On 07/03/2024 08:54, Huang, Ying wrote: > Ryan Roberts <ryan.roberts@xxxxxxx> writes: > >> On 07/03/2024 07:34, Huang, Ying wrote: >>> Miaohe Lin <linmiaohe@xxxxxxxxxx> writes: >>> >>>> On 2024/3/7 13:56, Huang, Ying wrote: >>>>> Miaohe Lin <linmiaohe@xxxxxxxxxx> writes: >>>>> >>>>>> On 2024/3/6 17:31, Ryan Roberts wrote: >>>>>>> On 06/03/2024 08:51, Miaohe Lin wrote: >>>>>>>> On 2024/3/6 10:52, Huang, Ying wrote: >>>>>>>>> Ryan Roberts <ryan.roberts@xxxxxxx> writes: >>>>>>>>> >>>>>>>>>> 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): >>>>>>>>>> >>>>>>>>>> --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<----- >>>>>>>>> >>>>>>>>> I think that this can be simplified. Even for a 4K folio, this could >>>>>>>>> happen. >>>>>>>>> >>>>>>>>> CPU0 CPU1 >>>>>>>>> ---- ---- >>>>>>>>> >>>>>>>>> zap_pte_range >>>>>>>>> free_swap_and_cache >>>>>>>>> __swap_entry_free >>>>>>>>> /* swap count become 0 */ >>>>>>>>> swapoff >>>>>>>>> try_to_unuse >>>>>>>>> filemap_get_folio >>>>>>>>> folio_free_swap >>>>>>>>> /* remove swap cache */ >>>>>>>>> /* free si->swap_map[] */ >>>>>>>>> >>>>>>>>> swap_page_trans_huge_swapped <-- access freed si->swap_map !!! >>>>>>>> >>>>>>>> Sorry for jumping the discussion here. IMHO, free_swap_and_cache is called with pte lock held. >>>>>>> >>>>>>> I don't beleive it has the PTL when called by shmem. >>>>>> >>>>>> In the case of shmem, folio_lock is used to guard against the race. >>>>> >>>>> I don't find folio is lock for shmem. find_lock_entries() will only >>>>> lock the folio if (!xa_is_value()), that is, not swap entry. Can you >>>>> point out where the folio is locked for shmem? >>>> >>>> You're right, folio is locked if not swap entry. That's my mistake. But it seems above race is still nonexistent. >>>> shmem_unuse() will first be called to read all the shared memory data that resides in the swap device back into >>>> memory when doing swapoff. In that case, all the swapped pages are moved to page cache thus there won't be any >>>> xa_is_value(folio) cases when calling shmem_undo_range(). free_swap_and_cache() even won't be called from >>>> shmem_undo_range() after shmem_unuse(). Or am I miss something? >>> >>> I think the following situation is possible. Right? >>> >>> CPU0 CPU1 >>> ---- ---- >>> shmem_undo_range >>> shmem_free_swap >>> xa_cmpxchg_irq >>> free_swap_and_cache >>> __swap_entry_free >>> /* swap count become 0 */ >>> swapoff >>> try_to_unuse >>> shmem_unuse /* cannot find swap entry */ >>> find_next_to_unuse >>> filemap_get_folio >>> folio_free_swap >>> /* remove swap cache */ >>> /* free si->swap_map[] */ >>> swap_page_trans_huge_swapped <-- access freed si->swap_map !!! >>> >>> shmem_undo_range can run earlier. >> >> Yes that's the shmem problem I've been trying to convey. Perhaps there are other >> (extremely subtle) mechanisms that make this impossible, I don't know. >> >> Either way, given the length of this discussion, and the subtleties in the >> syncrhonization mechanisms that have so far been identified, I think the safest >> thing to do is just apply the patch. Then we have explicit syncrhonization that >> we can trivially reason about. > > Yes. This is tricky and we can improve it. So I suggest to, > > - Revise the patch description to use shmem race as example except > someone found it's impossible. > > - Revise the comments of get_swap_device() about RCU reader side lock > (including IRQ off, spinlock, etc.) can prevent swapoff via > synchronize_rcu() in swapoff(). > > - Revise the comments of synchronize_rcu() in swapoff(), which can > prevent swapoff in parallel with RCU reader side lock including swap > cache operations, etc. The only problem with this is that Andrew has already put my v2 into mm-*stable* :-| So (1) from that list isn't possible. I could do a patch for (2) and (3), but to be honest, I think you would do a better job of writing it up than I would - any chance you could post the patch? > > -- > Best Regards, > Huang, Ying
