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uUFfvcC5PSfHgyQwO3ZRudSZMnBoabpnjdMlawzIVL3yRC6MvOQ= X-Proofpoint-ORIG-GUID: 5XRcDQ2XwFIfvRYpew20b4mXc-p6NDLN On Fri, Nov 14, 2025 at 12:06:57PM +0100, David Hildenbrand (Red Hat) wrote: > On 11.11.25 08:10, Harry Yoo wrote: > > # TL;DR > > > > previous discussion: https://lore.kernel.org/linux-mm/b41ea29e-6b48-4f64-859c-73be095453ae@redhat.com/ > > > > A "bad pmd" error occurs due to race condition between > > change_prot_numa() and THP migration. The mainline kernel does not have > > this bug as commit 670ddd8cdc fixes the race condition. 6.1.y, 5.15.y, > > 5.10.y, 5.4.y are affected by this bug. > > > > Fixing this in -stable kernels is tricky because pte_map_offset_lock() > > has different semantics in pre-6.5 and post-6.5 kernels. I am trying to > > backport the same mechanism we have in the mainline kernel. > > Since the code looks bit different due to different semantics of > > pte_map_offset_lock(), it'd be best to get this reviewed by MM folks. > > > > # Testing > > > > I verified that the bug described below is not reproduced anymore > > (on a downstream kernel) after applying this patch series. It used to > > trigger in few days of intensive numa balancing testing, but it survived > > 2 weeks with this applied. > > > > # Bug Description > > > > It was reported that a bad pmd is seen when automatic NUMA > > balancing is marking page table entries as prot_numa: > > [2437548.196018] mm/pgtable-generic.c:50: bad pmd 00000000af22fc02(dffffffe71fbfe02) > > [2437548.235022] Call Trace: > > [2437548.238234] > > [2437548.241060] dump_stack_lvl+0x46/0x61 > > [2437548.245689] panic+0x106/0x2e5 > > [2437548.249497] pmd_clear_bad+0x3c/0x3c > > [2437548.253967] change_pmd_range.isra.0+0x34d/0x3a7 > > [2437548.259537] change_p4d_range+0x156/0x20e > > [2437548.264392] change_protection_range+0x116/0x1a9 > > [2437548.269976] change_prot_numa+0x15/0x37 > > [2437548.274774] task_numa_work+0x1b8/0x302 > > [2437548.279512] task_work_run+0x62/0x95 > > [2437548.283882] exit_to_user_mode_loop+0x1a4/0x1a9 > > [2437548.289277] exit_to_user_mode_prepare+0xf4/0xfc > > [2437548.294751] ? sysvec_apic_timer_interrupt+0x34/0x81 > > [2437548.300677] irqentry_exit_to_user_mode+0x5/0x25 > > [2437548.306153] asm_sysvec_apic_timer_interrupt+0x16/0x1b > > > > This is due to a race condition between change_prot_numa() and > > THP migration because the kernel doesn't check is_swap_pmd() and > > pmd_trans_huge() atomically: > > > > change_prot_numa() THP migration > > ====================================================================== > > - change_pmd_range() > > -> is_swap_pmd() returns false, > > meaning it's not a PMD migration > > entry. > > - do_huge_pmd_numa_page() > > -> migrate_misplaced_page() sets > > migration entries for the THP. > > - change_pmd_range() > > -> pmd_none_or_clear_bad_unless_trans_huge() > > -> pmd_none() and pmd_trans_huge() returns false > > - pmd_none_or_clear_bad_unless_trans_huge() > > -> pmd_bad() returns true for the migration entry! > > > > The upstream commit 670ddd8cdcbd ("mm/mprotect: delete > > pmd_none_or_clear_bad_unless_trans_huge()") closes this race condition > > by checking is_swap_pmd() and pmd_trans_huge() atomically. > > > > # Backporting note > > > > commit a79390f5d6a7 ("mm/mprotect: use long for page accountings and retval") > > is backported to return an error code (negative value) in > > change_pte_range(). > > > > Unlike the mainline, pte_offset_map_lock() does not check if the pmd > > entry is a migration entry or a hugepage; acquires PTL unconditionally > > instead of returning failure. Therefore, it is necessary to keep the > > !is_swap_pmd() && !pmd_trans_huge() && !pmd_devmap() checks in > > change_pmd_range() before acquiring the PTL. > > > > After acquiring the lock, open-code the semantics of > > pte_offset_map_lock() in the mainline kernel; change_pte_range() fails > > if the pmd value has changed. This requires adding pmd_old parameter > > (pmd_t value that is read before calling the function) to > > change_pte_range(). > > Looks reasonable to me, so I assume the backporting diff makes sense. > > Acked-by: David Hildenbrand (Red Hat) Hi David, just wanted to say that I really appreciate your acknowledgement. Thanks! While this bug doesn't appear to cause severe damage on the system (only a "bad pmd" error printed to console due to race), that wasn't really clear before closer investigation and I think it is worth backporting to save others' time. I'll send v5.15, v5.10, v5.4 fix soon that does the same thing. -- Cheers, Harry / Hyeonggon