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A656460120; Wed, 1 Apr 2026 09:36:52 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id EBF7CC4CEF7; Wed, 1 Apr 2026 09:36:48 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1775036212; bh=cLLbkdmrgHcSVkludMmZDV4Dvq1Ehg21aAFt6YAwwL0=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=jgjsYPDjfNF/sZw+LlQ3aJ1qMpDN6LUPXTSMK7iT9KLZuBA/PiuqjD2iSi6D4WD2c RsjfvDVzbd/TH+WuRFXy/2fQuJr+VZ05k4UC+m0oXIYkbwQDOHpbmi9Z0OXnhB5u27 phkRDRSzMv1O1KzPVAGZOX6FTqFJfwtXRdAqAS97DSAwL0RuoO2zdtZcDjqaFCHR6E YtOBmAKnM2lvG1jhIu27KmeZh3nk5mmZsi1Bj5QYALq7o/RJSEz2HtVLakan8mS0Xc CUWTGG+rPyWit61hyiG9ovSlX8RMbrknvZXf/O6w5Yr6AElHATDCZ7F70XFthEdE5c 3DYLzvegxO9/g== Message-ID: Date: Wed, 1 Apr 2026 11:36:46 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [syzbot] [mm?] WARNING in deferred_split_folio To: Lance Yang , usama.arif@linux.dev, Liam.Howlett@oracle.com, ziy@nvidia.com Cc: syzbot+a7067a757858ac8eb085@syzkaller.appspotmail.com, akpm@linux-foundation.org, baohua@kernel.org, baolin.wang@linux.alibaba.com, dev.jain@arm.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, ljs@kernel.org, npache@redhat.com, ryan.roberts@arm.com, syzkaller-bugs@googlegroups.com, Deepanshu Kartikey References: <20260401081025.68951-1-lance.yang@linux.dev> <20260401085932.20945-1-lance.yang@linux.dev> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc 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THwFnjR4Zt04ueEJmVz3e/s0c0bAbfMrktAK9hf1SBk= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 4/1/26 10:59, Lance Yang wrote: > > On Wed, Apr 01, 2026 at 04:10:25PM +0800, Lance Yang wrote: >> >> +Cc Usama >> >> On Tue, Mar 31, 2026 at 11:08:27PM -0700, syzbot wrote: >>> Hello, >>> >>> syzbot found the following issue on: >>> >>> HEAD commit: cf7c3c02fdd0 Add linux-next specific files for 20260330 >>> git tree: linux-next >>> console output: https://syzkaller.appspot.com/x/log.txt?x=154ee46a580000 >>> kernel config: https://syzkaller.appspot.com/x/.config?x=3944d875fa9bfb67 >>> dashboard link: https://syzkaller.appspot.com/bug?extid=a7067a757858ac8eb085 >>> compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8 >>> syz repro: https://syzkaller.appspot.com/x/repro.syz?x=12c846ba580000 >>> >>> Downloadable assets: >>> disk image: https://storage.googleapis.com/syzbot-assets/053d3b49a360/disk-cf7c3c02.raw.xz >>> vmlinux: https://storage.googleapis.com/syzbot-assets/faabb37d41d0/vmlinux-cf7c3c02.xz >>> kernel image: https://storage.googleapis.com/syzbot-assets/8d47fe92aaa8/bzImage-cf7c3c02.xz >>> >>> IMPORTANT: if you fix the issue, please add the following tag to the commit: >>> Reported-by: syzbot+a7067a757858ac8eb085@syzkaller.appspotmail.com >>> >>> free_pages_and_swap_cache+0x2b9/0x490 mm/swap_state.c:401 >>> __tlb_batch_free_encoded_pages mm/mmu_gather.c:138 [inline] >>> tlb_batch_pages_flush mm/mmu_gather.c:151 [inline] >>> tlb_flush_mmu_free mm/mmu_gather.c:417 [inline] >>> tlb_flush_mmu+0x6d3/0xa30 mm/mmu_gather.c:424 >>> tlb_finish_mmu+0xf9/0x230 mm/mmu_gather.c:549 >>> exit_mmap+0x498/0x9e0 mm/mmap.c:1313 >>> __mmput+0x118/0x430 kernel/fork.c:1177 >>> exit_mm+0x18e/0x250 kernel/exit.c:581 >>> do_exit+0x6a2/0x22c0 kernel/exit.c:962 >>> do_group_exit+0x21b/0x2d0 kernel/exit.c:1116 >>> __do_sys_exit_group kernel/exit.c:1127 [inline] >>> __se_sys_exit_group kernel/exit.c:1125 [inline] >>> __x64_sys_exit_group+0x3f/0x40 kernel/exit.c:1125 >>> x64_sys_call+0x221a/0x2240 arch/x86/include/generated/asm/syscalls_64.h:232 >>> do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline] >>> do_syscall_64+0x15f/0xf80 arch/x86/entry/syscall_64.c:94 >>> entry_SYSCALL_64_after_hwframe+0x77/0x7f >>> ------------[ cut here ]------------ >>> 1 >>> WARNING: mm/huge_memory.c:4371 at deferred_split_folio+0x974/0xaa0 mm/huge_memory.c:4371, CPU#1: syz.3.1110/10500 >>> Modules linked in: >>> CPU: 1 UID: 0 PID: 10500 Comm: syz.3.1110 Not tainted syzkaller #0 PREEMPT(full) >>> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2026 >>> RIP: 0010:deferred_split_folio+0x974/0xaa0 mm/huge_memory.c:4371 >>> Code: 31 48 8d 65 d8 5b 41 5c 41 5d 41 5e 41 5f 5d e9 c2 67 8d 09 cc e8 8c 73 93 ff 48 89 df 48 c7 c6 20 5b fc 8b e8 dd 2b f5 fe 90 <0f> 0b 90 e9 d4 fe ff ff e8 9f 7a 8a 09 e8 6a 73 93 ff 48 89 df 48 >>> RSP: 0018:ffffc900047ef540 EFLAGS: 00010046 >>> RAX: 1c05fb65cfaab100 RBX: ffffea0001840000 RCX: 0000000080000001 >>> RDX: 0000000000000002 RSI: ffffffff8e4da1c7 RDI: ffff88807d6f9e80 >>> RBP: ffffc900047ef610 R08: ffff8880b87247d3 R09: 1ffff110170e48fa >>> R10: dffffc0000000000 R11: ffffed10170e48fb R12: ffffea0001840040 >>> R13: 0000000000000000 R14: 0000000000010000 R15: 1ffff920008fdeb0 >>> FS: 00007f32e32a76c0(0000) GS:ffff8881250e8000(0000) knlGS:0000000000000000 >>> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 >>> CR2: 00007f5825757930 CR3: 0000000034ad8000 CR4: 00000000003526f0 >>> Call Trace: >>> >>> migrate_folio_move mm/migrate.c:1411 [inline] >> >> Looks like a race introduced by commit[1] ("mm: migrate: requeue >> destination folio on deferred split queue"). >> >> Between folio migration (mbind) and rmap removal (exit_mmap), I guess :) >> >> migrate_folio_move() snapshots src_partially_mapped from src before >> migration: >> >> if (folio_order(src) > 1 && >> !data_race(list_empty(&src->_deferred_list))) { >> src_deferred_split = true; >> src_partially_mapped = folio_test_partially_mapped(src); >> } >> >> Then move_to_new_folio() eventually unqueues src in >> __folio_migrate_mapping(): >> >> folio_unqueue_deferred_split(src); >> >> After that, migration restores mappings to dst: >> >> if (old_page_state & PAGE_WAS_MAPPED) >> remove_migration_ptes(src, dst, 0); >> >> At that point, dst is already visible again. A concurrent unmap path >>from another sharer can then remove some of those mappings and reach >> deferred_split_folio(dst, true), which sets PG_partially_mapped on >> dst. >> >> Migration then resumes and does: >> >> if (src_deferred_split) >> deferred_split_folio(dst, src_partially_mapped); >> >> If the earlier snapshot from src was false, this becomes >> deferred_split_folio(dst, false), but dst may already have been marked >> partially mapped by the concurrent rmap-removal path, so the WARN in >> deferred_split_folio() fires: >> >> if (partially_mapped) { >> ... >> } else { >> /* partially mapped folios cannot become non-partially mapped */ >> VM_WARN_ON_FOLIO(folio_test_partially_mapped(folio), folio); >> } >> >> [1] https://lore.kernel.org/all/20260312104723.1351321-1-usama.arif@linux.dev/ >> > > Perhaps the WARN is simply too strict there :) > > Migration already holds the folio lock on dst, while the competing > rmap-removal path runs under the page-table lock. So once > remove_migration_ptes(src, dst, 0) makes dst visible again, this race > looks hard to avoid. > > So maybe the simplest fix is just to drop the WARN in the > !partially_mapped path: > > ---8<--- > Subject: [PATCH 1/1] mm/thp: avoid false warning in deferred_split_folio() > > From: Lance Yang > > migrate_folio_move() snapshots src_partially_mapped from src before > migration and later requeues dst after remove_migration_ptes(src, dst, 0). > > Once dst is visible again, a competing rmap-removal path can legally set > PG_partially_mapped before the migration path reaches > deferred_split_folio(dst, src_partially_mapped). > > Migration already holds the folio lock on dst, while the competing > rmap-removal path runs under the page-table lock. So once > remove_migration_ptes(src, dst, 0) makes dst visible again, this race > looks hard to avoid. > > So just drop the WARN in the !partially_mapped path and preserve an > already-set PG_partially_mapped bit. > > Link: https://lore.kernel.org/linux-mm/69ccb65b.050a0220.183828.003a.GAE@google.com/ > Fixes: 8a8ca142a488 ("mm: migrate: requeue destination folio on deferred split queue") > Reported-by: syzbot+a7067a757858ac8eb085@syzkaller.appspotmail.com > Signed-off-by: Lance Yang A fix just appeared: https://lore.kernel.org/r/20260401084116.22219-1-kartikey406@gmail.com Have to think about this :) -- Cheers, David