Re: [PATCH v1] drivers/base/memory.c: indicate all memory blocks as removable

[Dan Williams]

On Tue, Jan 28, 2020 at 1:44 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> We see multiple issues with the implementation/interface to compute
> whether a memory block can be offlined (exposed via
> /sys/devices/system/memory/memoryX/removable) and would like to simplify
> it (remove the implementation).
>
> 1. It runs basically lockless. While this might be good for performance,
>    we see possible races with memory offlining that will require at least
>    some sort of locking to fix.
>
> 2. Nowadays, more false positives are possible. No arch-specific checks
>    are performed that validate if memory offlining will not be denied
>    right away (and such check will require locking). For example, arm64
>    won't allow to offline any memory block that was added during boot -
>    which will imply a very high error rate. Other archs have other
>    constraints.
>
> 3. The interface is inherently racy. E.g., if a memory block is
>    detected to be removable (and was not a false positive at that time),
>    there is still no guarantee that offlining will actually succeed. So
>    any caller already has to deal with false positives.
>
> 4. It is unclear which performance benefit this interface actually
>    provides. The introducing commit 5c755e9fd813 ("memory-hotplug: add
>    sysfs removable attribute for hotplug memory remove") mentioned
>         "A user-level agent must be able to identify which sections of
>          memory are likely to be removable before attempting the
>          potentially expensive operation."
>    However, no actual performance comparison was included.
>
> Known users:
> - lsmem: Will group memory blocks based on the "removable" property. [1]
> - chmem: Indirect user. It has a RANGE mode where one can specify
>          removable ranges identified via lsmem to be offlined. However, it
>          also has a "SIZE" mode, which allows a sysadmin to skip the manual
>          "identify removable blocks" step. [2]
> - powerpc-utils: Uses the "removable" attribute to skip some memory
>                  blocks right away when trying to find some to
>                  offline+remove. However, with ballooning enabled, it
>                  already skips this information completely (because it
>                  once resulted in many false negatives). Therefore, the
>                  implementation can deal with false positives properly
>                  already. [3]
>
> According to Nathan Fontenot, DLPAR on powerpc is nowadays no longer
> driven from userspace via the drmgr command (powerpc-utils). Nowadays
> it's managed in the kernel - including onlining/offlining of memory
> blocks - triggered by drmgr writing to /sys/kernel/dlpar. So the
> affected legacy userspace handling is only active on old kernels. Only very
> old versions of drmgr on a new kernel (unlikely) might execute slower -
> totally acceptable.
>
> With CONFIG_MEMORY_HOTREMOVE, always indicating "removable" should not
> break any user space tool. We implement a very bad heuristic now.  Without
> CONFIG_MEMORY_HOTREMOVE we cannot offline anything, so report
> "not removable" as before.
>
> Original discussion can be found in [4] ("[PATCH RFC v1] mm:
> is_mem_section_removable() overhaul").
>
> Other users of is_mem_section_removable() will be removed next, so that
> we can remove is_mem_section_removable() completely.
>
> [1] http://man7.org/linux/man-pages/man1/lsmem.1.html
> [2] http://man7.org/linux/man-pages/man8/chmem.8.html
> [3] https://github.com/ibm-power-utilities/powerpc-utils
> [4] https://lkml.kernel.org/r/20200117105759.27905-1-david@xxxxxxxxxx
>
> Suggested-by: Michal Hocko <mhocko@xxxxxxxxxx>
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>

David, Andrew,

I'd like to recommend this patch for -stable as it likely (test
underway) solves this crash report from Steve:

[  148.796036] page dumped because: VM_BUG_ON_PAGE(PagePoisoned(p))
[  148.796074] ------------[ cut here ]------------
[  148.796098] kernel BUG at include/linux/mm.h:1087!
[  148.796126] invalid opcode: 0000 [#1] SMP NOPTI
[  148.796146] CPU: 63 PID: 5471 Comm: lsmem Not tainted 5.5.10-200.fc31.x8=
6_64+debug #1
[  148.796173] Hardware name: Intel Corporation S2600WFD/S2600WFD, BIOS SE5=
C620.86B.02.01.0010.010620200716 01/06/2020
[  148.796212] RIP: 0010:is_mem_section_removable+0x1a4/0x1b0
[  148.796561] Call Trace:
[  148.796591]  removable_show+0x6e/0xa0
[  148.796608]  dev_attr_show+0x19/0x40
[  148.796625]  sysfs_kf_seq_show+0xa9/0x100
[  148.796640]  seq_read+0xd5/0x450
[  148.796657]  vfs_read+0xc5/0x180
[  148.796672]  ksys_read+0x68/0xe0
[  148.796688]  do_syscall_64+0x5c/0xa0
[  148.796704]  entry_SYSCALL_64_after_hwframe+0x49/0xbe
[  148.796721] RIP: 0033:0x7f3ab1646412

...on a non-debug kernel it just crashes.

In this case lsmem is failing when reading memory96:

openat(3, "memory96/removable", O_RDONLY|O_CLOEXEC) = 4
fcntl(4, F_GETFL)                       = 0x8000 (flags O_RDONLY|O_LARGEFILE)
fstat(4, {st_mode=S_IFREG|0444, st_size=4096, ...}) = 0
read(4,  <unfinished ...>)              = ?
+++ killed by SIGSEGV +++
Segmentation fault (core dumped)

...which is phys_index 0x60 => memory address 0x3000000000

On this platform that lands us here:

100000000-303fffffff : System RAM
  291f000000-291fe00f70 : Kernel code
  2920000000-292051efff : Kernel rodata
  2920600000-292093b0bf : Kernel data
  29214f3000-2922dfffff : Kernel bss
3040000000-305fffffff : Reserved
3060000000-1aa5fffffff : Persistent Memory

...where the last memory block of System RAM is shared with persistent
memory. I.e. the block is only partially online which means that
page_to_nid() in is_mem_section_removable() will assert or crash for
some of the offline pages in that block.