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David Hildenbrand 53cdc1cb29 drivers/base/memory.c: indicate all memory blocks as removable
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 5c755e9fd8 ("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@redhat.com

Also, this patch probably fixes a crash reported by Steve.
http://lkml.kernel.org/r/CAPcyv4jpdaNvJ67SkjyUJLBnBnXXQv686BiVW042g03FUmWLXw@mail.gmail.com

Reported-by: "Scargall, Steve" <steve.scargall@intel.com>
Suggested-by: Michal Hocko <mhocko@kernel.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Nathan Fontenot <ndfont@gmail.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: Badari Pulavarty <pbadari@us.ibm.com>
Cc: Robert Jennings <rcj@linux.vnet.ibm.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Karel Zak <kzak@redhat.com>
Cc: <stable@vger.kernel.org>
Link: http://lkml.kernel.org/r/20200128093542.6908-1-david@redhat.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2020-03-29 09:47:05 -07:00
arch Merge branch 'parisc-5.6-2' of git://git.kernel.org/pub/scm/linux/kernel/git/deller/parisc-linux 2020-03-27 14:34:03 -07:00
block block: Fix partition support for host aware zoned block devices 2020-03-12 07:54:39 -06:00
certs certs: Add wrapper function to check blacklisted binary hash 2019-11-12 12:25:50 +11:00
crypto Merge branch 'next-integrity' of git://git.kernel.org/pub/scm/linux/kernel/git/zohar/linux-integrity 2020-02-20 15:15:16 -08:00
Documentation ARM: DT and driver fixes for v5.6 2020-03-27 13:52:32 -07:00
drivers drivers/base/memory.c: indicate all memory blocks as removable 2020-03-29 09:47:05 -07:00
fs afs: Fix unpinned address list during probing 2020-03-26 16:04:29 -07:00
include A handful of clk driver fixes. Mostly they're around the i.MX drivers 2020-03-27 09:33:48 -07:00
init int128: fix __uint128_t compiler test in Kconfig 2020-03-12 07:43:03 +09:00
ipc Revert "ipc,sem: remove uneeded sem_undo_list lock usage in exit_sem()" 2020-02-21 11:22:15 -08:00
kernel Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net 2020-03-25 13:58:05 -07:00
lib Merge branch 'linus' of git://git.kernel.org/pub/scm/linux/kernel/git/herbert/crypto-2.6 2020-03-23 15:55:21 -07:00
LICENSES LICENSES: Rename other to deprecated 2019-05-03 06:34:32 -06:00
mm mm/swapfile.c: move inode_lock out of claim_swapfile 2020-03-29 09:47:05 -07:00
net A patch for a rather old regression in fullness handling and two memory 2020-03-26 15:44:41 -07:00
samples Kbuild updates for v5.6 (2nd) 2020-02-09 16:05:50 -08:00
scripts Devicetree fix for 5.6, take 4: 2020-03-27 11:02:52 -07:00
security Merge branch 'next-integrity' of git://git.kernel.org/pub/scm/linux/kernel/git/zohar/linux-integrity 2020-02-20 15:15:16 -08:00
sound ALSA: hda/realtek - Enable the headset of Acer N50-600 with ALC662 2020-03-17 18:06:19 +01:00
tools Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net 2020-03-25 13:58:05 -07:00
usr initramfs: restore default compression behavior 2020-03-17 09:50:37 +09:00
virt KVM/arm fixes for 5.6, take #1 2020-02-28 11:50:06 +01:00
.clang-format clang-format: Update with the latest for_each macro list 2020-03-06 21:50:05 +01:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.get_maintainer.ignore Opt out of scripts/get_maintainer.pl 2019-05-16 10:53:40 -07:00
.gitattributes .gitattributes: use 'dts' diff driver for dts files 2019-12-04 19:44:11 -08:00
.gitignore selftest/lkdtm: Use local .gitignore 2020-03-02 08:39:39 -07:00
.mailmap mailmap: Update email address 2020-03-05 11:03:09 -08:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: Hand MIPS over to Thomas 2020-02-24 22:43:18 -08:00
Kbuild kbuild: rename hostprogs-y/always to hostprogs/always-y 2020-02-04 01:53:07 +09:00
Kconfig docs: kbuild: convert docs to ReST and rename to *.rst 2019-06-14 14:21:21 -06:00
MAINTAINERS MAINTAINERS: fix bad file pattern 2020-03-26 15:12:19 -07:00
Makefile Linux 5.6-rc7 2020-03-22 18:31:56 -07:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -06:00

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

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.