On Wed, Jul 23, 2014 at 09:52:22AM -0400, Matthew Wilcox wrote:
> 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).
See below.
>
> > 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()?
The most straight-forward way: extract body of pte cycle from
zap_pte_range() to separate function -- zap_pte() -- and use it.
> #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) {
Shouldn't this be ftruncate(fd, 0)? Otherwise the memcpy() below will
fault in page from backing storage, not hole and write will not replace
anything.
> 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);
> }
--
Kirill A. Shutemov
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