Prior to "percpu: implement partial chunk depopulation",
pcpu_depopulate_chunk() was called only on the destruction path. This
meant the virtual address range was on its way back to vmalloc which
will handle flushing the tlbs for us.

However, with pcpu_reclaim_populated(), we are now calling
pcpu_depopulate_chunk() during the active lifecycle of a chunk.
Therefore, we need to flush the tlb as well otherwise we can end up
accessing the wrong page through an invalid tlb mapping as reported in
[1].

[1] https://lore.kernel.org/lkml/20210702191140.GA3166599@roeck-us.net/

Fixes: f183324133ea ("percpu: implement partial chunk depopulation")
Reported-and-tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Dennis Zhou <dennis@kernel.org>

Pull percpu updates from Dennis Zhou:

 - percpu chunk depopulation - depopulate backing pages for chunks with
   empty pages when we exceed a global threshold without those pages.
   This lets us reclaim a portion of memory that would previously be
   lost until the full chunk would be freed (possibly never).

 - memcg accounting cleanup - previously separate chunks were managed
   for normal allocations and __GFP_ACCOUNT allocations. These are now
   consolidated which cleans up the code quite a bit.

 - a few misc clean ups for clang warnings

* 'for-5.14' of git://git.kernel.org/pub/scm/linux/kernel/git/dennis/percpu:
  percpu: optimize locking in pcpu_balance_workfn()
  percpu: initialize best_upa variable
  percpu: rework memcg accounting
  mm, memcg: introduce mem_cgroup_kmem_disabled()
  mm, memcg: mark cgroup_memory_nosocket, nokmem and noswap as __ro_after_init
  percpu: make symbol 'pcpu_free_slot' static
  percpu: implement partial chunk depopulation
  percpu: use pcpu_free_slot instead of pcpu_nr_slots - 1
  percpu: factor out pcpu_check_block_hint()
  percpu: split __pcpu_balance_workfn()
  percpu: fix a comment about the chunks ordering

pcpu_balance_workfn() unconditionally calls pcpu_balance_free(),
pcpu_reclaim_populated(), pcpu_balance_populated() and
pcpu_balance_free() again.

Each call to pcpu_balance_free() and pcpu_reclaim_populated() will
cause at least one acquisition of the pcpu_lock. So even if the
balancing was scheduled because of a failed atomic allocation,
pcpu_lock will be acquired at least 4 times. This obviously
increases the contention on the pcpu_lock.

To optimize the scheme let's grab the pcpu_lock on the upper level
(in pcpu_balance_workfn()) and keep it generally locked for the whole
duration of the scheduled work, but release conditionally to perform
any slow operations like chunk (de)population and creation of new
chunks.

Signed-off-by: Roman Gushchin <guro@fb.com>
Signed-off-by: Dennis Zhou <dennis@kernel.org>

Tom reported this finding from clang 10's static analysis [1].

Due to the way the code is written, it will always see a successful loop
iteration. Instead of setting an initial value, check that it was set
instead with BUG_ON() because 0 units per allocation is bogus.

[1] https://lore.kernel.org/lkml/20210515180817.1751084-1-trix@redhat.com/

Reported-by: Tom Rix <trix@redhat.com>
Signed-off-by: Dennis Zhou <dennis@kernel.org>

The current implementation of the memcg accounting of the percpu
memory is based on the idea of having two separate sets of chunks for
accounted and non-accounted memory. This approach has an advantage
of not wasting any extra memory for memcg data for non-accounted
chunks, however it complicates the code and leads to a higher chunks
number due to a lower chunk utilization.

Instead of having two chunk types it's possible to declare all* chunks
memcg-aware unless the kernel memory accounting is disabled globally
by a boot option. The size of objcg_array is usually small in
comparison to chunks themselves (it obviously depends on the number of
CPUs), so even if some chunk will have no accounted allocations, the
memory waste isn't significant and will likely be compensated by
a higher chunk utilization. Also, with time more and more percpu
allocations will likely become accounted.

* The first chunk is initialized before the memory cgroup subsystem,
  so we don't know for sure whether we need to allocate obj_cgroups.
  Because it's small, let's make it free for use. Then we don't need
  to allocate obj_cgroups for it.

Signed-off-by: Roman Gushchin <guro@fb.com>
Signed-off-by: Dennis Zhou <dennis@kernel.org>

The sparse tool complains as follows:

mm/percpu.c:138:5: warning:
 symbol 'pcpu_free_slot' was not declared. Should it be static?

This symbol is not used outside of percpu.c, so marks it static.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Dennis Zhou <dennis@kernel.org>

Fix ~94 single-word typos in locking code comments, plus a few
very obvious grammar mistakes.

Link: https://lkml.kernel.org/r/20210322212624.GA1963421@gmail.com
Link: https://lore.kernel.org/r/20210322205203.GB1959563@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Bhaskar Chowdhury <unixbhaskar@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

From Roman ("percpu: partial chunk depopulation"):
In our [Facebook] production experience the percpu memory allocator is
sometimes struggling with returning the memory to the system. A typical
example is a creation of several thousands memory cgroups (each has
several chunks of the percpu data used for vmstats, vmevents,
ref counters etc). Deletion and complete releasing of these cgroups
doesn't always lead to a shrinkage of the percpu memory, so that
sometimes there are several GB's of memory wasted.

The underlying problem is the fragmentation: to release an underlying
chunk all percpu allocations should be released first. The percpu
allocator tends to top up chunks to improve the utilization. It means
new small-ish allocations (e.g. percpu ref counters) are placed onto
almost filled old-ish chunks, effectively pinning them in memory.

This patchset solves this problem by implementing a partial depopulation
of percpu chunks: chunks with many empty pages are being asynchronously
depopulated and the pages are returned to the system.

To illustrate the problem the following script can be used:
--

cd /sys/fs/cgroup

mkdir percpu_test
echo "+memory" > percpu_test/cgroup.subtree_control

cat /proc/meminfo | grep Percpu

for i in `seq 1 1000`; do
    mkdir percpu_test/cg_"${i}"
    for j in `seq 1 10`; do
	mkdir percpu_test/cg_"${i}"_"${j}"
    done
done

cat /proc/meminfo | grep Percpu

for i in `seq 1 1000`; do
    for j in `seq 1 10`; do
	rmdir percpu_test/cg_"${i}"_"${j}"
    done
done

sleep 10

cat /proc/meminfo | grep Percpu

for i in `seq 1 1000`; do
    rmdir percpu_test/cg_"${i}"
done

rmdir percpu_test
--

It creates 11000 memory cgroups and removes every 10 out of 11.
It prints the initial size of the percpu memory, the size after
creating all cgroups and the size after deleting most of them.

Results:
  vanilla:
    ./percpu_test.sh
    Percpu:             7488 kB
    Percpu:           481152 kB
    Percpu:           481152 kB

  with this patchset applied:
    ./percpu_test.sh
    Percpu:             7488 kB
    Percpu:           481408 kB
    Percpu:           135552 kB

The total size of the percpu memory was reduced by more than 3.5 times.

This patch:

This patch implements partial depopulation of percpu chunks.

As of now, a chunk can be depopulated only as a part of the final
destruction, if there are no more outstanding allocations. However
to minimize a memory waste it might be useful to depopulate a
partially filed chunk, if a small number of outstanding allocations
prevents the chunk from being fully reclaimed.

This patch implements the following depopulation process: it scans
over the chunk pages, looks for a range of empty and populated pages
and performs the depopulation. To avoid races with new allocations,
the chunk is previously isolated. After the depopulation the chunk is
sidelined to a special list or freed. New allocations prefer using
active chunks to sidelined chunks. If a sidelined chunk is used, it is
reintegrated to the active lists.

The depopulation is scheduled on the free path if the chunk is all of
the following:
  1) has more than 1/4 of total pages free and populated
  2) the system has enough free percpu pages aside of this chunk
  3) isn't the reserved chunk
  4) isn't the first chunk
If it's already depopulated but got free populated pages, it's a good
target too. The chunk is moved to a special slot,
pcpu_to_depopulate_slot, chunk->isolated is set, and the balance work
item is scheduled. On isolation, these pages are removed from the
pcpu_nr_empty_pop_pages. It is constantly replaced to the
to_depopulate_slot when it meets these qualifications.

pcpu_reclaim_populated() iterates over the to_depopulate_slot until it
becomes empty. The depopulation is performed in the reverse direction to
keep populated pages close to the beginning. Depopulated chunks are
sidelined to preferentially avoid them for new allocations. When no
active chunk can suffice a new allocation, sidelined chunks are first
checked before creating a new chunk.

Signed-off-by: Roman Gushchin <guro@fb.com>
Co-developed-by: Dennis Zhou <dennis@kernel.org>
Signed-off-by: Dennis Zhou <dennis@kernel.org>
Tested-by: Pratik Sampat <psampat@linux.ibm.com>
Signed-off-by: Dennis Zhou <dennis@kernel.org>

This prepares for adding a to_depopulate list and sidelined list after
the free slot in the set of lists in pcpu_slot.

Signed-off-by: Dennis Zhou <dennis@kernel.org>
Acked-by: Roman Gushchin <guro@fb.com>
Signed-off-by: Dennis Zhou <dennis@kernel.org>

Factor out the pcpu_check_block_hint() helper, which will be useful
in the future. The new function checks if the allocation can likely
fit within the contig hint.

Signed-off-by: Roman Gushchin <guro@fb.com>
Signed-off-by: Dennis Zhou <dennis@kernel.org>
Signed-off-by: Dennis Zhou <dennis@kernel.org>