On Tue, Aug 11, 2020 at 12:23:31AM +0800, Ming Lei wrote:
On Mon, Aug 10, 2020 at 04:52:17PM +0900, Tetsuo Handa wrote:
On 2020/08/10 12:19, Ming Lei wrote:
Block layer usually doesn't support or allow zero-length bvec. Since commit 1bdc76aea115 ("iov_iter: use bvec iterator to implement iterate_bvec()"), iterate_bvec() switches to bvec iterator. However, Al mentioned that 'Zero-length segments are not disallowed' in iov_iter.
Fixes for_each_bvec() so that it can move on after seeing one zero length bvec.
Signed-off-by: Ming Lei ming.lei@redhat.com Link: https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg2262077.html Fixes: 1bdc76aea115 ("iov_iter: use bvec iterator to implement iterate_bvec()")
Is this Fixes: correct? That commit should be in RHEL8's 4.18 kernel but that kernel does not hit this bug.
Yeah, it is correct, see the following link:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?h...
Commit 1bdc76aea115 was merged to v4.8, so it is definitely in both RHEL8's 4.18 based kernel and upstream kernel.
Moreover, maybe nobody cares, but behavior of splice() differs when there are only zero-length pages. With this fix, splice() returns 0 despite there is still pipe writers.
It is another new issue, which isn't related with Commit 1bdc76aea115, see below.
Man page seems to say that splice() returns 0 when there is no pipe writers...
A return value of 0 means end of input. If fd_in refers to a pipe, then this means that there was no data to transfer, and it would not make sense to block because there are no writers connected to the write end of the pipe.----- test case ----- #define _GNU_SOURCE #include <stdio.h> #include <sys/types.h> #include <sys/stat.h> #include <fcntl.h> #include <unistd.h> #include <string.h>
int main(int argc, char *argv[]) { static char buffer[4096]; const int fd = open("/tmp/testfile", O_WRONLY | O_CREAT, 0600); int pipe_fd[2] = { EOF, EOF }; pipe(pipe_fd); write(pipe_fd[1], NULL, sizeof(buffer)); write(pipe_fd[1], NULL, sizeof(buffer)); memset(buffer, 'a', sizeof(buffer)); //write(pipe_fd[1], buffer, sizeof(buffer)); write(pipe_fd[1], NULL, sizeof(buffer)); write(pipe_fd[1], NULL, sizeof(buffer)); memset(buffer, 'b', sizeof(buffer)); //write(pipe_fd[1], buffer, sizeof(buffer)); write(pipe_fd[1], NULL, sizeof(buffer)); write(pipe_fd[1], NULL, sizeof(buffer)); memset(buffer, 'c', sizeof(buffer)); //write(pipe_fd[1], buffer, sizeof(buffer)); write(pipe_fd[1], NULL, sizeof(buffer)); write(pipe_fd[1], NULL, sizeof(buffer)); memset(buffer, 'd', sizeof(buffer)); //write(pipe_fd[1], buffer, sizeof(buffer)); write(pipe_fd[1], NULL, sizeof(buffer)); write(pipe_fd[1], NULL, sizeof(buffer)); splice(pipe_fd[0], NULL, fd, NULL, 65536, 0); return 0; }
The above test doesn't trigger the reported lockup issue, so this patch isn't related with the new issue you described.
BTW, for_each_bvec won't be called in the above splice test code.
Thanks, Ming