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@xxxxxxxxxxxxxxx>
> >
> > 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);
}