Re: [PATCH v3 1/4] mm: swap: Remove CLUSTER_FLAG_HUGE from swap_cluster_info:flags

[Ryan Roberts]

On 04/03/2024 20:50, David Hildenbrand wrote:
>>>>
>>>> This is the existing free_swap_and_cache(). I think _swap_info_get() would
>>>> break
>>>> if this could race with swapoff(), and __swap_entry_free() looks up the cluster
>>>> from an array, which would also be freed by swapoff if racing:
>>>>
>>>> int free_swap_and_cache(swp_entry_t entry)
>>>> {
>>>>      struct swap_info_struct *p;
>>>>      unsigned char count;
>>>>
>>>>      if (non_swap_entry(entry))
>>>>          return 1;
>>>>
>>>>      p = _swap_info_get(entry);
>>>>      if (p) {
>>>>          count = __swap_entry_free(p, entry);
>>>
>>> If count dropped to 0 and
>>>
>>>>          if (count == SWAP_HAS_CACHE)
>>>
>>>
>>> count is now SWAP_HAS_CACHE, there is in fact no swap entry anymore. We removed
>>> it. That one would have to be reclaimed asynchronously.
>>>
>>> The existing code we would call swap_page_trans_huge_swapped() with the SI it
>>> obtained via _swap_info_get().
>>>
>>> I also don't see what should be left protecting the SI. It's not locked anymore,
>>> the swapcounts are at 0. We don't hold the folio lock.
>>>
>>> try_to_unuse() will stop as soon as si->inuse_pages is at 0. Hm ...
>>
>> But, assuming the caller of free_swap_and_cache() acquires the PTL first, I
>> think this all works out ok? While free_swap_and_cache() is running,
>> try_to_unuse() will wait for the PTL. Or if try_to_unuse() runs first, then
>> free_swap_and_cache() will never be called because the swap entry will have been
>> removed from the PTE?
> 
> But can't try_to_unuse() run, detect !si->inuse_pages and not even bother about
> scanning any further page tables?
> 
> But my head hurts from digging through that code.

Yep, glad I'm not the only one that gets headaches from swapfile.c.

> 
> Let me try again:
> 
> __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 process 1 finished its call to swap_page_trans_huge_swapped()?

Assuming you are talking about anonymous memory, process 1 has the PTL while
it's executing free_swap_and_cache(). try_to_unuse() iterates over every vma in
every mm, and it swaps-in a page for every PTE that holds a swap entry for the
device being swapoff'ed. It takes the PTL while converting the swap entry to
present PTE - see unuse_pte(). Process 1 must have beaten try_to_unuse() to the
particular pte, because if try_to_unuse() got there first, it would have
converted it from a swap entry to present pte and process 1 would never even
have called free_swap_and_cache(). So try_to_unuse() will eventually wait on the
PTL until process 1 has released it after free_swap_and_cache() completes. Am I
missing something? Because that part feels pretty clear to me.

Its the shmem case that I'm struggling to explain.

> 
> 
> 
>>
>> That just leaves shmem... I suspected there might be some serialization between
>> shmem_unuse() (called from try_to_unuse()) and the shmem free_swap_and_cache()
>> callsites, but I can't see it. Hmm...
>>
>>>
>>> Would performing the overall operation under lock_cluster_or_swap_info help? Not
>>> so sure :(
>>
>> No - that function relies on being able to access the cluster from the array in
>> the swap_info and lock it. And I think that array has the same lifetime as
>> swap_map, so same problem. You'd need get_swap_device()/put_swap_device() and a
>> bunch of refactoring for the internals not to take the locks, I guess. I think
>> its doable, just not sure if neccessary...
> 
> Agreed.
>