Re: [PATCH v8 05/22] Add vm_replace_mixed()

[Matthew Wilcox]

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);
}