'stable' pages have always been a bit of a fiction. It's easy to
intentionally modify stable pages under io with some help from page
references that ignore mappings and page state.
Here's little test that uses O_DIRECT to get the pinned aio ring pages
under IO and then has event completion stores modify them while they're
in flight.
It's a nice quick way to test the consequences of stable pages being
modified. It can be used to burp out ratelimited csum failure kernel
messages with btrfs, for example.
- z
#define _GNU_SOURCE
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <limits.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/uio.h>
#include <assert.h>
#include <libaio.h>
int main(int argc, char **argv)
{
size_t total = 1 * 1024 * 1024;
size_t page_size = sysconf(_SC_PAGESIZE);
struct iovec *iov;
size_t iov_nr = total / page_size;
void *junk;
io_context_t ctx = NULL;
int nr_iocbs = 3;
struct iocb iocbs[nr_iocbs];
struct iocb *iocb_ptrs[nr_iocbs];
struct io_event events[nr_iocbs];
int ret;
int fd;
int nr;
int i;
if (argc != 2) {
fprintf(stderr, "usage: %s <file_to_overwrite>\n", argv[0]);
exit(1);
}
iov = calloc(iov_nr, sizeof(*iov));
junk = malloc(total);
assert(iov && junk);
fd = open(argv[1], O_RDWR|O_CREAT|O_DIRECT, 0644);
assert(fd >= 0);
ret = io_setup(nr_iocbs, &ctx);
assert(ret >= 0);
for (i = 0; i < iov_nr; i++) {
iov[i].iov_base = ctx;
iov[i].iov_len = page_size;
}
/* initial write to allocate the file region */
ret = writev(fd, iov, iov_nr);
assert(ret == total);
/*
* Keep one of each of these iocbs in flight:
*
* [0]: hopefully fast 0 byte read to keep churning events
* [1]: dio read of file bytes to trigger csum verification
* [2]: dio write of unstable event pages
*/
io_prep_pread(&iocbs[0], fd, junk, 0, 0);
io_prep_pread(&iocbs[1], fd, junk, total, 0);
io_prep_pwritev(&iocbs[2], fd, iov, iov_nr, 0);
for (i = 0; i < nr_iocbs; i++)
iocb_ptrs[i] = &iocbs[i];
nr = nr_iocbs;
for(;;) {
ret = io_submit(ctx, nr, iocb_ptrs);
assert(ret == nr);
nr = io_getevents(ctx, 1, nr_iocbs, events, NULL);
assert(nr > 0);
for (i = 0; i < nr; i++)
iocb_ptrs[i] = events[i].obj;
}
return 0;
}
--
To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
