linux-stable.git/mm, branch v4.4.201

mm/filemap.c: don't initiate writeback if mapping has no dirty pages

2019-11-12T18:13:30+00:00

commit c3aab9a0bd91b696a852169479b7db1ece6cbf8c upstream.

Functions like filemap_write_and_wait_range() should do nothing if inode
has no dirty pages or pages currently under writeback.  But they anyway
construct struct writeback_control and this does some atomic operations if
CONFIG_CGROUP_WRITEBACK=y - on fast path it locks inode->i_lock and
updates state of writeback ownership, on slow path might be more work.
Current this path is safely avoided only when inode mapping has no pages.

For example generic_file_read_iter() calls filemap_write_and_wait_range()
at each O_DIRECT read - pretty hot path.

This patch skips starting new writeback if mapping has no dirty tags set.
If writeback is already in progress filemap_write_and_wait_range() will
wait for it.

Link: http://lkml.kernel.org/r/156378816804.1087.8607636317907921438.stgit@buzz
Signed-off-by: Konstantin Khlebnikov 
Reviewed-by: Jan Kara 
Cc: Tejun Heo 
Cc: Jens Axboe 
Cc: Johannes Weiner 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds 
Signed-off-by: Greg Kroah-Hartman

mm, vmstat: hide /proc/pagetypeinfo from normal users

2019-11-12T18:13:18+00:00

commit abaed0112c1db08be15a784a2c5c8a8b3063cdd3 upstream.

/proc/pagetypeinfo is a debugging tool to examine internal page
allocator state wrt to fragmentation.  It is not very useful for any
other use so normal users really do not need to read this file.

Waiman Long has noticed that reading this file can have negative side
effects because zone->lock is necessary for gathering data and that a)
interferes with the page allocator and its users and b) can lead to hard
lockups on large machines which have very long free_list.

Reduce both issues by simply not exporting the file to regular users.

Link: http://lkml.kernel.org/r/20191025072610.18526-2-mhocko@kernel.org
Fixes: 467c996c1e19 ("Print out statistics in relation to fragmentation avoidance to /proc/pagetypeinfo")
Signed-off-by: Michal Hocko 
Reported-by: Waiman Long 
Acked-by: Mel Gorman 
Acked-by: Vlastimil Babka 
Acked-by: Waiman Long 
Acked-by: Rafael Aquini 
Acked-by: David Rientjes 
Reviewed-by: Andrew Morton 
Cc: David Hildenbrand 
Cc: Johannes Weiner 
Cc: Roman Gushchin 
Cc: Konstantin Khlebnikov 
Cc: Jann Horn 
Cc: Song Liu 
Cc: Greg Kroah-Hartman 
Cc: 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds 
Signed-off-by: Greg Kroah-Hartman

mm/slub: fix a deadlock in show_slab_objects()

2019-10-29T08:13:28+00:00

commit e4f8e513c3d353c134ad4eef9fd0bba12406c7c8 upstream.

A long time ago we fixed a similar deadlock in show_slab_objects() [1].
However, it is apparently due to the commits like 01fb58bcba63 ("slab:
remove synchronous synchronize_sched() from memcg cache deactivation
path") and 03afc0e25f7f ("slab: get_online_mems for
kmem_cache_{create,destroy,shrink}"), this kind of deadlock is back by
just reading files in /sys/kernel/slab which will generate a lockdep
splat below.

Since the "mem_hotplug_lock" here is only to obtain a stable online node
mask while racing with NUMA node hotplug, in the worst case, the results
may me miscalculated while doing NUMA node hotplug, but they shall be
corrected by later reads of the same files.

  WARNING: possible circular locking dependency detected
  ------------------------------------------------------
  cat/5224 is trying to acquire lock:
  ffff900012ac3120 (mem_hotplug_lock.rw_sem){++++}, at:
  show_slab_objects+0x94/0x3a8

  but task is already holding lock:
  b8ff009693eee398 (kn->count#45){++++}, at: kernfs_seq_start+0x44/0xf0

  which lock already depends on the new lock.

  the existing dependency chain (in reverse order) is:

  -> #2 (kn->count#45){++++}:
         lock_acquire+0x31c/0x360
         __kernfs_remove+0x290/0x490
         kernfs_remove+0x30/0x44
         sysfs_remove_dir+0x70/0x88
         kobject_del+0x50/0xb0
         sysfs_slab_unlink+0x2c/0x38
         shutdown_cache+0xa0/0xf0
         kmemcg_cache_shutdown_fn+0x1c/0x34
         kmemcg_workfn+0x44/0x64
         process_one_work+0x4f4/0x950
         worker_thread+0x390/0x4bc
         kthread+0x1cc/0x1e8
         ret_from_fork+0x10/0x18

  -> #1 (slab_mutex){+.+.}:
         lock_acquire+0x31c/0x360
         __mutex_lock_common+0x16c/0xf78
         mutex_lock_nested+0x40/0x50
         memcg_create_kmem_cache+0x38/0x16c
         memcg_kmem_cache_create_func+0x3c/0x70
         process_one_work+0x4f4/0x950
         worker_thread+0x390/0x4bc
         kthread+0x1cc/0x1e8
         ret_from_fork+0x10/0x18

  -> #0 (mem_hotplug_lock.rw_sem){++++}:
         validate_chain+0xd10/0x2bcc
         __lock_acquire+0x7f4/0xb8c
         lock_acquire+0x31c/0x360
         get_online_mems+0x54/0x150
         show_slab_objects+0x94/0x3a8
         total_objects_show+0x28/0x34
         slab_attr_show+0x38/0x54
         sysfs_kf_seq_show+0x198/0x2d4
         kernfs_seq_show+0xa4/0xcc
         seq_read+0x30c/0x8a8
         kernfs_fop_read+0xa8/0x314
         __vfs_read+0x88/0x20c
         vfs_read+0xd8/0x10c
         ksys_read+0xb0/0x120
         __arm64_sys_read+0x54/0x88
         el0_svc_handler+0x170/0x240
         el0_svc+0x8/0xc

  other info that might help us debug this:

  Chain exists of:
    mem_hotplug_lock.rw_sem --> slab_mutex --> kn->count#45

   Possible unsafe locking scenario:

         CPU0                    CPU1
         ----                    ----
    lock(kn->count#45);
                                 lock(slab_mutex);
                                 lock(kn->count#45);
    lock(mem_hotplug_lock.rw_sem);

   *** DEADLOCK ***

  3 locks held by cat/5224:
   #0: 9eff00095b14b2a0 (&p->lock){+.+.}, at: seq_read+0x4c/0x8a8
   #1: 0eff008997041480 (&of->mutex){+.+.}, at: kernfs_seq_start+0x34/0xf0
   #2: b8ff009693eee398 (kn->count#45){++++}, at:
  kernfs_seq_start+0x44/0xf0

  stack backtrace:
  Call trace:
   dump_backtrace+0x0/0x248
   show_stack+0x20/0x2c
   dump_stack+0xd0/0x140
   print_circular_bug+0x368/0x380
   check_noncircular+0x248/0x250
   validate_chain+0xd10/0x2bcc
   __lock_acquire+0x7f4/0xb8c
   lock_acquire+0x31c/0x360
   get_online_mems+0x54/0x150
   show_slab_objects+0x94/0x3a8
   total_objects_show+0x28/0x34
   slab_attr_show+0x38/0x54
   sysfs_kf_seq_show+0x198/0x2d4
   kernfs_seq_show+0xa4/0xcc
   seq_read+0x30c/0x8a8
   kernfs_fop_read+0xa8/0x314
   __vfs_read+0x88/0x20c
   vfs_read+0xd8/0x10c
   ksys_read+0xb0/0x120
   __arm64_sys_read+0x54/0x88
   el0_svc_handler+0x170/0x240
   el0_svc+0x8/0xc

I think it is important to mention that this doesn't expose the
show_slab_objects to use-after-free.  There is only a single path that
might really race here and that is the slab hotplug notifier callback
__kmem_cache_shrink (via slab_mem_going_offline_callback) but that path
doesn't really destroy kmem_cache_node data structures.

[1] http://lkml.iu.edu/hypermail/linux/kernel/1101.0/02850.html

[akpm@linux-foundation.org: add comment explaining why we don't need mem_hotplug_lock]
Link: http://lkml.kernel.org/r/1570192309-10132-1-git-send-email-cai@lca.pw
Fixes: 01fb58bcba63 ("slab: remove synchronous synchronize_sched() from memcg cache deactivation path")
Fixes: 03afc0e25f7f ("slab: get_online_mems for kmem_cache_{create,destroy,shrink}")
Signed-off-by: Qian Cai 
Acked-by: Michal Hocko 
Cc: Christoph Lameter 
Cc: Pekka Enberg 
Cc: David Rientjes 
Cc: Joonsoo Kim 
Cc: Tejun Heo 
Cc: Vladimir Davydov 
Cc: Roman Gushchin 
Cc: 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds 
Signed-off-by: Greg Kroah-Hartman

memfd: Fix locking when tagging pins

2019-10-29T08:13:23+00:00

The RCU lock is insufficient to protect the radix tree iteration as
a deletion from the tree can occur before we take the spinlock to
tag the entry.  In 4.19, this has manifested as a bug with the following
trace:

kernel BUG at lib/radix-tree.c:1429!
invalid opcode: 0000 [#1] SMP KASAN PTI
CPU: 7 PID: 6935 Comm: syz-executor.2 Not tainted 4.19.36 #25
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1ubuntu1 04/01/2014
RIP: 0010:radix_tree_tag_set+0x200/0x2f0 lib/radix-tree.c:1429
Code: 00 00 5b 5d 41 5c 41 5d 41 5e 41 5f c3 48 89 44 24 10 e8 a3 29 7e fe 48 8b 44 24 10 48 0f ab 03 e9 d2 fe ff ff e8 90 29 7e fe <0f> 0b 48 c7 c7 e0 5a 87 84 e8 f0 e7 08 ff 4c 89 ef e8 4a ff ac fe
RSP: 0018:ffff88837b13fb60 EFLAGS: 00010016
RAX: 0000000000040000 RBX: ffff8883c5515d58 RCX: ffffffff82cb2ef0
RDX: 0000000000000b72 RSI: ffffc90004cf2000 RDI: ffff8883c5515d98
RBP: ffff88837b13fb98 R08: ffffed106f627f7e R09: ffffed106f627f7e
R10: 0000000000000001 R11: ffffed106f627f7d R12: 0000000000000004
R13: ffffea000d7fea80 R14: 1ffff1106f627f6f R15: 0000000000000002
FS:  00007fa1b8df2700(0000) GS:ffff8883e2fc0000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fa1b8df1db8 CR3: 000000037d4d2001 CR4: 0000000000160ee0
Call Trace:
 memfd_tag_pins mm/memfd.c:51 [inline]
 memfd_wait_for_pins+0x2c5/0x12d0 mm/memfd.c:81
 memfd_add_seals mm/memfd.c:215 [inline]
 memfd_fcntl+0x33d/0x4a0 mm/memfd.c:247
 do_fcntl+0x589/0xeb0 fs/fcntl.c:421
 __do_sys_fcntl fs/fcntl.c:463 [inline]
 __se_sys_fcntl fs/fcntl.c:448 [inline]
 __x64_sys_fcntl+0x12d/0x180 fs/fcntl.c:448
 do_syscall_64+0xc8/0x580 arch/x86/entry/common.c:293

The problem does not occur in mainline due to the XArray rewrite which
changed the locking to exclude modification of the tree during iteration.
At the time, nobody realised this was a bugfix.  Backport the locking
changes to stable.

Cc: stable@vger.kernel.org
Reported-by: zhong jiang 
Signed-off-by: Matthew Wilcox (Oracle) 
Signed-off-by: Sasha Levin

mm/memcontrol.c: fix use after free in mem_cgroup_iter()

2019-08-25T08:52:56+00:00

commit 54a83d6bcbf8f4700013766b974bf9190d40b689 upstream.

This patch is sent to report an use after free in mem_cgroup_iter()
after merging commit be2657752e9e ("mm: memcg: fix use after free in
mem_cgroup_iter()").

I work with android kernel tree (4.9 & 4.14), and commit be2657752e9e
("mm: memcg: fix use after free in mem_cgroup_iter()") has been merged
to the trees.  However, I can still observe use after free issues
addressed in the commit be2657752e9e.  (on low-end devices, a few times
this month)

backtrace:
        css_tryget <- crash here
        mem_cgroup_iter
        shrink_node
        shrink_zones
        do_try_to_free_pages
        try_to_free_pages
        __perform_reclaim
        __alloc_pages_direct_reclaim
        __alloc_pages_slowpath
        __alloc_pages_nodemask

To debug, I poisoned mem_cgroup before freeing it:

  static void __mem_cgroup_free(struct mem_cgroup *memcg)
        for_each_node(node)
        free_mem_cgroup_per_node_info(memcg, node);
        free_percpu(memcg->stat);
  +     /* poison memcg before freeing it */
  +     memset(memcg, 0x78, sizeof(struct mem_cgroup));
        kfree(memcg);
  }

The coredump shows the position=0xdbbc2a00 is freed.

  (gdb) p/x ((struct mem_cgroup_per_node *)0xe5009e00)->iter[8]
  $13 = {position = 0xdbbc2a00, generation = 0x2efd}

  0xdbbc2a00:     0xdbbc2e00      0x00000000      0xdbbc2800      0x00000100
  0xdbbc2a10:     0x00000200      0x78787878      0x00026218      0x00000000
  0xdbbc2a20:     0xdcad6000      0x00000001      0x78787800      0x00000000
  0xdbbc2a30:     0x78780000      0x00000000      0x0068fb84      0x78787878
  0xdbbc2a40:     0x78787878      0x78787878      0x78787878      0xe3fa5cc0
  0xdbbc2a50:     0x78787878      0x78787878      0x00000000      0x00000000
  0xdbbc2a60:     0x00000000      0x00000000      0x00000000      0x00000000
  0xdbbc2a70:     0x00000000      0x00000000      0x00000000      0x00000000
  0xdbbc2a80:     0x00000000      0x00000000      0x00000000      0x00000000
  0xdbbc2a90:     0x00000001      0x00000000      0x00000000      0x00100000
  0xdbbc2aa0:     0x00000001      0xdbbc2ac8      0x00000000      0x00000000
  0xdbbc2ab0:     0x00000000      0x00000000      0x00000000      0x00000000
  0xdbbc2ac0:     0x00000000      0x00000000      0xe5b02618      0x00001000
  0xdbbc2ad0:     0x00000000      0x78787878      0x78787878      0x78787878
  0xdbbc2ae0:     0x78787878      0x78787878      0x78787878      0x78787878
  0xdbbc2af0:     0x78787878      0x78787878      0x78787878      0x78787878
  0xdbbc2b00:     0x78787878      0x78787878      0x78787878      0x78787878
  0xdbbc2b10:     0x78787878      0x78787878      0x78787878      0x78787878
  0xdbbc2b20:     0x78787878      0x78787878      0x78787878      0x78787878
  0xdbbc2b30:     0x78787878      0x78787878      0x78787878      0x78787878
  0xdbbc2b40:     0x78787878      0x78787878      0x78787878      0x78787878
  0xdbbc2b50:     0x78787878      0x78787878      0x78787878      0x78787878
  0xdbbc2b60:     0x78787878      0x78787878      0x78787878      0x78787878
  0xdbbc2b70:     0x78787878      0x78787878      0x78787878      0x78787878
  0xdbbc2b80:     0x78787878      0x78787878      0x00000000      0x78787878
  0xdbbc2b90:     0x78787878      0x78787878      0x78787878      0x78787878
  0xdbbc2ba0:     0x78787878      0x78787878      0x78787878      0x78787878

In the reclaim path, try_to_free_pages() does not setup
sc.target_mem_cgroup and sc is passed to do_try_to_free_pages(), ...,
shrink_node().

In mem_cgroup_iter(), root is set to root_mem_cgroup because
sc->target_mem_cgroup is NULL.  It is possible to assign a memcg to
root_mem_cgroup.nodeinfo.iter in mem_cgroup_iter().

        try_to_free_pages
        	struct scan_control sc = {...}, target_mem_cgroup is 0x0;
        do_try_to_free_pages
        shrink_zones
        shrink_node
        	 mem_cgroup *root = sc->target_mem_cgroup;
        	 memcg = mem_cgroup_iter(root, NULL, &reclaim);
        mem_cgroup_iter()
        	if (!root)
        		root = root_mem_cgroup;
        	...

        	css = css_next_descendant_pre(css, &root->css);
        	memcg = mem_cgroup_from_css(css);
        	cmpxchg(&iter->position, pos, memcg);

My device uses memcg non-hierarchical mode.  When we release a memcg:
invalidate_reclaim_iterators() reaches only dead_memcg and its parents.
If non-hierarchical mode is used, invalidate_reclaim_iterators() never
reaches root_mem_cgroup.

  static void invalidate_reclaim_iterators(struct mem_cgroup *dead_memcg)
  {
        struct mem_cgroup *memcg = dead_memcg;

        for (; memcg; memcg = parent_mem_cgroup(memcg)
        ...
  }

So the use after free scenario looks like:

  CPU1						CPU2

  try_to_free_pages
  do_try_to_free_pages
  shrink_zones
  shrink_node
  mem_cgroup_iter()
      if (!root)
      	root = root_mem_cgroup;
      ...
      css = css_next_descendant_pre(css, &root->css);
      memcg = mem_cgroup_from_css(css);
      cmpxchg(&iter->position, pos, memcg);

        				invalidate_reclaim_iterators(memcg);
        				...
        				__mem_cgroup_free()
        					kfree(memcg);

  try_to_free_pages
  do_try_to_free_pages
  shrink_zones
  shrink_node
  mem_cgroup_iter()
      if (!root)
      	root = root_mem_cgroup;
      ...
      mz = mem_cgroup_nodeinfo(root, reclaim->pgdat->node_id);
      iter = &mz->iter[reclaim->priority];
      pos = READ_ONCE(iter->position);
      css_tryget(&pos->css) <- use after free

To avoid this, we should also invalidate root_mem_cgroup.nodeinfo.iter
in invalidate_reclaim_iterators().

[cai@lca.pw: fix -Wparentheses compilation warning]
  Link: http://lkml.kernel.org/r/1564580753-17531-1-git-send-email-cai@lca.pw
Link: http://lkml.kernel.org/r/20190730015729.4406-1-miles.chen@mediatek.com
Fixes: 5ac8fb31ad2e ("mm: memcontrol: convert reclaim iterator to simple css refcounting")
Signed-off-by: Miles Chen 
Signed-off-by: Qian Cai 
Acked-by: Michal Hocko 
Cc: Johannes Weiner 
Cc: Vladimir Davydov 
Cc: 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds 
Signed-off-by: Greg Kroah-Hartman

mm/vmalloc: Sync unmappings in __purge_vmap_area_lazy()

2019-08-25T08:52:44+00:00

commit 3f8fd02b1bf1d7ba964485a56f2f4b53ae88c167 upstream.

On x86-32 with PTI enabled, parts of the kernel page-tables are not shared
between processes. This can cause mappings in the vmalloc/ioremap area to
persist in some page-tables after the region is unmapped and released.

When the region is re-used the processes with the old mappings do not fault
in the new mappings but still access the old ones.

This causes undefined behavior, in reality often data corruption, kernel
oopses and panics and even spontaneous reboots.

Fix this problem by activly syncing unmaps in the vmalloc/ioremap area to
all page-tables in the system before the regions can be re-used.

References: https://bugzilla.suse.com/show_bug.cgi?id=1118689
Fixes: 5d72b4fba40ef ('x86, mm: support huge I/O mapping capability I/F')
Signed-off-by: Joerg Roedel 
Signed-off-by: Thomas Gleixner 
Reviewed-by: Dave Hansen 
Link: https://lkml.kernel.org/r/20190719184652.11391-4-joro@8bytes.org
Signed-off-by: Greg Kroah-Hartman

mm/cma.c: fail if fixed declaration can't be honored

2019-08-06T16:28:27+00:00

[ Upstream commit c633324e311243586675e732249339685e5d6faa ]

The description of cma_declare_contiguous() indicates that if the
'fixed' argument is true the reserved contiguous area must be exactly at
the address of the 'base' argument.

However, the function currently allows the 'base', 'size', and 'limit'
arguments to be silently adjusted to meet alignment constraints.  This
commit enforces the documented behavior through explicit checks that
return an error if the region does not fit within a specified region.

Link: http://lkml.kernel.org/r/1561422051-16142-1-git-send-email-opendmb@gmail.com
Fixes: 5ea3b1b2f8ad ("cma: add placement specifier for "cma=" kernel parameter")
Signed-off-by: Doug Berger 
Acked-by: Michal Nazarewicz 
Cc: Yue Hu 
Cc: Mike Rapoport 
Cc: Laura Abbott 
Cc: Peng Fan 
Cc: Thomas Gleixner 
Cc: Marek Szyprowski 
Cc: Andrey Konovalov 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds 
Signed-off-by: Sasha Levin

mm, vmstat: make quiet_vmstat lighter

2019-08-04T07:35:01+00:00

commit f01f17d3705bb6081c9e5728078f64067982be36 upstream.

Mike has reported a considerable overhead of refresh_cpu_vm_stats from
the idle entry during pipe test:

    12.89%  [kernel]       [k] refresh_cpu_vm_stats.isra.12
     4.75%  [kernel]       [k] __schedule
     4.70%  [kernel]       [k] mutex_unlock
     3.14%  [kernel]       [k] __switch_to

This is caused by commit 0eb77e988032 ("vmstat: make vmstat_updater
deferrable again and shut down on idle") which has placed quiet_vmstat
into cpu_idle_loop.  The main reason here seems to be that the idle
entry has to get over all zones and perform atomic operations for each
vmstat entry even though there might be no per cpu diffs.  This is a
pointless overhead for _each_ idle entry.

Make sure that quiet_vmstat is as light as possible.

First of all it doesn't make any sense to do any local sync if the
current cpu is already set in oncpu_stat_off because vmstat_update puts
itself there only if there is nothing to do.

Then we can check need_update which should be a cheap way to check for
potential per-cpu diffs and only then do refresh_cpu_vm_stats.

The original patch also did cancel_delayed_work which we are not doing
here.  There are two reasons for that.  Firstly cancel_delayed_work from
idle context will blow up on RT kernels (reported by Mike):

  CPU: 1 PID: 0 Comm: swapper/1 Not tainted 4.5.0-rt3 #7
  Hardware name: MEDION MS-7848/MS-7848, BIOS M7848W08.20C 09/23/2013
  Call Trace:
    dump_stack+0x49/0x67
    ___might_sleep+0xf5/0x180
    rt_spin_lock+0x20/0x50
    try_to_grab_pending+0x69/0x240
    cancel_delayed_work+0x26/0xe0
    quiet_vmstat+0x75/0xa0
    cpu_idle_loop+0x38/0x3e0
    cpu_startup_entry+0x13/0x20
    start_secondary+0x114/0x140

And secondly, even on !RT kernels it might add some non trivial overhead
which is not necessary.  Even if the vmstat worker wakes up and preempts
idle then it will be most likely a single shot noop because the stats
were already synced and so it would end up on the oncpu_stat_off anyway.
We just need to teach both vmstat_shepherd and vmstat_update to stop
scheduling the worker if there is nothing to do.

[mgalbraith@suse.de: cancel pending work of the cpu_stat_off CPU]
Signed-off-by: Michal Hocko 
Reported-by: Mike Galbraith 
Acked-by: Christoph Lameter 
Signed-off-by: Mike Galbraith 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds 
Signed-off-by: Daniel Wagner 
Signed-off-by: Greg Kroah-Hartman

vmstat: Remove BUG_ON from vmstat_update

2019-08-04T07:35:01+00:00

commit 587198ba5206cdf0d30855f7361af950a4172cd6 upstream.

If we detect that there is nothing to do just set the flag and do not
check if it was already set before.  Races really do not matter.  If the
flag is set by any code then the shepherd will start dealing with the
situation and reenable the vmstat workers when necessary again.

Since commit 0eb77e988032 ("vmstat: make vmstat_updater deferrable again
and shut down on idle") quiet_vmstat might update cpu_stat_off and mark
a particular cpu to be handled by vmstat_shepherd.  This might trigger a
VM_BUG_ON in vmstat_update because the work item might have been
sleeping during the idle period and see the cpu_stat_off updated after
the wake up.  The VM_BUG_ON is therefore misleading and no more
appropriate.  Moreover it doesn't really suite any protection from real
bugs because vmstat_shepherd will simply reschedule the vmstat_work
anytime it sees a particular cpu set or vmstat_update would do the same
from the worker context directly.  Even when the two would race the
result wouldn't be incorrect as the counters update is fully idempotent.

Reported-by: Sasha Levin 
Signed-off-by: Christoph Lameter 
Acked-by: Michal Hocko 
Cc: Johannes Weiner 
Cc: Tetsuo Handa 
Cc: Andrew Morton 
Signed-off-by: Linus Torvalds 
Signed-off-by: Daniel Wagner 
Signed-off-by: Greg Kroah-Hartman

mm/mmu_notifier: use hlist_add_head_rcu()

2019-08-04T07:34:59+00:00

[ Upstream commit 543bdb2d825fe2400d6e951f1786d92139a16931 ]

Make mmu_notifier_register() safer by issuing a memory barrier before
registering a new notifier.  This fixes a theoretical bug on weakly
ordered CPUs.  For example, take this simplified use of notifiers by a
driver:

	my_struct->mn.ops = &my_ops; /* (1) */
	mmu_notifier_register(&my_struct->mn, mm)
		...
		hlist_add_head(&mn->hlist, &mm->mmu_notifiers); /* (2) */
		...

Once mmu_notifier_register() releases the mm locks, another thread can
invalidate a range:

	mmu_notifier_invalidate_range()
		...
		hlist_for_each_entry_rcu(mn, &mm->mmu_notifiers, hlist) {
			if (mn->ops->invalidate_range)

The read side relies on the data dependency between mn and ops to ensure
that the pointer is properly initialized.  But the write side doesn't have
any dependency between (1) and (2), so they could be reordered and the
readers could dereference an invalid mn->ops.  mmu_notifier_register()
does take all the mm locks before adding to the hlist, but those have
acquire semantics which isn't sufficient.

By calling hlist_add_head_rcu() instead of hlist_add_head() we update the
hlist using a store-release, ensuring that readers see prior
initialization of my_struct.  This situation is better illustated by
litmus test MP+onceassign+derefonce.

Link: http://lkml.kernel.org/r/20190502133532.24981-1-jean-philippe.brucker@arm.com
Fixes: cddb8a5c14aa ("mmu-notifiers: core")
Signed-off-by: Jean-Philippe Brucker 
Cc: Jérôme Glisse 
Cc: Michal Hocko 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds 
Signed-off-by: Sasha Levin