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linux/Documentation/translations/zh_CN/mm/vmalloced-kernel-stacks.rst
Linus Torvalds 6614a3c316 - The usual batches of cleanups from Baoquan He, Muchun Song, Miaohe 
Lin, Yang Shi, Anshuman Khandual and Mike Rapoport
 
 - Some kmemleak fixes from Patrick Wang and Waiman Long
 
 - DAMON updates from SeongJae Park
 
 - memcg debug/visibility work from Roman Gushchin
 
 - vmalloc speedup from Uladzislau Rezki
 
 - more folio conversion work from Matthew Wilcox
 
 - enhancements for coherent device memory mapping from Alex Sierra
 
 - addition of shared pages tracking and CoW support for fsdax, from
   Shiyang Ruan
 
 - hugetlb optimizations from Mike Kravetz
 
 - Mel Gorman has contributed some pagealloc changes to improve latency
   and realtime behaviour.
 
 - mprotect soft-dirty checking has been improved by Peter Xu
 
 - Many other singleton patches all over the place
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Merge tag 'mm-stable-2022-08-03' of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

Pull MM updates from Andrew Morton:
 "Most of the MM queue. A few things are still pending.

  Liam's maple tree rework didn't make it. This has resulted in a few
  other minor patch series being held over for next time.

  Multi-gen LRU still isn't merged as we were waiting for mapletree to
  stabilize. The current plan is to merge MGLRU into -mm soon and to
  later reintroduce mapletree, with a view to hopefully getting both
  into 6.1-rc1.

  Summary:

   - The usual batches of cleanups from Baoquan He, Muchun Song, Miaohe
     Lin, Yang Shi, Anshuman Khandual and Mike Rapoport

   - Some kmemleak fixes from Patrick Wang and Waiman Long

   - DAMON updates from SeongJae Park

   - memcg debug/visibility work from Roman Gushchin

   - vmalloc speedup from Uladzislau Rezki

   - more folio conversion work from Matthew Wilcox

   - enhancements for coherent device memory mapping from Alex Sierra

   - addition of shared pages tracking and CoW support for fsdax, from
     Shiyang Ruan

   - hugetlb optimizations from Mike Kravetz

   - Mel Gorman has contributed some pagealloc changes to improve
     latency and realtime behaviour.

   - mprotect soft-dirty checking has been improved by Peter Xu

   - Many other singleton patches all over the place"

 [ XFS merge from hell as per Darrick Wong in

   https://lore.kernel.org/all/YshKnxb4VwXycPO8@magnolia/ ]

* tag 'mm-stable-2022-08-03' of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm: (282 commits)
  tools/testing/selftests/vm/hmm-tests.c: fix build
  mm: Kconfig: fix typo
  mm: memory-failure: convert to pr_fmt()
  mm: use is_zone_movable_page() helper
  hugetlbfs: fix inaccurate comment in hugetlbfs_statfs()
  hugetlbfs: cleanup some comments in inode.c
  hugetlbfs: remove unneeded header file
  hugetlbfs: remove unneeded hugetlbfs_ops forward declaration
  hugetlbfs: use helper macro SZ_1{K,M}
  mm: cleanup is_highmem()
  mm/hmm: add a test for cross device private faults
  selftests: add soft-dirty into run_vmtests.sh
  selftests: soft-dirty: add test for mprotect
  mm/mprotect: fix soft-dirty check in can_change_pte_writable()
  mm: memcontrol: fix potential oom_lock recursion deadlock
  mm/gup.c: fix formatting in check_and_migrate_movable_page()
  xfs: fail dax mount if reflink is enabled on a partition
  mm/memcontrol.c: remove the redundant updating of stats_flush_threshold
  userfaultfd: don't fail on unrecognized features
  hugetlb_cgroup: fix wrong hugetlb cgroup numa stat
  ...
2022-08-05 16:32:45 -07:00

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.. SPDX-License-Identifier: GPL-2.0
.. include:: ../disclaimer-zh_CN.rst
:Original: Documentation/mm/vmalloced-kernel-stacks.rst
:翻译:
司延腾 Yanteng Si <siyanteng@loongson.cn>
:校译:
====================
支持虚拟映射的内核栈
====================
:作者: Shuah Khan <skhan@linuxfoundation.org>
.. contents:: :local:
概览
----
这是介绍 `虚拟映射内核栈功能 <https://lwn.net/Articles/694348/>` 的代码
和原始补丁系列的信息汇总。
简介
----
内核堆栈溢出通常难以调试，并使内核容易被（恶意）利用。问题可能在稍后的时间出现，使其难以
隔离和究其根本原因。
带有保护页的虚拟映射内核堆栈如果溢出，会被立即捕获，而不会放任其导致难以诊断的损
坏。
HAVE_ARCH_VMAP_STACK和VMAP_STACK配置选项能够支持带有保护页的虚拟映射堆栈。
当堆栈溢出时，这个特性会引发可靠的异常。溢出后堆栈跟踪的可用性以及对溢出本身的
响应取决于架构。
.. note::
截至本文撰写时， arm64, powerpc, riscv, s390, um, 和 x86 支持VMAP_STACK。
HAVE_ARCH_VMAP_STACK
--------------------
能够支持虚拟映射内核栈的架构应该启用这个bool配置选项。要求是:
- vmalloc空间必须大到足以容纳许多内核堆栈。这可能排除了许多32位架构。
- vmalloc空间的堆栈需要可靠地工作。例如，如果vmap页表是按需创建的，当堆栈指向
具有未填充页表的虚拟地址时，这种机制需要工作，或者架构代码（switch_to()和
switch_mm()，很可能）需要确保堆栈的页表项在可能未填充的堆栈上运行之前已经填
充。
- 如果堆栈溢出到一个保护页，就应该发生一些合理的事情。“合理”的定义是灵活的，但
在没有记录任何东西的情况下立即重启是不友好的。
VMAP_STACK
----------
VMAP_STACK bool配置选项在启用时分配虚拟映射的任务栈。这个选项依赖于
HAVE_ARCH_VMAP_STACK。
- 如果你想使用带有保护页的虚拟映射的内核堆栈，请启用该选项。这将导致内核栈溢出
被立即捕获，而不是难以诊断的损坏。
.. note::
使用KASAN的这个功能需要架构支持用真实的影子内存来支持虚拟映射，并且
必须启用KASAN_VMALLOC。
.. note::
启用VMAP_STACK时，无法在堆栈分配的数据上运行DMA。
内核配置选项和依赖性不断变化。请参考最新的代码库:
`Kconfig <https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/Kconfig>`
分配方法
--------
当一个新的内核线程被创建时，线程堆栈是由页级分配器分配的虚拟连续的内存页组成。这
些页面被映射到有PAGE_KERNEL保护的连续的内核虚拟空间。
alloc_thread_stack_node()调用__vmalloc_node_range()来分配带有PAGE_KERNEL
保护的栈。
- 分配的堆栈被缓存起来，以后会被新的线程重用，所以在分配/释放堆栈给任务时，要手动
进行memcg核算。因此，__vmalloc_node_range被调用时没有__GFP_ACCOUNT。
- vm_struct被缓存起来，以便能够找到在中断上下文中启动的空闲线程。 free_thread_stack()
可以在中断上下文中调用。
- 在arm64上，所有VMAP的堆栈都需要有相同的对齐方式，以确保VMAP的堆栈溢出检测正常
工作。架构特定的vmap堆栈分配器照顾到了这个细节。
- 这并不涉及中断堆栈--参考原始补丁
线程栈分配是由clone()、fork()、vfork()、kernel_thread()通过kernel_clone()
启动的。留点提示在这，以便搜索代码库，了解线程栈何时以及如何分配。
大量的代码是在:
`kernel/fork.c <https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/fork.c>`.
task_struct中的stack_vm_area指针可以跟踪虚拟分配的堆栈，一个非空的stack_vm_area
指针可以表明虚拟映射的内核堆栈已经启用。
::
struct vm_struct *stack_vm_area;
堆栈溢出处理
------------
前守护页和后守护页有助于检测堆栈溢出。当堆栈溢出到守护页时，处理程序必须小心不要再
次溢出堆栈。当处理程序被调用时，很可能只留下很少的堆栈空间。
在x86上，这是通过处理表明内核堆栈溢出的双异常堆栈的缺页异常来实现的。
用守护页测试VMAP分配
--------------------
我们如何确保VMAP_STACK在分配时确实有前守护页和后守护页的保护？下面的 lkdtm 测试
可以帮助检测任何回归。
::
void lkdtm_STACK_GUARD_PAGE_LEADING()
void lkdtm_STACK_GUARD_PAGE_TRAILING()
结论
----
- vmalloced堆栈的percpu缓存似乎比高阶堆栈分配要快一些，至少在缓存命中时是这样。
- THREAD_INFO_IN_TASK完全摆脱了arch-specific thread_info，并简单地将
thread_info（仅包含标志）和'int cpu'嵌入task_struct中。
- 一旦任务死亡，线程栈就可以被释放（无需等待RCU），然后，如果使用vmapped栈，就
可以将整个栈缓存起来，以便在同一cpu上重复使用。
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