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Subject[PATCH v12 00/31] Speculative page faults
This is a port on kernel 5.1 of the work done by Peter Zijlstra to handle
page fault without holding the mm semaphore [1].

The idea is to try to handle user space page faults without holding the
mmap_sem. This should allow better concurrency for massively threaded
process since the page fault handler will not wait for other threads memory
layout change to be done, assuming that this change is done in another part
of the process's memory space. This type of page fault is named speculative
page fault. If the speculative page fault fails because a concurrency has
been detected or because underlying PMD or PTE tables are not yet
allocating, it is failing its processing and a regular page fault is then

The speculative page fault (SPF) has to look for the VMA matching the fault
address without holding the mmap_sem, this is done by protecting the MM RB
tree with RCU and by using a reference counter on each VMA. When fetching a
VMA under the RCU protection, the VMA's reference counter is incremented to
ensure that the VMA will not freed in our back during the SPF
processing. Once that processing is done the VMA's reference counter is
decremented. To ensure that a VMA is still present when walking the RB tree
locklessly, the VMA's reference counter is incremented when that VMA is
linked in the RB tree. When the VMA is unlinked from the RB tree, its
reference counter will be decremented at the end of the RCU grace period,
ensuring it will be available during this time. This means that the VMA
freeing could be delayed and could delay the file closing for file
mapping. Since the SPF handler is not able to manage file mapping, file is
closed synchronously and not during the RCU cleaning. This is safe since
the page fault handler is aborting if a file pointer is associated to the

Using RCU fixes the overhead seen by Haiyan Song using the will-it-scale
benchmark [2].

The VMA's attributes checked during the speculative page fault processing
have to be protected against parallel changes. This is done by using a per
VMA sequence lock. This sequence lock allows the speculative page fault
handler to fast check for parallel changes in progress and to abort the
speculative page fault in that case.

Once the VMA has been found, the speculative page fault handler would check
for the VMA's attributes to verify that the page fault has to be handled
correctly or not. Thus, the VMA is protected through a sequence lock which
allows fast detection of concurrent VMA changes. If such a change is
detected, the speculative page fault is aborted and a *classic* page fault
is tried. VMA sequence lockings are added when VMA attributes which are
checked during the page fault are modified.

When the PTE is fetched, the VMA is checked to see if it has been changed,
so once the page table is locked, the VMA is valid, so any other changes
leading to touching this PTE will need to lock the page table, so no
parallel change is possible at this time.

The locking of the PTE is done with interrupts disabled, this allows
checking for the PMD to ensure that there is not an ongoing collapsing
operation. Since khugepaged is firstly set the PMD to pmd_none and then is
waiting for the other CPU to have caught the IPI interrupt, if the pmd is
valid at the time the PTE is locked, we have the guarantee that the
collapsing operation will have to wait on the PTE lock to move
forward. This allows the SPF handler to map the PTE safely. If the PMD
value is different from the one recorded at the beginning of the SPF
operation, the classic page fault handler will be called to handle the
operation while holding the mmap_sem. As the PTE lock is done with the
interrupts disabled, the lock is done using spin_trylock() to avoid dead
lock when handling a page fault while a TLB invalidate is requested by
another CPU holding the PTE.

In pseudo code, this could be seen as:
vma = find_vma_rcu()
check vma sequence count
check vma's support
disable interrupt
check pgd,p4d,...,pte
save pmd and pte in vmf
save vma sequence counter in vmf
enable interrupt
check vma sequence count
page = alloc_page()
disable interrupt
abort if sequence counter has changed
abort if pmd or pte has changed
pte map and lock
enable interrupt
if abort
free page

if (speculative_page_fault(&vma))
goto done
vma = find_vma();
if retry
goto again;
handle fault error

Support for THP is not done because when checking for the PMD, we can be
confused by an in progress collapsing operation done by khugepaged. The
issue is that pmd_none() could be true either if the PMD is not already
populated or if the underlying PTE are in the way to be collapsed. So we
cannot safely allocate a PMD if pmd_none() is true.

This series add a new software performance event named 'speculative-faults'
or 'spf'. It counts the number of successful page fault event handled
speculatively. When recording 'faults,spf' events, the faults one is
counting the total number of page fault events while 'spf' is only counting
the part of the faults processed speculatively.

There are some trace events introduced by this series. They allow
identifying why the page faults were not processed speculatively. This
doesn't take in account the faults generated by a monothreaded process
which directly processed while holding the mmap_sem. This trace events are
grouped in a system named 'pagefault', they are:

- pagefault:spf_vma_changed : if the VMA has been changed in our back
- pagefault:spf_vma_noanon : the vma->anon_vma field was not yet set.
- pagefault:spf_vma_notsup : the VMA's type is not supported
- pagefault:spf_vma_access : the VMA's access right are not respected
- pagefault:spf_pmd_changed : the upper PMD pointer has changed in our

To record all the related events, the easier is to run perf with the
following arguments :
$ perf stat -e 'faults,spf,pagefault:*' <command>

There is also a dedicated vmstat counter showing the number of successful
page fault handled speculatively. I can be seen this way:
$ grep speculative_pgfault /proc/vmstat

It is possible to deactivate the speculative page fault handler by echoing
0 in /proc/sys/vm/speculative_page_fault.

This series builds on top of v5.1-rc4-mmotm-2019-04-09-17-51 and is
functional on x86, PowerPC. I cross built it on arm64 but I was not able to
test it.

This series is also available on github [4].

Real Workload results

Test using a "popular in memory multithreaded database product" on 128cores
SMT8 Power system are in progress and I will come back with performance
mesurement as soon as possible. With the previous series we seen up to 30%
improvements in the number of transaction processed per second, and we hope
this will be the case with this series too.

Benchmarks results

Base kernel is v5.1-rc4-mmotm-2019-04-09-17-51
SPF is BASE + this series

Here are the results on a 48 CPUs X86 system using kernbench on a 5.0
kernel (kernel is build 5 times):

Average Half load -j 24
Run (std deviation)
Elapsed Time 56.52 (1.39185) 56.256 (1.15106) 0.47%
User Time 980.018 (2.94734) 984.958 (1.98518) -0.50%
System Time 130.744 (1.19148) 133.616 (0.873573) -2.20%
Percent CPU 1965.6 (49.682) 1988.4 (40.035) -1.16%
Context Switches 29926.6 (272.789) 30472.4 (109.569) -1.82%
Sleeps 124793 (415.87) 125003 (591.008) -0.17%

Average Optimal load -j 48
Run (std deviation)
Elapsed Time 46.354 (0.917949) 45.968 (1.42786) 0.83%
User Time 1193.42 (224.96) 1196.78 (223.28) -0.28%
System Time 143.306 (13.2726) 146.177 (13.2659) -2.00%
Percent CPU 2668.6 (743.157) 2699.9 (753.767) -1.17%
Context Switches 62268.3 (34097.1) 62721.7 (33999.1) -0.73%
Sleeps 132556 (8222.99) 132607 (8077.6) -0.04%

During a run on the SPF, perf events were captured:
Performance counter stats for '../kernbench -M':
525,873,132 faults
242 spf
0 pagefault:spf_vma_changed
0 pagefault:spf_vma_noanon
441 pagefault:spf_vma_notsup
0 pagefault:spf_vma_access
0 pagefault:spf_pmd_changed

Very few speculative page faults were recorded as most of the processes
involved are monothreaded (sounds that on this architecture some threads
were created during the kernel build processing).

Here are the kerbench results on a 1024 CPUs Power8 VM:

5.1.0-rc4-mm1+ 5.1.0-rc4-mm1-spf-rcu+
Average Half load -j 512 Run (std deviation):
Elapsed Time 52.52 (0.906697) 52.778 (0.510069) -0.49%
User Time 3855.43 (76.378) 3890.44 (73.0466) -0.91%
System Time 1977.24 (182.316) 1974.56 (166.097) 0.14%
Percent CPU 11111.6 (540.461) 11115.2 (458.907) -0.03%
Context Switches 83245.6 (3061.44) 83651.8 (1202.31) -0.49%
Sleeps 613459 (23091.8) 628378 (27485.2) -2.43%

Average Optimal load -j 1024 Run (std deviation):
Elapsed Time 52.964 (0.572346) 53.132 (0.825694) -0.32%
User Time 4058.22 (222.034) 4070.2 (201.646) -0.30%
System Time 2672.81 (759.207) 2712.13 (797.292) -1.47%
Percent CPU 12756.7 (1786.35) 12806.5 (1858.89) -0.39%
Context Switches 88818.5 (6772) 87890.6 (5567.72) 1.04%
Sleeps 618658 (20842.2) 636297 (25044) -2.85%

During a run on the SPF, perf events were captured:
Performance counter stats for '../kernbench -M':
149 375 832 faults
1 spf
0 pagefault:spf_vma_changed
0 pagefault:spf_vma_noanon
561 pagefault:spf_vma_notsup
0 pagefault:spf_vma_access
0 pagefault:spf_pmd_changed

Most of the processes involved are monothreaded so SPF is not activated but
there is no impact on the performance.

The test is counting the number of records per second it can manage, the
higher is the best. I run it like this 'ebizzy -mTt <nrcpus>'. To get
consistent result I repeated the test 100 times and measure the average
result. The number is the record processes per second, the higher is the best.

BASE SPF delta
24 CPUs x86 5492.69 9383.07 70.83%
1024 CPUS P8 VM 8476.74 17144.38 102%

Here are the performance counter read during a run on a 48 CPUs x86 node:
Performance counter stats for './ebizzy -mTt 48':
11,846,569 faults
10,886,706 spf
957,702 pagefault:spf_vma_changed
0 pagefault:spf_vma_noanon
815 pagefault:spf_vma_notsup
0 pagefault:spf_vma_access
0 pagefault:spf_pmd_changed

And the ones captured during a run on a 1024 CPUs Power VM:
Performance counter stats for './ebizzy -mTt 1024':
1 359 789 faults
1 284 910 spf
72 085 pagefault:spf_vma_changed
0 pagefault:spf_vma_noanon
2 669 pagefault:spf_vma_notsup
0 pagefault:spf_vma_access
0 pagefault:spf_pmd_changed

In ebizzy's case most of the page fault were handled in a speculative way,
leading the ebizzy performance boost.

Changes since v11 [3]
- Check vm_ops.fault instead of vm_ops since now all the VMA as a vm_ops.
- Abort speculative page fault when doing swap readhead because VMA's
boundaries are not protected at this time. Doing this the first swap in
is doing a readhead, the next fault should be handled in a speculative
way as the page is present in the swap read page.
- Handle a race between copy_pte_range() and the wp_page_copy called by
the speculative page fault handler.
- Ported to Kernel v5.0
- Moved VM_FAULT_PTNOTSAME define in mm_types.h
- Use RCU to protect the MM RB tree instead of a rwlock.
- Add a toggle interface: /proc/sys/vm/speculative_page_fault


Laurent Dufour (25):
mm: introduce pte_spinlock for FAULT_FLAG_SPECULATIVE
mm: make pte_unmap_same compatible with SPF
mm: introduce INIT_VMA()
mm: protect VMA modifications using VMA sequence count
mm: protect mremap() against SPF hanlder
mm: protect SPF handler against anon_vma changes
mm: cache some VMA fields in the vm_fault structure
mm/migrate: Pass vm_fault pointer to migrate_misplaced_page()
mm: introduce __lru_cache_add_active_or_unevictable
mm: introduce __vm_normal_page()
mm: introduce __page_add_new_anon_rmap()
mm: protect against PTE changes done by dup_mmap()
mm: protect the RB tree with a sequence lock
mm: introduce vma reference counter
mm: Introduce find_vma_rcu()
mm: don't do swap readahead during speculative page fault
mm: adding speculative page fault failure trace events
perf: add a speculative page fault sw event
perf tools: add support for the SPF perf event
mm: add speculative page fault vmstats
powerpc/mm: add speculative page fault
mm: Add a speculative page fault switch in sysctl

Mahendran Ganesh (2):
arm64/mm: add speculative page fault

Peter Zijlstra (4):
mm: VMA sequence count
mm: provide speculative fault infrastructure
x86/mm: add speculative pagefault handling

arch/arm64/Kconfig | 1 +
arch/arm64/mm/fault.c | 12 +
arch/powerpc/Kconfig | 1 +
arch/powerpc/mm/fault.c | 16 +
arch/x86/Kconfig | 1 +
arch/x86/mm/fault.c | 14 +
fs/exec.c | 1 +
fs/proc/task_mmu.c | 5 +-
fs/userfaultfd.c | 17 +-
include/linux/hugetlb_inline.h | 2 +-
include/linux/migrate.h | 4 +-
include/linux/mm.h | 138 +++++-
include/linux/mm_types.h | 16 +-
include/linux/pagemap.h | 4 +-
include/linux/rmap.h | 12 +-
include/linux/swap.h | 10 +-
include/linux/vm_event_item.h | 3 +
include/trace/events/pagefault.h | 80 ++++
include/uapi/linux/perf_event.h | 1 +
kernel/fork.c | 35 +-
kernel/sysctl.c | 9 +
mm/Kconfig | 22 +
mm/huge_memory.c | 6 +-
mm/hugetlb.c | 2 +
mm/init-mm.c | 3 +
mm/internal.h | 45 ++
mm/khugepaged.c | 5 +
mm/madvise.c | 6 +-
mm/memory.c | 631 ++++++++++++++++++++++----
mm/mempolicy.c | 51 ++-
mm/migrate.c | 6 +-
mm/mlock.c | 13 +-
mm/mmap.c | 249 ++++++++--
mm/mprotect.c | 4 +-
mm/mremap.c | 13 +
mm/nommu.c | 1 +
mm/rmap.c | 5 +-
mm/swap.c | 6 +-
mm/swap_state.c | 10 +-
mm/vmstat.c | 5 +-
tools/include/uapi/linux/perf_event.h | 1 +
tools/perf/util/evsel.c | 1 +
tools/perf/util/parse-events.c | 4 +
tools/perf/util/parse-events.l | 1 +
tools/perf/util/python.c | 1 +
45 files changed, 1277 insertions(+), 196 deletions(-)
create mode 100644 include/trace/events/pagefault.h


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