Message-ID: <129e6d6e-5b1d-4761-b5f4-b4448daf33bc@redhat.com>
Date: Wed, 26 Feb 2025 09:03:44 +0100
From: David Hildenbrand <david@...hat.com>
To: Barry Song <21cnbao@...il.com>
Cc: Liam.Howlett@...cle.com, aarcange@...hat.com, akpm@...ux-foundation.org,
 axelrasmussen@...gle.com, bgeffon@...gle.com, brauner@...nel.org,
 hughd@...gle.com, jannh@...gle.com, kaleshsingh@...gle.com,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org, lokeshgidra@...gle.com,
 mhocko@...e.com, ngeoffray@...gle.com, peterx@...hat.com, rppt@...nel.org,
 ryan.roberts@....com, shuah@...nel.org, surenb@...gle.com,
 v-songbaohua@...o.com, viro@...iv.linux.org.uk, willy@...radead.org,
 zhangpeng362@...wei.com, zhengtangquan@...o.com, yuzhao@...gle.com,
 stable@...r.kernel.org
Subject: Re: [PATCH RFC] mm: Fix kernel BUG when userfaultfd_move encounters
 swapcache

On 26.02.25 06:37, Barry Song wrote:
> On Thu, Feb 20, 2025 at 11:24 PM David Hildenbrand <david@...hat.com> wrote:
>>
>> On 20.02.25 10:21, Barry Song wrote:
>>> On Thu, Feb 20, 2025 at 9:40 PM David Hildenbrand <david@...hat.com> wrote:
>>>>
>>>> On 19.02.25 19:58, Suren Baghdasaryan wrote:
>>>>> On Wed, Feb 19, 2025 at 10:30 AM David Hildenbrand <david@...hat.com> wrote:
>>>>>>
>>>>>> On 19.02.25 19:26, Suren Baghdasaryan wrote:
>>>>>>> On Wed, Feb 19, 2025 at 3:25 AM Barry Song <21cnbao@...il.com> wrote:
>>>>>>>>
>>>>>>>> From: Barry Song <v-songbaohua@...o.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.
> 
> I actually don't quite understand the changelog in  commit 0f8975ec4db2
> (" mm: soft-dirty bits for user memory changes tracking").
> 
> "    Another thing to note, is that when mremap moves PTEs they are marked
>      with soft-dirty as well, since from the user perspective mremap modifies
>      the virtual memory at mremap's new address."
> 
> Why is the hardware-dirty bit not relevant? From the user's perspective,
> the memory at the destination virtual address of mremap/userfaultfd_move
> has changed.

Yes, but it did not change from the system POV. For example, if the page 
was R/O clean and we moved it, why should it suddenly be R/O dirty and 
e.g., require writeback again.

Nobody modified *page content*, but from a user perspective the memory 
at that *virtual memory location* (dst) changed, for example, for 
logical zero (no page mapped) to non-zero (page mapped). That's what 
soft-dirty is about.


> 
> For systems where CONFIG_HAVE_ARCH_SOFT_DIRTY is false, how can the dirty status
> be determined?

No soft-dirty tracking, so nothing to maintain.

> 
> Or is the answer that we only care about soft-dirty changes?
 > > For the hardware-dirty bit, do we only care about actual 
modifications to the
> physical page content rather than changes at the virtual address level?

Exactly!

-- 
Cheers,

David / dhildenb

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