Ritesh Harjani (IBM) bbcbf045d6 powerpc/64s: Fix unmap race with PMD migration entries
The following race is possible with migration swap entries or
device-private THP entries. e.g. when move_pages is called on a PMD THP
page, then there maybe an intermediate state, where PMD entry acts as
a migration swap entry (pmd_present() is true). Then if an munmap
happens at the same time, then this VM_BUG_ON() can happen in
pmdp_huge_get_and_clear_full().

This patch fixes that.

Thread A: move_pages() syscall
  add_folio_for_migration()
    mmap_read_lock(mm)
    folio_isolate_lru(folio)
    mmap_read_unlock(mm)

  do_move_pages_to_node()
    migrate_pages()
      try_to_migrate_one()
        spin_lock(ptl)
        set_pmd_migration_entry()
          pmdp_invalidate()     # PMD: _PAGE_INVALID | _PAGE_PTE | pfn
          set_pmd_at()          # PMD: migration swap entry (pmd_present=0)
        spin_unlock(ptl)
        [page copy phase]       # <--- RACE WINDOW -->

Thread B: munmap()
  mmap_write_downgrade(mm)
  unmap_vmas() -> zap_pmd_range()
    zap_huge_pmd()
      __pmd_trans_huge_lock()
        pmd_is_huge():          # !pmd_present && !pmd_none -> TRUE (swap entry)
        pmd_lock() -> 		# spin_lock(ptl), waits for Thread A to release ptl
      pmdp_huge_get_and_clear_full()
        VM_BUG_ON(!pmd_present(*pmdp))  # HITS!

[  287.738700][ T1867] ------------[ cut here ]------------
[  287.743843][ T1867] kernel BUG at arch/powerpc/mm/book3s64/pgtable.c:187!
cpu 0x0: Vector: 700 (Program Check) at [c00000044037f4f0]
    pc: c000000000094ca4: pmdp_huge_get_and_clear_full+0x6c/0x23c
    lr: c000000000645dec: zap_huge_pmd+0xb0/0x868
    sp: c00000044037f790
   msr: 800000000282b033
  current = 0xc0000004032c1a00
  paca    = 0xc000000004fe0000   irqmask: 0x03   irq_happened: 0x09
    pid   = 1867, comm = a.out
kernel BUG at :187!
Linux version 6.19.0-12136-g14360d4f917c-dirty (powerpc64le-linux-gnu-gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40) #27 SMP PREEMPT Sun Feb 22 10:38:56 IST 2026
enter ? for help
[link register   ] c000000000645dec zap_huge_pmd+0xb0/0x868
[c00000044037f790] c00000044037f7d0 (unreliable)
[c00000044037f7d0] c000000000645dcc zap_huge_pmd+0x90/0x868
[c00000044037f840] c0000000005724cc unmap_page_range+0x176c/0x1f40
[c00000044037fa00] c000000000572ea0 unmap_vmas+0xb0/0x1d8
[c00000044037fa90] c0000000005af254 unmap_region+0xb4/0x128
[c00000044037fb50] c0000000005af400 vms_complete_munmap_vmas+0x138/0x310
[c00000044037fbe0] c0000000005b0f1c do_vmi_align_munmap+0x1ec/0x238
[c00000044037fd30] c0000000005b3688 __vm_munmap+0x170/0x1f8
[c00000044037fdf0] c000000000587f74 sys_munmap+0x2c/0x40
[c00000044037fe10] c000000000032668 system_call_exception+0x128/0x350
[c00000044037fe50] c00000000000d05c system_call_vectored_common+0x15c/0x2ec
---- Exception: 3000 (System Call Vectored) at 0000000010064a2c
SP (7fff9b1ee9c0) is in userspace
0:mon> zh

commit a30b48bf1b ("mm/migrate_device: implement THP migration of zone device pages"),
enabled migration for device-private PMD entries. Hence this is one
other path where this warning could get trigger from.

 ------------[ cut here ]------------
 WARNING: arch/powerpc/mm/book3s64/hash_pgtable.c:199 at hash__pmd_hugepage_update+0x48/0x284, CPU#3: hmm-tests/1905
 Modules linked in: test_hmm
 CPU: 3 UID: 0 PID: 1905 Comm: hmm-tests Tainted: G    B   W    L   N  7.0.0-rc1-01438-g7e2f0ee7581c #21 PREEMPT
 Tainted: [B]=BAD_PAGE, [W]=WARN, [L]=SOFTLOCKUP, [N]=TEST
 Hardware name: IBM pSeries (emulated by qemu) POWER10 (architected) 0x801200 0xf000006 of:SLOF,git-ee03ae pSeries
 NIP [c000000000096b70] hash__pmd_hugepage_update+0x48/0x284
 LR [c000000000096e7c] hash__pmdp_huge_get_and_clear+0xd0/0xd4
 Call Trace:
 [c000000604707670] [c000000004e102b8] 0xc000000004e102b8 (unreliable)
 [c000000604707700] [c00000000064ec3c] set_pmd_migration_entry+0x414/0x498
 [c000000604707760] [c00000000063e5a4] migrate_vma_collect_pmd+0x12e8/0x16c4
 [c000000604707890] [c00000000059282c] walk_pgd_range+0x7fc/0xd2c
 [c000000604707990] [c000000000592e40] __walk_page_range+0xe4/0x2ac
 [c000000604707a10] [c000000000593534] walk_page_range_mm_unsafe+0x204/0x2a4
 [c000000604707ab0] [c00000000063af10] migrate_vma_setup+0x1dc/0x2e8
 [c000000604707b10] [c008000006a21838] dmirror_migrate_to_system.constprop.0+0x210/0x4b0 [test_hmm]
 [c000000604707c30] [c008000006a245b0] dmirror_fops_unlocked_ioctl+0x454/0xa5c [test_hmm]
 [c000000604707d20] [c0000000006aab84] sys_ioctl+0x4ec/0x1178
 [c000000604707e10] [c0000000000326a8] system_call_exception+0x128/0x350
 [c000000604707e50] [c00000000000d05c] system_call_vectored_common+0x15c/0x2ec
 ---- interrupt: 3000 at 0x7fffbe44f50c

Fixes: 75358ea359 ("powerpc/mm/book3s64: Fix MADV_DONTNEED and parallel page fault race")
Fixes: a30b48bf1b ("mm/migrate_device: implement THP migration of zone device pages")
Reported-by: Pavithra Prakash <pavrampu@linux.vnet.ibm.com>
Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/9437e5ef28d1e2f5cbdd7f8286350ce93c1d43c5.1773078178.git.ritesh.list@gmail.com
2026-03-17 13:56:37 +05:30
2026-01-26 19:07:09 -08:00
2022-09-28 09:02:20 +02:00
2025-02-19 14:53:27 -07:00
2026-03-15 13:52:05 -07:00

Linux kernel
============

The Linux kernel is the core of any Linux operating system. It manages hardware,
system resources, and provides the fundamental services for all other software.

Quick Start
-----------

* Report a bug: See Documentation/admin-guide/reporting-issues.rst
* Get the latest kernel: https://kernel.org
* Build the kernel: See Documentation/admin-guide/quickly-build-trimmed-linux.rst
* Join the community: https://lore.kernel.org/

Essential Documentation
-----------------------

All users should be familiar with:

* Building requirements: Documentation/process/changes.rst
* Code of Conduct: Documentation/process/code-of-conduct.rst
* License: See COPYING

Documentation can be built with make htmldocs or viewed online at:
https://www.kernel.org/doc/html/latest/


Who Are You?
============

Find your role below:

* New Kernel Developer - Getting started with kernel development
* Academic Researcher - Studying kernel internals and architecture
* Security Expert - Hardening and vulnerability analysis
* Backport/Maintenance Engineer - Maintaining stable kernels
* System Administrator - Configuring and troubleshooting
* Maintainer - Leading subsystems and reviewing patches
* Hardware Vendor - Writing drivers for new hardware
* Distribution Maintainer - Packaging kernels for distros
* AI Coding Assistant - LLMs and AI-powered development tools


For Specific Users
==================

New Kernel Developer
--------------------

Welcome! Start your kernel development journey here:

* Getting Started: Documentation/process/development-process.rst
* Your First Patch: Documentation/process/submitting-patches.rst
* Coding Style: Documentation/process/coding-style.rst
* Build System: Documentation/kbuild/index.rst
* Development Tools: Documentation/dev-tools/index.rst
* Kernel Hacking Guide: Documentation/kernel-hacking/hacking.rst
* Core APIs: Documentation/core-api/index.rst

Academic Researcher
-------------------

Explore the kernel's architecture and internals:

* Researcher Guidelines: Documentation/process/researcher-guidelines.rst
* Memory Management: Documentation/mm/index.rst
* Scheduler: Documentation/scheduler/index.rst
* Networking Stack: Documentation/networking/index.rst
* Filesystems: Documentation/filesystems/index.rst
* RCU (Read-Copy Update): Documentation/RCU/index.rst
* Locking Primitives: Documentation/locking/index.rst
* Power Management: Documentation/power/index.rst

Security Expert
---------------

Security documentation and hardening guides:

* Security Documentation: Documentation/security/index.rst
* LSM Development: Documentation/security/lsm-development.rst
* Self Protection: Documentation/security/self-protection.rst
* Reporting Vulnerabilities: Documentation/process/security-bugs.rst
* CVE Procedures: Documentation/process/cve.rst
* Embargoed Hardware Issues: Documentation/process/embargoed-hardware-issues.rst
* Security Features: Documentation/userspace-api/seccomp_filter.rst

Backport/Maintenance Engineer
-----------------------------

Maintain and stabilize kernel versions:

* Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst
* Backporting Guide: Documentation/process/backporting.rst
* Applying Patches: Documentation/process/applying-patches.rst
* Subsystem Profile: Documentation/maintainer/maintainer-entry-profile.rst
* Git for Maintainers: Documentation/maintainer/configure-git.rst

System Administrator
--------------------

Configure, tune, and troubleshoot Linux systems:

* Admin Guide: Documentation/admin-guide/index.rst
* Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst
* Sysctl Tuning: Documentation/admin-guide/sysctl/index.rst
* Tracing/Debugging: Documentation/trace/index.rst
* Performance Security: Documentation/admin-guide/perf-security.rst
* Hardware Monitoring: Documentation/hwmon/index.rst

Maintainer
----------

Lead kernel subsystems and manage contributions:

* Maintainer Handbook: Documentation/maintainer/index.rst
* Pull Requests: Documentation/maintainer/pull-requests.rst
* Managing Patches: Documentation/maintainer/modifying-patches.rst
* Rebasing and Merging: Documentation/maintainer/rebasing-and-merging.rst
* Development Process: Documentation/process/maintainer-handbooks.rst
* Maintainer Entry Profile: Documentation/maintainer/maintainer-entry-profile.rst
* Git Configuration: Documentation/maintainer/configure-git.rst

Hardware Vendor
---------------

Write drivers and support new hardware:

* Driver API Guide: Documentation/driver-api/index.rst
* Driver Model: Documentation/driver-api/driver-model/driver.rst
* Device Drivers: Documentation/driver-api/infrastructure.rst
* Bus Types: Documentation/driver-api/driver-model/bus.rst
* Device Tree Bindings: Documentation/devicetree/bindings/
* Power Management: Documentation/driver-api/pm/index.rst
* DMA API: Documentation/core-api/dma-api.rst

Distribution Maintainer
-----------------------

Package and distribute the kernel:

* Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst
* ABI Documentation: Documentation/ABI/README
* Kernel Configuration: Documentation/kbuild/kconfig.rst
* Module Signing: Documentation/admin-guide/module-signing.rst
* Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst
* Tainted Kernels: Documentation/admin-guide/tainted-kernels.rst

AI Coding Assistant
-------------------

CRITICAL: If you are an LLM or AI-powered coding assistant, you MUST read and
follow the AI coding assistants documentation before contributing to the Linux
kernel:

* Documentation/process/coding-assistants.rst

This documentation contains essential requirements about licensing, attribution,
and the Developer Certificate of Origin that all AI tools must comply with.


Communication and Support
=========================

* Mailing Lists: https://lore.kernel.org/
* IRC: #kernelnewbies on irc.oftc.net
* Bugzilla: https://bugzilla.kernel.org/
* MAINTAINERS file: Lists subsystem maintainers and mailing lists
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