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syzbot found the following issue on: >>>> >>>> HEAD commit:    627277ba7c23 Merge tag 'arm64_cbpf_mitigation_2025_05_08' .. >>>> git tree:       upstream >>>> console output: https://syzkaller.appspot.com/x/log.txt?x=1150f670580000 >>>> kernel config:  https://syzkaller.appspot.com/x/.config?x=5929ac65be9baf3c >>>> dashboard link: https://syzkaller.appspot.com/bug?extid=2b99589e33edbe9475ca >>>> compiler:       Debian clang version 20.1.2 (++20250402124445+58df0ef89dd6-1~exp1~20250402004600.97), Debian LLD 20.1.2 >>>> >>>> Unfortunately, I don't have any reproducer for this issue yet. >>>> >>>> Downloadable assets: >>>> disk image: https://storage.googleapis.com/syzbot-assets/0a42ae72fe0e/disk-627277ba.raw.xz >>>> vmlinux: https://storage.googleapis.com/syzbot-assets/0be88297bb66/vmlinux-627277ba.xz >>>> kernel image: https://storage.googleapis.com/syzbot-assets/31808a4b1210/bzImage-627277ba.xz >>>> >>>> IMPORTANT: if you fix the issue, please add the following tag to the commit: >>>> Reported-by: syzbot+2b99589e33edbe9475ca@syzkaller.appspotmail.com >>>> >>>> ------------[ cut here ]------------ >>>> WARNING: CPU: 1 PID: 38 at ./include/linux/mm.h:1335 folio_large_mapcount+0xd0/0x110 include/linux/mm.h:1335 >>> >>> This should be >>> >>> VM_WARN_ON_FOLIO(!folio_test_large(folio), folio); >>> >>>> Modules linked in: >>>> CPU: 1 UID: 0 PID: 38 Comm: khugepaged Not tainted 6.15.0-rc6-syzkaller-00025-g627277ba7c23 #0 PREEMPT(full) >>>> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/07/2025 >>>> RIP: 0010:folio_large_mapcount+0xd0/0x110 include/linux/mm.h:1335 >>>> Code: 04 38 84 c0 75 29 8b 03 ff c0 5b 41 5e 41 5f e9 96 d2 2b 09 cc e8 d0 cb 99 ff 48 89 df 48 c7 c6 20 de 77 8b e8 a1 dc de ff 90 <0f> 0b 90 eb b6 89 d9 80 e1 07 80 c1 03 38 c1 7c cb 48 89 df e8 87 >>>> RSP: 0018:ffffc90000af77e0 EFLAGS: 00010246 >>>> RAX: e1fcb38c0ff8ce00 RBX: ffffea00014c8000 RCX: e1fcb38c0ff8ce00 >>>> RDX: 0000000000000001 RSI: ffffffff8d9226df RDI: ffff88801e2fbc00 >>>> RBP: ffffc90000af7b50 R08: ffff8880b8923e93 R09: 1ffff110171247d2 >>>> R10: dffffc0000000000 R11: ffffed10171247d3 R12: 1ffffd4000299000 >>>> R13: dffffc0000000000 R14: 0000000000000000 R15: dffffc0000000000 >>>> FS:  0000000000000000(0000) GS:ffff8881261fb000(0000) knlGS:0000000000000000 >>>> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033 >>>> CR2: 00007ffe58f12dc0 CR3: 0000000030e04000 CR4: 00000000003526f0 >>>> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 >>>> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 >>>> Call Trace: >>>>    >>>>    folio_mapcount include/linux/mm.h:1369 [inline] >>> >>> And here we come through >>> >>> if (likely(!folio_test_large(folio))) { >>>      ... >>> } >>> return folio_large_mapcount(folio); >>> >>> >>> So the folio is split concurrently. And I think there is nothing stopping it from getting freed. >>> >>> We do a xas_for_each() under RCU. So yes, this is racy. >>> >>> In  collapse_file(), we re-validate everything. >>> >>> We could >>> >>> (A) Take proper pagecache locks >>> >>> (B) Try grabbing a temporary folio reference >>> >>> (C) Try snapshotting the folio >>> >>> Probably, in this code, (B) might be cleanest for now? Handling it just like other code in mm/filemap.c. >>> >> >> Hi, > > Hi, > >> >> I've implemented your suggestion (B) using folio_try_get(). >> Could you please review if my patch looks correct? > > You should probably drop both comments, the code merely mimics what filemap.c does. > > Apart from that, nothing jumped at me. > Thank you. I have attached revised patch. 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