@@ -1232,6 +1232,11 @@ static unsigned char __swap_entry_free_locked(struct swap_info_struct *p,
* 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 entry)
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;
}
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<----- Fixes: 7c00bafee87c ("mm/swap: free swap slots in batch") Closes: https://lore.kernel.org/linux-mm/65a66eb9-41f8-4790-8db2-0c70ea15979f@redhat.com/ Cc: stable@vger.kernel.org Signed-off-by: Ryan Roberts <ryan.roberts@arm.com> --- Hi Andrew, Please replace v1 of this patch in mm-unstable with this version. Changes since v1: - Added comments for get_swap_device() as suggested by David - Moved check that swap entry is not free from get_swap_device() to free_swap_and_cache() since there are some paths that legitimately call with a free offset. I haven't addressed the recommendation by Huang Ying [1] to also revert commit 23b230ba8ac3 ("mm/swap: print bad swap offset entry in get_swap_device"). It should be done separately to this, and and we need to conclude discussion first. [1] https://lore.kernel.org/all/875xy0842q.fsf@yhuang6-desk2.ccr.corp.intel.com/ Thanks, Ryan mm/swapfile.c | 13 ++++++++++++- 1 file changed, 12 insertions(+), 1 deletion(-) -- 2.25.1