Re: [PATCH v1] mm: swap: Fix race between free_swap_and_cache() and swapoff()

[Ryan Roberts <ryan.roberts@arm.com>]

On 05/03/2024 15:50, David Hildenbrand wrote:
> On 05.03.24 16:13, Ryan Roberts wrote:
>> 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 Hilenbrand
>> for deriving this):
> 
> Almost
> 
> "s/Hilenbrand/Hildenbrand/" :)

Ahh sorry... I even explicitly checked it against your name on emails... fat
fingers...

> 
>>
>> --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>
>> ---
>>
>> Applies on top of v6.8-rc6 and mm-unstable (b38c34939fe4).
>>
>> Thanks,
>> Ryan
>>
>>   mm/swapfile.c | 14 +++++++++++---
>>   1 file changed, 11 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/swapfile.c b/mm/swapfile.c
>> index 2b3a2d85e350..f580e6abc674 100644
>> --- a/mm/swapfile.c
>> +++ b/mm/swapfile.c
>> @@ -1281,7 +1281,9 @@ struct swap_info_struct *get_swap_device(swp_entry_t entry)
>>       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 @@ struct swap_info_struct *get_swap_device(swp_entry_t
>> entry)
>>   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 entry)
>>       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;
>>   }
>> -- 
>> 2.25.1
>>
> 
> LGTM
> 
> Are you planning on sending a doc extension for get_swap_device()?

I saw your comment:

We should likely update the documentation of get_swap_device(), that after
decrementing the refcount, the SI might become stale and should not be touched
without a prior get_swap_device().

But when I went to make the changes, I saw the documentation already said:

...we need to enclose all swap related functions with get_swap_device() and
put_swap_device()... Notice that swapoff ... can still happen before the
percpu_ref_tryget_live() in get_swap_device() or after the percpu_ref_put() in
put_swap_device()... The caller must be prepared for that.

I thought that already covered it? I'm sure as usual, I've misunderstood your
point. Happy to respin if you have something in mind?