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...

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@arm.com Subject: mm: swap: fix race between free_swap_and_cache() and swapoff() Date: Tue, 5 Mar 2024 15:13:49 +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 valid. This wasn't present in get_swap_device() so I've added it. I couldn't find any existing get_swap_device() call sites where this extra check would cause any false alarms.

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/20240305151349.3781428-1-ryan.roberts@arm.com Fixes: 7c00bafee87c ("mm/swap: free swap slots in batch") Closes: https://lore.kernel.org/linux-mm/65a66eb9-41f8-4790-8db2-0c70ea15979f@redhat... Signed-off-by: Ryan Roberts ryan.roberts@arm.com Cc: David Hildenbrand david@redhat.com Cc: "Huang, Ying" ying.huang@intel.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/swapfile.c | 14 +++++++++++--- 1 file changed, 11 insertions(+), 3 deletions(-)

--- a/mm/swapfile.c~mm-swap-fix-race-between-free_swap_and_cache-and-swapoff +++ a/mm/swapfile.c @@ -1281,7 +1281,9 @@ struct swap_info_struct *get_swap_device smp_rmb(); offset = swp_offset(entry); if (offset >= si->max) - goto put_out; + goto bad_offset; + if (data_race(!si->swap_map[swp_offset(entry)])) + goto bad_free;

return si; bad_nofile: @@ -1289,9 +1291,14 @@ bad_nofile: out: return NULL; put_out: - pr_err("%s: %s%08lx\n", __func__, Bad_offset, entry.val); percpu_ref_put(&si->users); return NULL; +bad_offset: + pr_err("%s: %s%08lx\n", __func__, Bad_offset, entry.val); + goto put_out; +bad_free: + pr_err("%s: %s%08lx\n", __func__, Unused_offset, entry.val); + goto put_out; }

static unsigned char __swap_entry_free(struct swap_info_struct *p, @@ -1609,13 +1616,14 @@ 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) { 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@arm.com are

mm-swap-fix-race-between-free_swap_and_cache-and-swapoff.patch