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Date: Fri, 08 Mar 2024 08:55:18 +0800 From: "Huang, Ying" <ying.huang@...el.com> To: Ryan Roberts <ryan.roberts@....com> Cc: Miaohe Lin <linmiaohe@...wei.com>, Andrew Morton <akpm@...ux-foundation.org>, David Hildenbrand <david@...hat.com>, <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>, <stable@...r.kernel.org> Subject: Re: [PATCH v1] mm: swap: Fix race between free_swap_and_cache() and swapoff() Ryan Roberts <ryan.roberts@....com> writes: > On 07/03/2024 08:54, Huang, Ying wrote: >> Ryan Roberts <ryan.roberts@....com> writes: >> >>> On 07/03/2024 07:34, Huang, Ying wrote: >>>> Miaohe Lin <linmiaohe@...wei.com> writes: >>>> >>>>> On 2024/3/7 13:56, Huang, Ying wrote: >>>>>> Miaohe Lin <linmiaohe@...wei.com> 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@....com> 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? > Sure. I will do that. -- Best Regards, Huang, Ying
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