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[2003:cb:c706:2000:e44c:bc46:d8d3:be5]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-43982bcc607sm117006065e9.16.2025.02.20.02.24.01 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 20 Feb 2025 02:24:02 -0800 (PST) Message-ID: <02f14ee1-923f-47e3-a994-4950afb9afcc@redhat.com> Date: Thu, 20 Feb 2025 11:24:01 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH RFC] mm: Fix kernel BUG when userfaultfd_move encounters swapcache To: Barry Song <21cnbao@gmail.com> Cc: Liam.Howlett@oracle.com, aarcange@redhat.com, akpm@linux-foundation.org, axelrasmussen@google.com, bgeffon@google.com, brauner@kernel.org, hughd@google.com, jannh@google.com, kaleshsingh@google.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, lokeshgidra@google.com, mhocko@suse.com, ngeoffray@google.com, peterx@redhat.com, rppt@kernel.org, ryan.roberts@arm.com, shuah@kernel.org, surenb@google.com, v-songbaohua@oppo.com, viro@zeniv.linux.org.uk, willy@infradead.org, zhangpeng362@huawei.com, zhengtangquan@oppo.com, yuzhao@google.com, stable@vger.kernel.org References: <69dbca2b-cf67-4fd8-ba22-7e6211b3e7c4@redhat.com> <20250220092101.71966-1-21cnbao@gmail.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 20.02.25 10:21, Barry Song wrote: > On Thu, Feb 20, 2025 at 9:40 PM David Hildenbrand wrote: >> >> On 19.02.25 19:58, Suren Baghdasaryan wrote: >>> On Wed, Feb 19, 2025 at 10:30 AM David Hildenbrand 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 >>>>>> >>>>>> 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