linux-kernel - Re: [PATCH v1] mm: swap: Fix race between free_swap_and

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Date: Tue, 5 Mar 2024 16:50:00 +0100
From: David Hildenbrand <david@...hat.com>
To: Ryan Roberts <ryan.roberts@....com>,
 Andrew Morton <akpm@...ux-foundation.org>, "Huang, Ying"
 <ying.huang@...el.com>
Cc: 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()

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/" :)

> 
> --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@...r.kernel.org
> Signed-off-by: Ryan Roberts <ryan.roberts@....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()?

-- 
Cheers,

David / dhildenb