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@xxxxxxxxx/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@xxxxxxxxx/ --- 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. 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 v6.1-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: Break COW PTE in 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 | 3 + include/linux/mm.h | 20 + 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 | 5 + kernel/sys.c | 11 + mm/gup.c | 2 + mm/khugepaged.c | 23 ++ mm/ksm.c | 4 +- mm/madvise.c | 13 + mm/memory.c | 582 ++++++++++++++++++++++++++++- 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 | 2 + mm/rmap.c | 12 +- mm/swapfile.c | 2 + mm/userfaultfd.c | 6 + mm/vmscan.c | 7 +- 25 files changed, 725 insertions(+), 16 deletions(-) -- 2.37.3
