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

[Pasha Tatashin]

On Mon, Feb 6, 2023 at 10:52 PM Chih-En Lin <shiyn.lin@xxxxxxxxx> wrote:
>
> 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@xxxxxxxxxx/
>
>
> v3: https://lore.kernel.org/linux-mm/20221220072743.3039060-1-shiyn.lin@xxxxxxxxx/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@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:

For some reason, I was not able to reproduce performance improvements
with a simple fork() performance measurement program. The results that
I saw are the following:

Base:
Fork latency per gigabyte: 0.004416 seconds
Fork latency per gigabyte: 0.004382 seconds
Fork latency per gigabyte: 0.004442 seconds
COW kernel:
Fork latency per gigabyte: 0.004524 seconds
Fork latency per gigabyte: 0.004764 seconds
Fork latency per gigabyte: 0.004547 seconds

AMD EPYC 7B12 64-Core Processor
Base:
Fork latency per gigabyte: 0.003923 seconds
Fork latency per gigabyte: 0.003909 seconds
Fork latency per gigabyte: 0.003955 seconds
COW kernel:
Fork latency per gigabyte: 0.004221 seconds
Fork latency per gigabyte: 0.003882 seconds
Fork latency per gigabyte: 0.003854 seconds

Given, that page table for child is not copied, I was expecting the
performance to be better with COW kernel, and also not to depend on
the size of the parent.

Test program:

#include <time.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/time.h>
#include <sys/mman.h>
#include <sys/types.h>

#define USEC    1000000
#define GIG     (1ul << 30)
#define NGIG    32
#define SIZE    (NGIG * GIG)
#define NPROC   16

void main() {
        int page_size = getpagesize();
        struct timeval start, end;
        long duration, i;
        char *p;

        p = mmap(NULL, SIZE, PROT_READ | PROT_WRITE,
                 MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
        if (p == MAP_FAILED) {
                perror("mmap");
                exit(1);
        }
        madvise(p, SIZE, MADV_NOHUGEPAGE);

        /* Touch every page */
        for (i = 0; i < SIZE; i += page_size)
                p[i] = 0;

        gettimeofday(&start, NULL);
        for (i = 0; i < NPROC; i++) {
                int pid = fork();

                if (pid == 0) {
                        sleep(30);
                        exit(0);
                }
        }
        gettimeofday(&end, NULL);
        /* Normolize per proc and per gig */
        duration = ((end.tv_sec - start.tv_sec) * USEC
                + (end.tv_usec - start.tv_usec)) / NPROC / NGIG;
        printf("Fork latency per gigabyte: %ld.%06ld seconds\n",
                duration / USEC, duration % USEC);
}