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    SubjectRe: [PATCH 1/2] mm,migration: During fork(), wait for migration to end if migration PTE is encountered
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    On Tue, Apr 27, 2010 at 7:37 AM, Mel Gorman <mel@csn.ul.ie> wrote:
    > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    >
    > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    >
    > At page migration, we replace pte with migration_entry, which has
    > similar format as swap_entry and replace it with real pfn at the
    > end of migration. But there is a race with fork()'s copy_page_range().
    >
    > Assume page migraion on CPU A and fork in CPU B. On CPU A, a page of
    > a process is under migration. On CPU B, a page's pte is under copy.
    >
    >        CPUA                    CPU B
    >                                do_fork()
    >                                copy_mm() (from process 1 to process2)
    >                                insert new vma to mmap_list (if inode/anon_vma)
    >        pte_lock(process1)
    >        unmap a page
    >        insert migration_entry
    >        pte_unlock(process1)
    >
    >        migrate page copy
    >                                copy_page_range
    >        remap new page by rmap_walk()
    >        pte_lock(process2)
    >        found no pte.
    >        pte_unlock(process2)
    >                                pte lock(process2)
    >                                pte lock(process1)
    >                                copy migration entry to process2
    >                                pte unlock(process1)
    >                                pte unlokc(process2)
    >        pte_lock(process1)
    >        replace migration entry
    >        to new page's pte.
    >        pte_unlock(process1)
    >
    > Then, some serialization is necessary. IIUC, this is very rare event but
    > it is reproducible if a lot of migration is happening a lot with the
    > following program running in parallel.
    >
    >    #include <stdio.h>
    >    #include <string.h>
    >    #include <stdlib.h>
    >    #include <sys/mman.h>
    >
    >    #define SIZE (24*1048576UL)
    >    #define CHILDREN 100
    >    int main()
    >    {
    >            int i = 0;
    >            pid_t pids[CHILDREN];
    >            char *buf = mmap(NULL, SIZE, PROT_READ|PROT_WRITE,
    >                            MAP_PRIVATE|MAP_ANONYMOUS,
    >                            0, 0);
    >            if (buf == MAP_FAILED) {
    >                    perror("mmap");
    >                    exit(-1);
    >            }
    >
    >            while (++i) {
    >                    int j = i % CHILDREN;
    >
    >                    if (j == 0) {
    >                            printf("Waiting on children\n");
    >                            for (j = 0; j < CHILDREN; j++) {
    >                                    memset(buf, i, SIZE);
    >                                    if (pids[j] != -1)
    >                                            waitpid(pids[j], NULL, 0);
    >                            }
    >                            j = 0;
    >                    }
    >
    >                    if ((pids[j] = fork()) == 0) {
    >                            memset(buf, i, SIZE);
    >                            exit(EXIT_SUCCESS);
    >                    }
    >            }
    >
    >            munmap(buf, SIZE);
    >    }
    >
    > copy_page_range() can wait for the end of migration.
    >
    > Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    > Signed-off-by: Mel Gorman <mel@csn.ul.ie>
    Reviewed-by : Minchan Kim <minchan.kim@gmail.com>

    --
    Kind regards,
    Minchan Kim
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