On Tue, May 22, 2018 at 07:17:48PM +0200, David Sterba wrote:
> On Tue, May 22, 2018 at 09:47:58AM -0700, Omar Sandoval wrote:
> > From: Omar Sandoval <osandov@xxxxxx>
> >
> > Jun Wu at Facebook reported that an internal service was seeing a return
> > value of 1 from ftruncate() on Btrfs in some cases.
>
> Do you have a reproducer? To estimate how likely is to hit the problem
> in practice.
This reproduces it every time when mounted with compress-force=zstd:
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <unistd.h>
int main() {
int i;
for (i = 0; i < 2; i++) {
char buf[256] = { 0 };
int fd, ret;
char *p;
fd = open("test", O_CREAT | O_WRONLY | O_TRUNC, 0666);
if (fd == -1) {
perror("open");
return EXIT_FAILURE;
}
if (write(fd, buf, sizeof(buf)) != sizeof(buf)) {
perror("write");
close(fd);
return EXIT_FAILURE;
}
close(fd);
fd = open("test", O_RDONLY, 0666);
if (fd == -1) {
perror("open");
return EXIT_FAILURE;
}
p = mmap(NULL, 256, PROT_READ, MAP_SHARED, fd, 0);
if (p == MAP_FAILED) {
perror("mmap");
close(fd);
return EXIT_FAILURE;
}
if (p[0] != 0)
return 1;
close(fd);
fd = open("test", O_WRONLY, 0666);
if (fd == -1) {
perror("open");
return EXIT_FAILURE;
}
ret = ftruncate(fd, 128);
if (ret) {
printf("ftruncate() returned %d\n", ret);
close(fd);
return EXIT_FAILURE;
}
close(fd);
}
return EXIT_SUCCESS;
}
This happens any time NEED_TRUNCATE_BLOCK is returned. The file has to
be inline and compressed, and there's some other condition that I
haven't figured out yet which the mmap() is there for.
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