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    SubjectRe: [PATCH v8 05/22] Add vm_replace_mixed()
    On Wed, Jul 23, 2014 at 02:45:40PM +0300, Kirill A. Shutemov wrote:
    > On Tue, Jul 22, 2014 at 03:47:53PM -0400, Matthew Wilcox wrote:
    > > From: Matthew Wilcox <willy@linux.intel.com>
    > >
    > > vm_insert_mixed() will fail if there is already a valid PTE at that
    > > location. The DAX code would rather replace the previous value with
    > > the new PTE.

    > > @@ -1492,8 +1492,12 @@ static int insert_page(struct vm_area_struct *vma, unsigned long addr,
    > > if (!pte)
    > > goto out;
    > > retval = -EBUSY;
    > > - if (!pte_none(*pte))
    > > - goto out_unlock;
    > > + if (!pte_none(*pte)) {
    > > + if (!replace)
    > > + goto out_unlock;
    > > + VM_BUG_ON(!mutex_is_locked(&vma->vm_file->f_mapping->i_mmap_mutex));
    > > + zap_page_range_single(vma, addr, PAGE_SIZE, NULL);
    >
    > zap_page_range_single() takes ptl by itself in zap_pte_range(). It's not
    > going to work.

    I have a test program that exercises this path ... it seems to work!
    Following the code, I don't understand why it does. Maybe it's not
    exercising this path after all? I've attached the program (so that I
    have an "oh, duh" moment about 5 seconds after sending the email).

    > And zap_page_range*() is pretty heavy weapon to shoot down one pte, which
    > we already have pointer to. Why?

    I'd love to use a lighter-weight weapon! What would you recommend using,
    zap_pte_range()?

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

    int
    main(int argc, char *argv[])
    {
    int fd;
    void *addr;
    char buf[4096];

    if (argc != 2) {
    fprintf(stderr, "usage: %s filename\n", argv[0]);
    exit(1);
    }

    if ((fd = open(argv[1], O_CREAT|O_RDWR, 0666)) < 0) {
    perror(argv[1]);
    exit(1);
    }

    if (ftruncate(fd, 4096) < 0) {
    perror("ftruncate");
    exit(1);
    }

    if ((addr = mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED,
    fd, 0)) == MAP_FAILED) {
    perror("mmap");
    exit(1);
    }

    close(fd);

    /* first read */
    memcpy(buf, addr, 4096);

    /* now write a bit */
    memcpy(addr, buf, 8);

    printf("%s: test passed.\n", argv[0]);
    exit(0);
    }
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