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[2003:cb:c747:ff00:9d85:4afb:a7df:6c45]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-43aba5396desm12089565e9.20.2025.02.26.00.03.45 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 26 Feb 2025 00:03:46 -0800 (PST) Message-ID: <129e6d6e-5b1d-4761-b5f4-b4448daf33bc@redhat.com> Date: Wed, 26 Feb 2025 09:03:44 +0100 Precedence: bulk X-Mailing-List: stable@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> <02f14ee1-923f-47e3-a994-4950afb9afcc@redhat.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 26.02.25 06:37, Barry Song wrote: > On Thu, Feb 20, 2025 at 11:24 PM David Hildenbrand wrote: >> >> 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. > > 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