[PATCH v4 00/14] Introduce Copy-On-Write to Page Table

Chih-En Lin
2023-02-07T03:52:37Z

v3 -> v4
- Add Kconfig, CONFIG_COW_PTE, since some of the architectures, e.g.,
s390 and powerpc32, don't support the PMD entry and PTE table
operations.
- Fix unmatch type of break_cow_pte_range() in
migrate_vma_collect_pmd().
- Don’t break COW PTE in folio_referenced_one().
- Fix the wrong VMA range checking in break_cow_pte_range().
- Only break COW when we modify the soft-dirty bit in
clear_refs_pte_range().
- Handle do_swap_page() with COW PTE in mm/memory.c and mm/khugepaged.c.
- Change the tlb flush from flush_tlb_mm_range() (x86 specific) to
tlb_flush_pmd_range().
- Handle VM_DONTCOPY with COW PTE fork.
- Fix the wrong address and invalid vma in recover_pte_range().
- Fix the infinite page fault loop in GUP routine.
In mm/gup.c:follow_pfn_pte(), instead of calling the break COW PTE
handler, we return -EMLINK to let the GUP handles the page fault
(call faultin_page() in __get_user_pages()).
- return not_found(pvmw) if the break COW PTE failed in
page_vma_mapped_walk().
- Since COW PTE has the same result as the normal COW selftest, it
probably passed the COW selftest.

# [RUN] vmsplice() + unmap in child ... with hugetlb (2048 kB)
not ok 33 No leak from parent into child
# [RUN] vmsplice() + unmap in child with mprotect() optimization ... with hugetlb (2048 kB)
not ok 44 No leak from parent into child
# [RUN] vmsplice() before fork(), unmap in parent after fork() ... with hugetlb (2048 kB)
not ok 55 No leak from child into parent
# [RUN] vmsplice() + unmap in parent after fork() ... with hugetlb (2048 kB)
not ok 66 No leak from child into parent

Bail out! 4 out of 147 tests failed
# Totals: pass:143 fail:4 xfail:0 xpass:0 skip:0 error:0
See the more information about anon cow hugetlb tests:
https://patchwork.kernel.org/project/linux-mm/patch/20220927110120.106906-5-david@redhat.com/

v3: https://lore.kernel.org/linux-mm/20221220072743.3039060-1-shiyn.lin@gmail.com/T/

RFC v2 -> v3
- Change the sysctl with PID to prctl(PR_SET_COW_PTE).
- Account all the COW PTE mapped pages in fork() instead of defer it to
page fault (break COW PTE).
- If there is an unshareable mapped page (maybe pinned or private
device), recover all the entries that are already handled by COW PTE
fork, then copy to the new one.
- Remove COW_PTE_OWNER_EXCLUSIVE flag and handle the only case of GUP,
follow_pfn_pte().
- Remove the PTE ownership since we don't need it.
- Use pte lock to protect the break COW PTE and free COW-ed PTE.
- Do TLB flushing in break COW PTE handler.
- Handle THP, KSM, madvise, mprotect, uffd and migrate device.
- Handle the replacement page of uprobe.
- Handle the clear_refs_write() of fs/proc.
- All of the benchmarks dropped since the accounting and pte lock.
The benchmarks of v3 is worse than RFC v2, most of the cases are
similar to the normal fork, but there still have an use case
(TriforceAFL) is better than the normal fork version.

RFC v2: https://lore.kernel.org/linux-mm/20220927162957.270460-1-shiyn.lin@gmail.com/T/

RFC v1 -> RFC v2
- Change the clone flag method to sysctl with PID.
- Change the MMF_COW_PGTABLE flag to two flags, MMF_COW_PTE and
MMF_COW_PTE_READY, for the sysctl.
- Change the owner pointer to use the folio padding.
- Handle all the VMAs that cover the PTE table when doing the break COW PTE.
- Remove the self-defined refcount to use the _refcount for the page
table page.
- Add the exclusive flag to let the page table only own by one task in
some situations.
- Invalidate address range MMU notifier and start the write_seqcount
when doing the break COW PTE.
- Handle the swap cache and swapoff.

RFC v1: https://lore.kernel.org/all/20220519183127.3909598-1-shiyn.lin@gmail.com/

---

Currently, copy-on-write is only used for the mapped memory; the child
process still needs to copy the entire page table from the parent
process during forking. The parent process might take a lot of time and
memory to copy the page table when the parent has a big page table
allocated. For example, the memory usage of a process after forking with
1 GB mapped memory is as follows:

DEFAULT FORK
parent child
VmRSS: 1049688 kB 1048688 kB
VmPTE: 2096 kB 2096 kB

This patch introduces copy-on-write (COW) for the PTE level page tables.
COW PTE improves performance in the situation where the user needs
copies of the program to run on isolated environments. Feedback-based
fuzzers (e.g., AFL) and serverless/microservice frameworks are two major
examples. For instance, COW PTE achieves a 1.03x throughput increase
when running TriforceAFL.

After applying COW to PTE, the memory usage after forking is as follows:

COW PTE
parent child
VmRSS: 1049968 kB 2576 kB
VmPTE: 2096 kB 44 kB

The results show that this patch significantly decreases memory usage.
The other number of latencies are discussed later.

Real-world application benchmarks
=================================

We run benchmarks of fuzzing and VM cloning. The experiments were
done with the normal fork or the fork with COW PTE.

With AFL (LLVM mode) and SQLite, COW PTE (52.15 execs/sec) is a
little bit worse than the normal fork version (53.50 execs/sec).

fork
execs_per_sec unix_time time
count 28.000000 2.800000e+01 28.000000
mean 53.496786 1.671270e+09 96.107143
std 3.625060 7.194717e+01 71.947172
min 35.350000 1.671270e+09 0.000000
25% 53.967500 1.671270e+09 33.750000
50% 54.235000 1.671270e+09 92.000000
75% 54.525000 1.671270e+09 149.250000
max 55.100000 1.671270e+09 275.000000

COW PTE
execs_per_sec unix_time time
count 34.000000 3.400000e+01 34.000000
mean 52.150000 1.671268e+09 103.323529
std 3.218271 7.507682e+01 75.076817
min 34.250000 1.671268e+09 0.000000
25% 52.500000 1.671268e+09 42.250000
50% 52.750000 1.671268e+09 94.500000
75% 52.952500 1.671268e+09 150.750000
max 53.680000 1.671268e+09 285.000000

With TriforceAFL which is for kernel fuzzing with QEMU, COW PTE
(105.54 execs/sec) achieves a 1.05x throughput increase over the
normal fork version (102.30 execs/sec).

fork
execs_per_sec unix_time time
count 38.000000 3.800000e+01 38.000000
mean 102.299737 1.671269e+09 156.289474
std 20.139268 8.717113e+01 87.171130
min 6.600000 1.671269e+09 0.000000
25% 95.657500 1.671269e+09 82.250000
50% 109.950000 1.671269e+09 176.500000
75% 113.972500 1.671269e+09 223.750000
max 118.790000 1.671269e+09 281.000000

COW PTE
execs_per_sec unix_time time
count 42.000000 4.200000e+01 42.000000
mean 105.540714 1.671269e+09 163.476190
std 19.443517 8.858845e+01 88.588453
min 6.200000 1.671269e+09 0.000000
25% 96.585000 1.671269e+09 123.500000
50% 113.925000 1.671269e+09 180.500000
75% 116.940000 1.671269e+09 233.500000
max 121.090000 1.671269e+09 286.000000

Microbenchmark - syscall latency
================================

We run microbenchmarks to measure the latency of a fork syscall with
sizes of mapped memory ranging from 0 to 512 MB. The results show that
the latency of a normal fork reaches 10 ms. The latency of a fork with
COW PTE is also around 10 ms.

Microbenchmark - page fault latency
====================================

We conducted some microbenchmarks to measure page fault latency with
different patterns of accesses to a 512 MB memory buffer after forking.

In the first experiment, the program accesses the entire 512 MB memory
by writing to all the pages consecutively. The experiment is done with
normal fork, fork with COW PTE and calculates the single access average
latency. COW PTE page fault latency (0.000795 ms) and the normal fork
fault latency (0.000770 ms). Here are the raw numbers:

Page fault - Access to the entire 512 MB memory

fork mean: 0.000770 ms
fork median: 0.000769 ms
fork std: 0.000010 ms

COW PTE mean: 0.000795 ms
COW PTE median: 0.000795 ms
COW PTE std: 0.000009 ms

The second experiment simulates real-world applications with sparse
accesses. The program randomly accesses the memory by writing to one
random page 1 million times and calculates the average access time,
after that, we run both 100 times to get the averages. The result shows
that COW PTE (0.000029 ms) is similar to the normal fork (0.000026 ms).

Page fault - Random access

fork mean: 0.000026 ms
fork median: 0.000025 ms
fork std: 0.000002 ms

COW PTE mean: 0.000029 ms
COW PTE median: 0.000026 ms
COW PTE std: 0.000004 ms

All the tests were run with QEMU and the kernel was built with
the x86_64 default config (v3 patch set).

Summary
=======

In summary, COW PTE reduces the memory footprint of processes and
improves the performance for some use cases.

This patch is based on the paper "On-demand-fork: a microsecond fork
for memory-intensive and latency-sensitive applications" [1] from
Purdue University.

Any comments and suggestions are welcome.

Thanks,
Chih-En Lin

---

[1] https://dl.acm.org/doi/10.1145/3447786.3456258

This patch is based on v6.2-rc7.

---

Chih-En Lin (14):
mm: Allow user to control COW PTE via prctl
mm: Add Copy-On-Write PTE to fork()
mm: Add break COW PTE fault and helper functions
mm/rmap: Break COW PTE in rmap walking
mm/khugepaged: Break COW PTE before scanning pte
mm/ksm: Break COW PTE before modify shared PTE
mm/madvise: Handle COW-ed PTE with madvise()
mm/gup: Trigger break COW PTE before calling follow_pfn_pte()
mm/mprotect: Break COW PTE before changing protection
mm/userfaultfd: Support COW PTE
mm/migrate_device: Support COW PTE
fs/proc: Support COW PTE with clear_refs_write
events/uprobes: Break COW PTE before replacing page
mm: fork: Enable COW PTE to fork system call

fs/proc/task_mmu.c | 5 +
include/linux/mm.h | 37 ++
include/linux/pgtable.h | 6 +
include/linux/rmap.h | 2 +
include/linux/sched/coredump.h | 12 +-
include/trace/events/huge_memory.h | 1 +
include/uapi/linux/prctl.h | 6 +
kernel/events/uprobes.c | 2 +-
kernel/fork.c | 7 +
kernel/sys.c | 11 +
mm/Kconfig | 9 +
mm/gup.c | 8 +-
mm/khugepaged.c | 35 +-
mm/ksm.c | 4 +-
mm/madvise.c | 13 +
mm/memory.c | 642 ++++++++++++++++++++++++++++-
mm/migrate.c | 3 +-
mm/migrate_device.c | 2 +
mm/mmap.c | 4 +
mm/mprotect.c | 9 +
mm/mremap.c | 2 +
mm/page_vma_mapped.c | 4 +
mm/rmap.c | 9 +-
mm/swapfile.c | 2 +
mm/userfaultfd.c | 6 +
mm/vmscan.c | 3 +-
26 files changed, 826 insertions(+), 18 deletions(-)

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2.34.1