hildenbrand
for deriving this):
--8<-----
__swap_entry_free() might be the last user and result in
"count == SWAP_HAS_CACHE".
swapoff->try_to_unuse() will stop as soon as soon as si->inuse_pages==0.
So the question is: could someone reclaim the folio and turn
si->inuse_pages==0, before we completed swap_page_trans_huge_swapped().
Imagine the following: 2 MiB folio in the swapcache. Only 2 subpages are
still references by swap entries.
Process 1 still references subpage 0 via swap entry.
Process 2 still references subpage 1 via swap entry.
Process 1 quits. Calls free_swap_and_cache().
-> count == SWAP_HAS_CACHE
[then, preempted in the hypervisor etc.]
Process 2 quits. Calls free_swap_and_cache().
-> count == SWAP_HAS_CACHE
Process 2 goes ahead, passes swap_page_trans_huge_swapped(), and calls
__try_to_reclaim_swap().
__try_to_reclaim_swap()->folio_free_swap()->delete_from_swap_cache()->
put_swap_folio()->free_swap_slot()->swapcache_free_entries()->
swap_entry_free()->swap_range_free()->
...
WRITE_ONCE(si->inuse_pages, si->inuse_pages - nr_entries);
What stops swapoff to succeed after process 2 reclaimed the swap cache
but before process1 finished its call to swap_page_trans_huge_swapped()?
--8<-----
Link: https://lkml.kernel.org/r/20240306140356.3974886-1-ryan.roberts@xxxxxxx
Fixes: 7c00bafee87c ("mm/swap: free swap slots in batch")
Closes: https://lore.kernel.org/linux-mm/65a66eb9-41f8-4790-8db2-0c70ea15979f@xxxxxxxxxx/
Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: "Huang, Ying" <ying.huang@xxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/swapfile.c | 13 ++++++++++++-
1 file changed, 12 insertions(+), 1 deletion(-)
--- a/mm/swapfile.c~mm-swap-fix-race-between-free_swap_and_cache-and-swapoff
+++ a/mm/swapfile.c
@@ -1232,6 +1232,11 @@ static unsigned char __swap_entry_free_l
* with get_swap_device() and put_swap_device(), unless the swap
* functions call get/put_swap_device() by themselves.
*
+ * Note that when only holding the PTL, swapoff might succeed immediately
+ * after freeing a swap entry. Therefore, immediately after
+ * __swap_entry_free(), the swap info might become stale and should not
+ * be touched without a prior get_swap_device().
+ *
* Check whether swap entry is valid in the swap device. If so,
* return pointer to swap_info_struct, and keep the swap entry valid
* via preventing the swap device from being swapoff, until
@@ -1609,13 +1614,19 @@ int free_swap_and_cache(swp_entry_t entr
if (non_swap_entry(entry))
return 1;
- p = _swap_info_get(entry);
+ p = get_swap_device(entry);
if (p) {
+ if (WARN_ON(data_race(!p->swap_map[swp_offset(entry)]))) {
+ put_swap_device(p);
+ return 0;
+ }
+
count = __swap_entry_free(p, entry);
if (count == SWAP_HAS_CACHE &&
!swap_page_trans_huge_swapped(p, entry))
__try_to_reclaim_swap(p, swp_offset(entry),
TTRS_UNMAPPED | TTRS_FULL);
+ put_swap_device(p);
}
return p != NULL;
}
_
Patches currently in -mm which might be from ryan.roberts@xxxxxxx are
mm-swap-fix-race-between-free_swap_and_cache-and-swapoff.patch
