On Wed, Aug 21, 2019 at 10:23:49PM +0200, Andreas Grünbacher wrote: > Hi Darrick, > > Am So., 2. Dez. 2018 um 19:13 Uhr schrieb Darrick J. Wong > <darrick.wong@xxxxxxxxxx>: > > From: Darrick J. Wong <darrick.wong@xxxxxxxxxx> > > > > In commit 4721a601099, we tried to fix a problem wherein directio reads > > into a splice pipe will bounce EFAULT/EAGAIN all the way out to > > userspace by simulating a zero-byte short read. This happens because > > some directio read implementations (xfs) will call > > bio_iov_iter_get_pages to grab pipe buffer pages and issue asynchronous > > reads, but as soon as we run out of pipe buffers that _get_pages call > > returns EFAULT, which the splice code translates to EAGAIN and bounces > > out to userspace. > > > > In that commit, the iomap code catches the EFAULT and simulates a > > zero-byte read, but that causes assertion errors on regular splice reads > > because xfs doesn't allow short directio reads. This causes infinite > > splice() loops and assertion failures on generic/095 on overlayfs > > because xfs only permit total success or total failure of a directio > > operation. The underlying issue in the pipe splice code has now been > > fixed by changing the pipe splice loop to avoid avoid reading more data > > than there is space in the pipe. > > > > Therefore, it's no longer necessary to simulate the short directio, so > > remove the hack from iomap. > > > > Fixes: 4721a601099 ("iomap: dio data corruption and spurious errors when pipes fill") > > Reported-by: Amir Goldstein <amir73il@xxxxxxxxx> > > Reviewed-by: Christoph Hellwig <hch@xxxxxx> > > Signed-off-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx> > > --- > > v2: split into two patches per hch request > > --- > > fs/iomap.c | 9 --------- > > 1 file changed, 9 deletions(-) > > > > diff --git a/fs/iomap.c b/fs/iomap.c > > index 3ffb776fbebe..d6bc98ae8d35 100644 > > --- a/fs/iomap.c > > +++ b/fs/iomap.c > > @@ -1877,15 +1877,6 @@ iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter, > > dio->wait_for_completion = true; > > ret = 0; > > } > > - > > - /* > > - * Splicing to pipes can fail on a full pipe. We have to > > - * swallow this to make it look like a short IO > > - * otherwise the higher splice layers will completely > > - * mishandle the error and stop moving data. > > - */ > > - if (ret == -EFAULT) > > - ret = 0; > > break; > > } > > pos += ret; > > I'm afraid this breaks the following test case on xfs and gfs2, the > two current users of iomap_dio_rw. Hmm, I had kinda wondered if regular pipes still needed this help. Evidently we don't have a lot of splice tests in fstests. :( > Here, the splice system call fails with errno = EAGAIN when trying to > "move data" from a file opened with O_DIRECT into a pipe. > > The test case can be run with option -d to not use O_DIRECT, which > makes the test succeed. > > The -r option switches from reading from the pipe sequentially to > reading concurrently with the splice, which doesn't change the > behavior. > > Any thoughts? This would be great as an xfstest! :) Do you have one ready to go, or should I just make one from the source code? --D > Thanks, > Andreas > > =================================== 8< =================================== > #define _GNU_SOURCE > #include <sys/types.h> > #include <sys/stat.h> > #include <sys/wait.h> > #include <unistd.h> > #include <fcntl.h> > #include <err.h> > > #include <stdlib.h> > #include <stdio.h> > #include <stdbool.h> > #include <string.h> > #include <errno.h> > > #define SECTOR_SIZE 512 > #define BUFFER_SIZE (150 * SECTOR_SIZE) > > void read_from_pipe(int fd, const char *filename, size_t size) > { > char buffer[SECTOR_SIZE]; > size_t sz; > ssize_t ret; > > while (size) { > sz = size; > if (sz > sizeof buffer) > sz = sizeof buffer; > ret = read(fd, buffer, sz); > if (ret < 0) > err(1, "read: %s", filename); > if (ret == 0) { > fprintf(stderr, "read: %s: unexpected EOF\n", filename); > exit(1); > } > size -= sz; > } > } > > void do_splice1(int fd, const char *filename, size_t size) > { > bool retried = false; > int pipefd[2]; > > if (pipe(pipefd) == -1) > err(1, "pipe"); > while (size) { > ssize_t spliced; > > spliced = splice(fd, NULL, pipefd[1], NULL, size, SPLICE_F_MOVE); > if (spliced == -1) { > if (errno == EAGAIN && !retried) { > retried = true; > fprintf(stderr, "retrying splice\n"); > sleep(1); > continue; > } > err(1, "splice"); > } > read_from_pipe(pipefd[0], filename, spliced); > size -= spliced; > } > close(pipefd[0]); > close(pipefd[1]); > } > > void do_splice2(int fd, const char *filename, size_t size) > { > bool retried = false; > int pipefd[2]; > int pid; > > if (pipe(pipefd) == -1) > err(1, "pipe"); > > pid = fork(); > if (pid == 0) { > close(pipefd[1]); > read_from_pipe(pipefd[0], filename, size); > exit(0); > } else { > close(pipefd[0]); > while (size) { > ssize_t spliced; > > spliced = splice(fd, NULL, pipefd[1], NULL, size, SPLICE_F_MOVE); > if (spliced == -1) { > if (errno == EAGAIN && !retried) { > retried = true; > fprintf(stderr, "retrying splice\n"); > sleep(1); > continue; > } > err(1, "splice"); > } > size -= spliced; > } > close(pipefd[1]); > waitpid(pid, NULL, 0); > } > } > > void usage(const char *argv0) > { > fprintf(stderr, "USAGE: %s [-rd] {filename}\n", basename(argv0)); > exit(2); > } > > int main(int argc, char *argv[]) > { > void (*do_splice)(int fd, const char *filename, size_t size); > const char *filename; > char *buffer; > int opt, open_flags, fd; > ssize_t ret; > > do_splice = do_splice1; > open_flags = O_CREAT | O_TRUNC | O_RDWR | O_DIRECT; > > while ((opt = getopt(argc, argv, "rd")) != -1) { > switch(opt) { > case 'r': > do_splice = do_splice2; > break; > case 'd': > open_flags &= ~O_DIRECT; > break; > default: /* '?' */ > usage(argv[0]); > } > } > > if (optind >= argc) > usage(argv[0]); > filename = argv[optind]; > > printf("%s reader %s O_DIRECT\n", > do_splice == do_splice1 ? "sequential" : "concurrent", > (open_flags & O_DIRECT) ? "with" : "without"); > > buffer = aligned_alloc(SECTOR_SIZE, BUFFER_SIZE); > if (buffer == NULL) > err(1, "aligned_alloc"); > > fd = open(filename, open_flags, 0666); > if (fd == -1) > err(1, "open: %s", filename); > > memset(buffer, 'x', BUFFER_SIZE); > ret = write(fd, buffer, BUFFER_SIZE); > if (ret < 0) > err(1, "write: %s", filename); > if (ret != BUFFER_SIZE) { > fprintf(stderr, "%s: short write\n", filename); > exit(1); > } > > ret = lseek(fd, 0, SEEK_SET); > if (ret != 0) > err(1, "lseek: %s", filename); > > do_splice(fd, filename, BUFFER_SIZE); > > if (unlink(filename) == -1) > err(1, "unlink: %s", filename); > > return 0; > } > =================================== 8< ===================================