> > Just using cp to read the file is enough to cause problems but I included
> > a very basic program below that produces the BUG_ON checks. Is this a known
> > issue or am I using the interface incorrectly?
>
> I'd say you're using it correctly but you've found a hitherto unknown bug.
> On i386 highmem machines with CONFIG_HIGHPTE (at least) pte_offset_map()
> takes kmap_atomic(), so pagemap_pte_range() can't do copy_to_user() as it
> presently does.
>
> Drat.
>
> Still, that shouldn't really disrupt the testing which you're doing. You
> could disable CONFIG_HIGHPTE to shut it up.
>
Yes, that did the trick. Using pagemap, it was trivial to show that the
2.6.24-rc5-mm1 kernel was placing pages in reverse physical order like
the following output shows
b: 32763 v: 753091 p: 65559 . 65558 contig: 1
b: 32764 v: 753092 p: 65558 . 65557 contig: 1
b: 32765 v: 753093 p: 65557 . 65556 contig: 1
b: 32766 v: 753094 p: 65556 . 65555 contig: 1
b: 32767 v: 753095 p: 65555 . 65555 contig: 1
p: is the PFN of the page v: is the page offset within an anonymous
mapping and b: is the number of non-contiguous blocks in the anonymous
mapping. With the patch applied, it looks more like;
b: 1232 v: 752964 p: 58944 ................ 87328 contig: 15
b: 1233 v: 752980 p: 87328 ................ 91200 contig: 15
b: 1234 v: 752996 p: 91200 ................ 40272 contig: 15
b: 1235 v: 753012 p: 40272 ................ 85664 contig: 15
b: 1236 v: 753028 p: 85664 ................ 87312 contig: 15
so mappings are using contiguous pages again. This was the final test
program I used in case it's of any interest.
Thanks
/*
* showcontiguous.c
*
* Use the /proc/pid/pagemap interface to give an indication of how contiguous
* physical memory is in an anonymous virtual memory mapping
*/
#include <stdio.h>
#include <sys/mman.h>
#include <stdlib.h>
#include <unistd.h>
#include <linux/types.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#define MAPSIZE (128*1048576)
#define PM_ENTRY_BYTES sizeof(__u64)
int main(int argc, char **argv)
{
int pagemap_fd;
unsigned long *anonmapping;
__u64 pagemap_entry = 0ULL;
unsigned long vpfn, ppfn, ppfn_last;
int block_number = 0;
int contig_count = 1;
size_t mmap_offset;
int pagesize = getpagesize();
if (sizeof(pagemap_entry) < PM_ENTRY_BYTES) {
printf("ERROR: Failed assumption on size of pagemap_entry\n");
exit(EXIT_FAILURE);
}
/* Open the pagemap interface */
pagemap_fd = open("/proc/self/pagemap", O_RDONLY);
if (pagemap_fd == -1) {
perror("fopen");
exit(EXIT_FAILURE);
}
/* Create an anonymous mapping */
anonmapping = mmap(NULL, MAPSIZE,
PROT_READ|PROT_WRITE,
MAP_PRIVATE|MAP_ANONYMOUS|MAP_POPULATE,
-1, 0);
if (anonmapping == MAP_FAILED) {
perror("mmap");
exit(1);
}
/* Work out the VPN the mapping is at and seek to it in pagemap */
vpfn = ((unsigned long)anonmapping) / pagesize;
mmap_offset = lseek(pagemap_fd, vpfn * PM_ENTRY_BYTES, SEEK_SET);
if (mmap_offset == -1) {
perror("fseek");
exit(EXIT_FAILURE);
}
ppfn_last = 0;
/* Read the PFN of each page in the mapping */
for (mmap_offset = 0; mmap_offset < MAPSIZE; mmap_offset += pagesize) {
vpfn = ((unsigned long)anonmapping + mmap_offset) / pagesize;
if (read(pagemap_fd, &pagemap_entry, PM_ENTRY_BYTES) == 0) {
perror("fread");
exit(EXIT_FAILURE);
}
ppfn = (unsigned long)pagemap_entry;
if (ppfn == ppfn_last + 1) {
printf(".");
contig_count++;
} else {
printf(" %lu contig: %d\nb: %6d v: %8lu p: %8lu .",
ppfn, contig_count,
block_number, vpfn, ppfn);
contig_count = 1;
block_number++;
}
ppfn_last = ppfn;
}
printf(" %lu config: %d\n", ppfn, contig_count);
close(pagemap_fd);
munmap(anonmapping, MAPSIZE);
exit(EXIT_SUCCESS);
}
--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab
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