Hi Al,
On Sun, May 20, 2018 at 05:09:40PM +0200, Enrico Zini wrote:
> Hello,
>
> thank you for your work on Linux. I'm trying to report what looks to me like a
> kernel issue: the code below, if run on a normal file system prints:
>
> After creation: 0/1048576
> After read: 0/1048576
>
> but if run on a tmpfs it prints:
>
> After creation: 0/1048576
> After read: 1048576/1048576
>
> This unexpected allocation happens only when using sendfile(2), not
> while reading the file with read(2).
>
> I observed this behaviour on a range of kernels, from Centos7's 3.10.0
> to 4.16 currently running in my machine.
>
> This is the code to reproduce it:
>
> #include <sys/fcntl.h>
> #include <sys/types.h>
> #include <sys/stat.h>
> #include <sys/sendfile.h>
> #include <unistd.h>
> #include <stdio.h>
>
> int main()
> {
> int fd = open("test", O_WRONLY | O_CREAT, 0644);
> if (fd == -1)
> {
> perror("cannot open for writing");
> return 1;
> }
>
> if (ftruncate(fd, 1024*1024) == -1)
> {
> perror("cannot ftruncate");
> return 1;
> }
>
> close(fd);
>
> struct stat st;
> if (stat("test", &st) == -1)
> {
> perror("cannot stat after creation");
> return 1;
> }
> fprintf(stdout, "After creation: %d/%d\n", st.st_blocks * 512, st.st_size);
>
> fd = open("test", O_RDONLY);
> if (fd == -1)
> {
> perror("cannot open for reading");
> return 1;
> }
>
> int out = open("/dev/null", O_WRONLY);
> if (out == -1)
> {
> perror("cannot open /dev/null");
> return 1;
> }
>
> off_t offset = 0;
> ssize_t res = sendfile(out, fd, &offset, st.st_size);
> if (res == -1)
> {
> perror("cannot sendfile");
> return 1;
> }
> if (res != st.st_size)
> fprintf(stderr, "warning: partial sendfile\n");
>
> if (stat("test", &st) == -1)
> {
> perror("cannot stat after read");
> return 1;
> }
> fprintf(stdout, "After read: %d/%d\n", st.st_blocks * 512, st.st_size);
>
> close(out);
> close(fd);
> unlink("test");
> return 0;
> }
This still seems reproducible in same way as well with 5.10.38. Is
this intentional behaviour in some way?
Note, this was originally reported downstream in Debian as
https://bugs.debian.org/899027 .
Regards,
Salvatore
