Re: [PATCH] mm/swapfile: drop a large unmapped swap-in folio in unuse_pte()

[David Hildenbrand (Arm)]

On 10/30/26 20:11, Mani Tofigh via B4 Relay wrote:
> From: Mani Tofigh <mt4022@xxxxxxxxxxxx>
> 

Thanks for the report!

> Since commit f1879e8a0c60 ("mm, swap: never bypass the swap cache even
> for SWP_SYNCHRONOUS_IO"), a fault can read a swapped-out mTHP range into
> one large folio in the swap cache.  The folio only becomes anon once
> do_swap_page() maps all of it, so if the faulting PTE changes in the
> meantime the folio stays in the swap cache while not being mapped or
> anon:
> 
>   Task A (fault)                       Task B
>   do_swap_page()
>     /* swaps in large folio F */
>                                        madvise(MADV_DONTNEED)
>                                          /* zaps the faulting PTE */
>     /* PTE changed (F not mapped) */
> 
> swapoff then maps it, which marks all of its pages as mapped and
> exclusive, and the next fault on one of the other PTEs crashes in
> do_swap_page():

Ugh.

Why is this special to large folios? I guess because a single PTE keeps the
large folio alive, right?

The problem really is that unuse_pte() open-codes whet do_swap_page() would
usually do. And it doesn't handle large folios in this scenario correctly.

What a mess.

I guess we could just teach unuse_pte() to only map a single PTE.

folio_add_new_anon_rmap() assumes the folios is fully mapped. We could add
up to twp folio_remove_rmap_ptes() calls with everything except the page we want
to map.

But reading below ...

> 
>   [    6.424485] kernel BUG at mm/memory.c:5195!
>   [    6.424503] Oops: invalid opcode: 0000 [#1] SMP KASAN NOPTI
>   [    6.427654] CPU: 0 UID: 0 PID: 67 Comm: swapmthp Tainted: G        W            7.3.0-17087-gd865f2677c44 #1 PREEMPT
>   [    6.432665] RIP: 0010:do_swap_page+0x24eb/0x2fa0
>   [    6.451035] Call Trace:
>   [    6.451567]  <TASK>
>   [    6.453841]  __handle_mm_fault+0x13b5/0x3b40
>   [    6.458525]  handle_mm_fault+0x16f/0x480
>   [    6.461143]  do_user_addr_fault+0x312/0xa30
>   [    6.462044]  exc_page_fault+0x66/0xc0
>   [    6.462846]  asm_exc_page_fault+0x26/0x30
>   [    6.476708]  </TASK>
> 
> do_swap_page() already drops such a folio from the swap cache when it
> can't map all of it.  Do the same in unuse_pte() so that swapoff swaps
> the entries in again.  With the fix, nothing is reported.

We indeed have this

if (!folio_test_anon(folio) && folio_test_large(folio) &&
    nr_pages != folio_nr_pages(folio)) {
	...
}

check.

The nr_pages check makes sense as in your case below it's simply nr_pages == 1.

> 
> This bug was identified and confirmed with the help of a deterministic
> hypervisor currently under development.  The C reproducer below however
> hits it within a second.

Rings a bell :)

> 
> Fixes: f1879e8a0c60 ("mm, swap: never bypass the swap cache even for SWP_SYNCHRONOUS_IO")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Mani Tofigh <mt4022@xxxxxxxxxxxx>
> ---
> This conflicts in context with patch 41/47 of David's v2 series [1],
> which changes the line right below in unuse_pte() for KSM folios.  I
> can rebase on top of it once it's applied if needed.

It will likely get applied on top of

	git git://git.kernel.org/pub/scm/linux/kernel/git/mm/core.git for-next

Tomorrow and then quickly find its way into next. So if a resend is required,
best to send against that.

> 
> [1] https://lore.kernel.org/all/20261029-fixes_core-v2-41-a64ce7f83abb@xxxxxxxxxx/
> 
> Reproducer: https://github.com/manitofigh/files/blob/main/linux-repros/swapmthp.c
> ---
>  mm/swapfile.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 189f04d8838f..b0b75ff16835 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -2518,6 +2518,18 @@ static int unuse_pte(struct vm_area_struct *vma, pmd_t *pmd,
>  	if (!folio_matches_swap_entry(folio, entry))
>  		return 0;
>  
> +	/*
> +	 * A large folio that is not anon yet was swapped in by a page fault
> +	 * that did not map it (yet). It can only be mapped as a whole, by
> +	 * do_swap_page(). Drop it from the swap cache and swap in small
> +	 * folios instead.
> +	 */
> +	if (unlikely(folio_test_large(folio) && !folio_test_anon(folio))) {
> +		if (!WARN_ON_ONCE(folio_test_dirty(folio)))
> +			swap_cache_del_folio(folio);
> +		return 0;
> +	}

I don't enjoy the additional code duplication, including the comment.

Can you pack the existing handling in do_swap_page() into a helper, pass
nr_pages, and reuse it here?

I guess finding a good name is the main challenge ...


static bool swapcache_folio_mappable(struct folio *folio,
		unsigned int nr_pages)
{
	if (!folio_test_anon(folio) || !folio_test_large(folio) ||
	    nr_pages == folio_nr_pages(folio))
		return true;

	if (!WARN_ON_ONCE(folio_test_dirty(folio)))
		swap_cache_del_folio(folio);

	return false;
}


Just a thought.

But it does look like the right fix.

-- 
Cheers,

David