Re: [PATCH RFC] mm: Fix kernel BUG when userfaultfd_move encounters swapcache

[David Hildenbrand <david@redhat.com>]

On 20.02.25 10:21, Barry Song wrote:
> On Thu, Feb 20, 2025 at 9:40 PM David Hildenbrand <david@redhat.com> wrote:
>>
>> On 19.02.25 19:58, Suren Baghdasaryan wrote:
>>> On Wed, Feb 19, 2025 at 10:30 AM David Hildenbrand <david@redhat.com> wrote:
>>>>
>>>> On 19.02.25 19:26, Suren Baghdasaryan wrote:
>>>>> On Wed, Feb 19, 2025 at 3:25 AM Barry Song <21cnbao@gmail.com> wrote:
>>>>>>
>>>>>> From: Barry Song <v-songbaohua@oppo.com>
>>>>>>
>>>>>> userfaultfd_move() checks whether the PTE entry is present or a
>>>>>> swap entry.
>>>>>>
>>>>>> - If the PTE entry is present, move_present_pte() handles folio
>>>>>>      migration by setting:
>>>>>>
>>>>>>      src_folio->index = linear_page_index(dst_vma, dst_addr);
>>>>>>
>>>>>> - If the PTE entry is a swap entry, move_swap_pte() simply copies
>>>>>>      the PTE to the new dst_addr.
>>>>>>
>>>>>> This approach is incorrect because even if the PTE is a swap
>>>>>> entry, it can still reference a folio that remains in the swap
>>>>>> cache.
>>>>>>
>>>>>> If do_swap_page() is triggered, it may locate the folio in the
>>>>>> swap cache. However, during add_rmap operations, a kernel panic
>>>>>> can occur due to:
>>>>>>     page_pgoff(folio, page) != linear_page_index(vma, address)
>>>>>
>>>>> Thanks for the report and reproducer!
>>>>>
>>>>>>
>>>>>> $./a.out > /dev/null
>>>>>> [   13.336953] page: refcount:6 mapcount:1 mapping:00000000f43db19c index:0xffffaf150 pfn:0x4667c
>>>>>> [   13.337520] head: order:2 mapcount:1 entire_mapcount:0 nr_pages_mapped:1 pincount:0
>>>>>> [   13.337716] memcg:ffff00000405f000
>>>>>> [   13.337849] anon flags: 0x3fffc0000020459(locked|uptodate|dirty|owner_priv_1|head|swapbacked|node=0|zone=0|lastcpupid=0xffff)
>>>>>> [   13.338630] raw: 03fffc0000020459 ffff80008507b538 ffff80008507b538 ffff000006260361
>>>>>> [   13.338831] raw: 0000000ffffaf150 0000000000004000 0000000600000000 ffff00000405f000
>>>>>> [   13.339031] head: 03fffc0000020459 ffff80008507b538 ffff80008507b538 ffff000006260361
>>>>>> [   13.339204] head: 0000000ffffaf150 0000000000004000 0000000600000000 ffff00000405f000
>>>>>> [   13.339375] head: 03fffc0000000202 fffffdffc0199f01 ffffffff00000000 0000000000000001
>>>>>> [   13.339546] head: 0000000000000004 0000000000000000 00000000ffffffff 0000000000000000
>>>>>> [   13.339736] page dumped because: VM_BUG_ON_PAGE(page_pgoff(folio, page) != linear_page_index(vma, address))
>>>>>> [   13.340190] ------------[ cut here ]------------
>>>>>> [   13.340316] kernel BUG at mm/rmap.c:1380!
>>>>>> [   13.340683] Internal error: Oops - BUG: 00000000f2000800 [#1] PREEMPT SMP
>>>>>> [   13.340969] Modules linked in:
>>>>>> [   13.341257] CPU: 1 UID: 0 PID: 107 Comm: a.out Not tainted 6.14.0-rc3-gcf42737e247a-dirty #299
>>>>>> [   13.341470] Hardware name: linux,dummy-virt (DT)
>>>>>> [   13.341671] pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
>>>>>> [   13.341815] pc : __page_check_anon_rmap+0xa0/0xb0
>>>>>> [   13.341920] lr : __page_check_anon_rmap+0xa0/0xb0
>>>>>> [   13.342018] sp : ffff80008752bb20
>>>>>> [   13.342093] x29: ffff80008752bb20 x28: fffffdffc0199f00 x27: 0000000000000001
>>>>>> [   13.342404] x26: 0000000000000000 x25: 0000000000000001 x24: 0000000000000001
>>>>>> [   13.342575] x23: 0000ffffaf0d0000 x22: 0000ffffaf0d0000 x21: fffffdffc0199f00
>>>>>> [   13.342731] x20: fffffdffc0199f00 x19: ffff000006210700 x18: 00000000ffffffff
>>>>>> [   13.342881] x17: 6c203d2120296567 x16: 6170202c6f696c6f x15: 662866666f67705f
>>>>>> [   13.343033] x14: 6567617028454741 x13: 2929737365726464 x12: ffff800083728ab0
>>>>>> [   13.343183] x11: ffff800082996bf8 x10: 0000000000000fd7 x9 : ffff80008011bc40
>>>>>> [   13.343351] x8 : 0000000000017fe8 x7 : 00000000fffff000 x6 : ffff8000829eebf8
>>>>>> [   13.343498] x5 : c0000000fffff000 x4 : 0000000000000000 x3 : 0000000000000000
>>>>>> [   13.343645] x2 : 0000000000000000 x1 : ffff0000062db980 x0 : 000000000000005f
>>>>>> [   13.343876] Call trace:
>>>>>> [   13.344045]  __page_check_anon_rmap+0xa0/0xb0 (P)
>>>>>> [   13.344234]  folio_add_anon_rmap_ptes+0x22c/0x320
>>>>>> [   13.344333]  do_swap_page+0x1060/0x1400
>>>>>> [   13.344417]  __handle_mm_fault+0x61c/0xbc8
>>>>>> [   13.344504]  handle_mm_fault+0xd8/0x2e8
>>>>>> [   13.344586]  do_page_fault+0x20c/0x770
>>>>>> [   13.344673]  do_translation_fault+0xb4/0xf0
>>>>>> [   13.344759]  do_mem_abort+0x48/0xa0
>>>>>> [   13.344842]  el0_da+0x58/0x130
>>>>>> [   13.344914]  el0t_64_sync_handler+0xc4/0x138
>>>>>> [   13.345002]  el0t_64_sync+0x1ac/0x1b0
>>>>>> [   13.345208] Code: aa1503e0 f000f801 910f6021 97ff5779 (d4210000)
>>>>>> [   13.345504] ---[ end trace 0000000000000000 ]---
>>>>>> [   13.345715] note: a.out[107] exited with irqs disabled
>>>>>> [   13.345954] note: a.out[107] exited with preempt_count 2
>>>>>>
>>>>>> Fully fixing it would be quite complex, requiring similar handling
>>>>>> of folios as done in move_present_pte.
>>>>>
>>>>> How complex would that be? Is it a matter of adding
>>>>> folio_maybe_dma_pinned() checks, doing folio_move_anon_rmap() and
>>>>> folio->index = linear_page_index like in move_present_pte() or
>>>>> something more?
>>>>
>>>> If the entry is pte_swp_exclusive(), and the folio is order-0, it cannot
>>>> be pinned and we may be able to move it I think.
>>>>
>>>> So all that's required is to check pte_swp_exclusive() and the folio size.
>>>>
>>>> ... in theory :) Not sure about the swap details.
>>>
>>> Looking some more into it, I think we would have to perform all the
>>> folio and anon_vma locking and pinning that we do for present pages in
>>> move_pages_pte(). If that's correct then maybe treating swapcache
>>> pages like a present page inside move_pages_pte() would be simpler?
>>
>> I'd be more in favor of not doing that. Maybe there are parts we can
>> move out into helper functions instead, so we can reuse them?
> 
> I actually have a v2 ready. Maybe we can discuss if some of the code can be
> extracted as a helper based on the below before I send it formally?
> 
> I’d say there are many parts that can be shared with present PTE, but there
> are two major differences:
> 
> 1. Page exclusivity – swapcache doesn’t require it (try_to_unmap_one has remove
> Exclusive flag;)
> 2. src_anon_vma and its lock – swapcache doesn’t require it（folio is not mapped）
> 

That's a lot of complicated code you have there (not your fault, it's 
complicated stuff ... ) :)

Some of it might be compressed/simplified by the use of "else if".

I'll try to take a closer look later (will have to apply it to see the 
context better). Just one independent comment because I stumbled over 
this recently:

[...]

> @@ -1062,10 +1063,13 @@ static int move_present_pte(struct mm_struct *mm,
>   	folio_move_anon_rmap(src_folio, dst_vma);
>   	src_folio->index = linear_page_index(dst_vma, dst_addr);
>   
> -	orig_dst_pte = mk_pte(&src_folio->page, dst_vma->vm_page_prot);
> -	/* Follow mremap() behavior and treat the entry dirty after the move */
> -	orig_dst_pte = pte_mkwrite(pte_mkdirty(orig_dst_pte), dst_vma);
> -
> +	if (pte_present(orig_src_pte)) {
> +		orig_dst_pte = mk_pte(&src_folio->page, dst_vma->vm_page_prot);
> +		/* Follow mremap() behavior and treat the entry dirty after the move */
> +		orig_dst_pte = pte_mkwrite(pte_mkdirty(orig_dst_pte), dst_vma);

I'll note that the comment and mkdirty is misleading/wrong. It's 
softdirty that we care about only. But that is something independent of 
this change.

For swp PTEs, we maybe also would want to set softdirty.

See move_soft_dirty_pte() on what is actually done on the mremap path.

-- 
Cheers,

David / dhildenb