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I'm resurrecting an old thread here because I >>>> think I ran into the same problem with this assertion failing on Linux >>>> 6.7: >>>> >>>> static inline struct page *pfn_swap_entry_to_page(swp_entry_t entry) >>>> { >>>>           struct page *p = pfn_to_page(swp_offset_pfn(entry)); >>>> >>>>           /* >>>>            * Any use of migration entries may only occur while the >>>>            * corresponding page is locked >>>>            */ >>>> -->     BUG_ON(is_migration_entry(entry) && !PageLocked(p)); >>>> >>>>           return p; >>>> } >>>> >>>> It looks like this thread just fizzled two years ago. Did anything >>>> ever come of this? >>>> >>>> Maybe I should add that I saw this in a pre-silicon test environment. >>>> I've never seen this on real hardware. Maybe something >>>> timing-sensitive. >>> >>> In the past, it indicated a swp pte corruption, that would e.g., mess up >>> the stored PFN ot the swap entry type. >>> >>> On which call chain do you see that? >>> >> >> This is the backtrace, it's coming from hmm_range_fault. Looks like the >> swap entries are from migrated DEVICE_PRIVATE pages. > > Thanks, on which kernel version can you reproduce this? This is on a branch based on v6.7: $ git describe HEAD v6.7-2677-g065851796b25 The branch mostly changes code in drivers. No changes in kernel/ or mm/. A few changes in include/linux, but nothing that looks related to core memory management. > >> >> [Apr 3 20:11] ------------[ cut here ]------------ >> [  +0.000041] kernel BUG at include/linux/swapops.h:466! >> [  +0.000691] invalid opcode: 0000 [#1] PREEMPT SMP NOPTI >> [  +0.000342] CPU: 2 PID: 49 Comm: kworker/2:1 Not tainted >> 6.7.0-kfd-compute-rocm-npi-186 #1 >> [  +0.000556] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS >> rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014 >> [  +0.000703] Workqueue: events amdgpu_irq_handle_ih_soft [amdgpu] >> [  +0.000501] RIP: 0010:migration_entry_wait_on_locked+0x26b/0x2b0 >> [  +0.000389] Code: fe ff ff 48 8d 7c 24 07 e8 02 7e f0 ff e9 58 fe ff >> ff 48 8b 43 08 a8 01 75 3f 66 90 48 89 d8 48 8b 00 a8 01 0f 85 f1 fd ff >> ff <0f> 0b 48 8d 58 ff e9 f7 fd ff ff 48 89 d8 f7 c3 ff 0f 00 00 75 df >> [  +0.001161] RSP: 0018:ffffb211c01bb788 EFLAGS: 00010246 >> [  +0.000339] RAX: 017fff8000080018 RBX: fffff682c40ce8c0 RCX: >> 0000000000000001 >> [  +0.000463] RDX: 0000000000000000 RSI: ffff977a45034840 RDI: >> 000000000000001a >> [  +0.000454] RBP: ffff977a45034840 R08: 68000000001033a3 R09: >> 0000000000000030 >> [  +0.000451] R10: ffffb211c01bb6a8 R11: 0000000000000001 R12: >> ffff977a46bd1318 >> [  +0.000461] R13: 0000000000000003 R14: 4000000000000000 R15: >> ffffb211c01bb9b8 >> [  +0.000454] FS:  0000000000000000(0000) GS:ffff977dafd00000(0000) >> knlGS:0000000000000000 >> [  +0.000518] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033 >> [  +0.000372] CR2: 00007fa2d1cba000 CR3: 00000001030d2004 CR4: >> 0000000000770ef0 >> [  +0.000453] PKRU: 55555554 >> [  +0.000182] Call Trace: >> [  +0.000171]  >> [  +0.000147]  ? die+0x37/0x90 >> [  +0.000211]  ? do_trap+0xe0/0x110 >> [  +0.000221]  ? migration_entry_wait_on_locked+0x26b/0x2b0 >> [  +0.000351]  ? do_error_trap+0x98/0x120 >> [  +0.000252]  ? migration_entry_wait_on_locked+0x26b/0x2b0 >> [  +0.000346]  ? migration_entry_wait_on_locked+0x26b/0x2b0 >> [  +0.000355]  ? exc_invalid_op+0x52/0x70 >> [  +0.000254]  ? migration_entry_wait_on_locked+0x26b/0x2b0 >> [  +0.000345]  ? asm_exc_invalid_op+0x1a/0x20 >> [  +0.000274]  ? migration_entry_wait_on_locked+0x26b/0x2b0 >> [  +0.000361]  ? migration_entry_wait+0x4e/0x160 >> [  +0.000293]  ? lock_release+0x119/0x260 >> [  +0.000255]  migration_entry_wait+0x105/0x160 >> [  +0.000290]  hmm_vma_walk_pmd+0x822/0x8a0 >> [  +0.000263]  walk_pgd_range+0x40b/0x900 >> [  +0.000268]  __walk_page_range+0x205/0x220 > > I wonder if that is coming from pmd_migration_entry_wait() or > migration_entry_wait() --  the "?" above adds uncertainty :) This is weird. I only see a call to pmd_migration_entry_wait in hmm_vma_walk_pmd. > > Likely it's from migration_entry_wait(). > > I was first concerned about the lack of PTL in this function, but > migration_entry_wait() will take the PTL and re-read the PTE. > > So when we call into migration_entry_wait_on_locked(), we are holding > the PTL and we verified that we indeed have a migration entry. > > So if we fail in > migration_entry_wait_on_locked()->pfn_swap_entry_folio(), we verified > under PTL and still have a migration entry. > > The referenced folio is indeed not locked then. I must admit, I'm not familiar with this code at all, so my observations and questions are probably naive. So is the BUG_ON bad, or is migration_entry_wait_on_locked missing some page locking? I see that migration_entry_wait_on_locked does a folio_trylock_flag(folio, PG_locked, wait), but _after_ getting the folio with page_folio(pfn_swap_entry_to_page(entry)). Maybe as a workaround for the team stumbling over this, I'll suggest disabling THP. Regards, Felix