Matt Helsley [matthltc@xxxxxxxxxx] wrote: | | On Tue, 2008-06-17 at 17:30 -0500, Serge E. Hallyn wrote: | > Quoting sukadev@xxxxxxxxxx (sukadev@xxxxxxxxxx): | > > | > > >From fd13986de32af31621b1badbcf7bfb5626648e0e Mon Sep 17 00:00:00 2001 | > > From: Sukadev Bhattiprolu <sukadev@xxxxxxxxxxxxxxxxxx> | > > Date: Mon, 16 Jun 2008 18:41:05 -0700 | > > Subject: [PATCH] Save/restore state of unnamed pipes | > > | > > Design: | > > | > > Current Linux kernels provide ability to read/write contents of FIFOs | > > using /proc. i.e 'cat /proc/pid/fd/read-side-fd' prints the unread data | > > in the FIFO. Similarly, 'cat foo > /proc/pid/fd/read-sid-fd' appends | > > the contents of 'foo' to the unread contents of the FIFO. | > > | > > So to save/restore the state of the pipe, a simple implementation is | > > to read the from the unnamed pipe's fd and save to the checkpoint-file. | > > When restoring, create a pipe (using PT_PIPE()) in the child process, | > > read the contents of the pipe from the checkpoint file and write it to | > > the newly created pipe. | > > | > > Its fairly straightforward, except for couple of notes: | > > | > > - when we read contents of '/proc/pid/fd/read-side-fd' we drain | > > the pipe such that when the checkpointed application resumes, | > > it will not find any data. To fix this, we read from the | > > 'read-side-fd' and write it back to the 'read-side-fd' in | > > addition to writing to the checkpoint file. | > > | > > - there does not seem to be a mechanism to determine the count | > > of unread bytes in the file. Current implmentation assumes a | > > maximum of 64K bytes (PIPE_BUFS * PAGE_SIZE on i386) and fails | > > if the pipe is not fully drained. | > > | > > Basic unit-testing done at this point (using tests/pipe.c). | > > | > > TODO: | > > - Additional testing (with multiple-processes and multiple-pipes) | > > - Named-pipes | > > | > > Signed-off-by: Sukadev Bhattiprolu <sukadev@xxxxxxxxxx> | > > --- | > > cr.c | 215 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++---- | > > 1 files changed, 203 insertions(+), 12 deletions(-) | > > | > > diff --git a/cr.c b/cr.c | > > index 5163a3d..0cb9774 100644 | > > --- a/cr.c | > > +++ b/cr.c | > > @@ -84,6 +84,11 @@ typedef struct fdinfo_t { | > > char name[128]; /* file name. NULL if anonymous (pipe, socketpair) */ | > > } fdinfo_t; | > > | > > +typedef struct fifoinfo_t { | > > + int fi_fd; /* fifo's read-side fd */ | > > + int fi_length; /* number of bytes in the fifo */ | > > +} fifofdinfo_t; | > > + | > > typedef struct memseg_t { | > > unsigned long start; /* memory segment start address */ | > > unsigned long end; /* memory segment end address */ | > > @@ -468,6 +473,128 @@ out: | > > return rc; | > > } | > > | > > +static int estimate_fifo_unread_bytes(pinfo_t *pi, int fd) | > > +{ | > > + /* | > > + * Is there a way to find the number of bytes remaining to be | > > + * read in a fifo ? If not, can we print it in fdinfo ? | > > + * | > > + * Return 64K (PIPE_BUFS * PAGE_SIZE) for now. | > > + */ | > > + return 65536; | > > +} | > > + | > > +static void ensure_fifo_has_drained(char *fname, int fifo_fd) | > > +{ | > > + int rc, c; | > > + | > > + rc = read(fifo_fd, &c, 1); | > > + if (rc != -1 && errno != EAGAIN) { | > | > Won't errno only be set if rc == -1? Did you mean || here? | > | > > + ERROR("FIFO '%s' not drained fully. rc %d, c %d " | > > + "errno %d\n", fname, rc, c, errno); | > > + } | > > + | > > +} | > > + | > > +static int save_process_fifo_info(pinfo_t *pi, int fd) | > > +{ | > > + int i; | > > + int rc; | > > + int nbytes; | > > + int fifo_fd; | > > + int pbuf_size; | > > + pid_t pid = pi->pid; | > > + char fname[256]; | > > + fdinfo_t *fi = pi->fi; | > > + char *pbuf; | > > + fifofdinfo_t fifofdinfo; | > > + | > > + write_item(fd, "FIFO", NULL, 0); | > > + | > > + for (i = 0; i < pi->nf; i++) { | > > + if (! S_ISFIFO(fi[i].mode)) | > > + continue; | > > + | > > + DEBUG("FIFO fd %d (%s), flag 0x%x\n", fi[i].fdnum, fi[i].name, | > > + fi[i].flag); | > > + | > > + if (!(fi[i].flag & O_WRONLY)) | > > + continue; | > > + | > > + pbuf_size = estimate_fifo_unread_bytes(pi, fd); | > > + | > > + pbuf = (char *)malloc(pbuf_size); | > > + if (!pbuf) { | > > + ERROR("Unable to allocate FIFO buffer of size %d\n", | > > + pbuf_size); | > > + } | > > + memset(pbuf, 0, pbuf_size); | > > + | > > + sprintf(fname, "/proc/%u/fd/%u", pid, fi[i].fdnum); | > > + | > > + /* | > > + * Open O_NONBLOCK so read does not block if fifo has fewer | > > + * bytes than our estimate. | > > + */ | > > + fifo_fd = open(fname, O_RDWR|O_NONBLOCK); | > > + if (fifo_fd < 0) | > > + ERROR("Error %d opening FIFO '%s'\n", errno, fname); | > > + | > > + nbytes = read(fifo_fd, pbuf, pbuf_size); | > > + if (nbytes < 0) { | > > + if (errno != EAGAIN) { | > > + ERROR("Error %d reading FIFO '%s'\n", errno, | > > + fname); | > > + } | > > + nbytes = 0; /* empty fifo */ | > > + } | > > + | > > + /* | > > + * Ensure FIFO has been drained. | > > + * | > > + * TODO: If FIFO has not fully drained, our estimate of | > > + * unread-bytes is wrong. We could: | > > + * | > > + * - have kernel print exact number of unread-bytes | > > + * in /proc/pid/fdinfo/<fd> | > > + * | > > + * - read in contents multiple times and write multiple | > > + * fifobufs or assemble them into a single, large | > > + * buffer. | > > + */ | > > + ensure_fifo_has_drained(fname, fifo_fd); | > > + | > > + /* | > > + * Save FIFO data to checkpoint file | > > + */ | > > + fifofdinfo.fi_fd = fi[i].fdnum; | > > + fifofdinfo.fi_length = nbytes; | > > + write_item(fd, "fifofdinfo", &fifofdinfo, sizeof(fifofdinfo)); | > > + | > > + if (nbytes) { | > > + write_item(fd, "fifobufs", pbuf, nbytes); | > > + | > > + /* | > > + * Restore FIFO's contents so checkpointed application | > > + * won't miss a thing. | > > + */ | > > + errno = 0; | > > + rc = write(fifo_fd, pbuf, nbytes); | > > + if (rc != nbytes) { | > > + ERROR("Wrote-back only %d of %d bytes to FIFO, " | > > + "error %d\n", rc, nbytes, errno); | > > + } | > > + } | > > + | > > + close(fifo_fd); | > > + free(pbuf); | > > + } | > > + | > > + write_item(fd, "END FIFO", NULL, 0); | > > + | > > + return 0; | > > +} | > > + | > > static int save_process_data(pid_t pid, int fd, lh_list_t *ptree) | > > { | > > char fname[256], exe[256], cwd[256], *argv, *env, *buf; | > > @@ -587,6 +714,8 @@ static int save_process_data(pid_t pid, int fd, lh_list_t *ptree) | > > } | > > write_item(fd, "END FD", NULL, 0); | > > | > > + save_process_fifo_info(pi, fd); | > > + | > > /* sockets */ | > > write_item(fd, "SOCK", NULL, 0); | > > for (i = 0; i < pi->ns; i++) | > > @@ -839,6 +968,29 @@ int restore_fd(int fd, pid_t pid) | > > } | > > if (pfd != fdinfo->fdnum) t_d(PT_CLOSE(pid, pfd)); | > > } | > > + } else if (S_ISFIFO(fdinfo->mode)) { | > > + int pipefds[2] = { 0, 0 }; | > > + | > > + /* | > > + * We create the pipe when we see the pipe's read-fd. | > > + * Just ignore the pipe's write-fd. | > > + */ | > > + if (fdinfo->flag == O_WRONLY) | > > + continue; | > > + | > > + DEBUG("Creating pipe for fd %d\n", fdinfo->fdnum); | > > + | > > + t_d(PT_PIPE(pid, pipefds)); | > > + t_d(pipefds[0]); | > > + t_d(pipefds[1]); | > > + | > > + if (pipefds[0] != fdinfo->fdnum) { | > > + DEBUG("Hmm, new pipe has fds %d, %d " | > > + "Old pipe had fd %d\n", pipefds[0], | > > + pipefds[1], fdinfo->fdnum); getchar(); | > | > Can you explain what you're doing here? I would have expected you to | > dup2() to get back the correct fd, so maybe I'm missing something... | | Yes, I agree. | | Though I wonder if it's possible that the two fds returned could be | swapped during restart. Does anyone know if POSIX makes any guarantees | about the numeric relationship between pipefds[0] and pipefds[1] (like | "pipefds[0] < pipefds[1]")? If there are no guarantees then it may be | possible for a simple dup2() to break the new pipe. Suppose, for | example, that the original pipe used fds 4 and 5 in elements 0 and 1 of | the fd array respectively and then we restart: Yes, I was just thinking about this assumption and was wondering if I could find the peer fd by walking the list of fds in /proc/pid/fd and doing an lstat() and comparing the inode numbers. Then save the peer fd in fdinf. On restore, we could create the pipe and dup2() both read and write-side fds. | | | t_d(PT_PIPE(pid, pipefds)); /* returns 5 and 4 in elements 0 and 1 */ | if (pipefds[0] != fdinfo->fdnum) | PT_DUP2(pid, pipefds[0], fdinfo->fdnum); /* accidentally closes | pipefds[1] */ | | | I don't see anything in the pipe man page, at least, that suggests we | can safely assume pipefds[0] < pipefds[1]. | | The solution could be to use "trampoline" fds. Suppose last_fd is the | largest fd that exists in the final checkpointed/restarting application. | We could do (Skipping the PT_FUNC "notation" for clarity): | | | pipe(pipefds); /* returns 5 and 4 in elements 0 and 1 */ | /* use fds after last_fd as trampolines for fds we want to create */ | dup2(pipefds[0], last_fd + 1); | dup2(pipefds[1], last_fd + 2); | close(pipefds[0]); | close(pipefds[1]); | dup2(last_fd + 1, <orig pipefd[0]>); | dup2(last_fd + 2, <orig pipefd[1]>); | close(last_fd + 1); | close(last_fd + 2); | | | Which is alot more code but should work no matter which fds we get back | from pipe(). Of course this assumes the checkpointed application hasn't | used all of its fds. :( | This sounds like a good idea too, but we could use any fd that has not yet been used in the restart-process right ? It would break if all fds are used AND one of the pipe fds is the very last one :-) In that case, we could maybe create all pipe fds first and then go back to creating the rest ? _______________________________________________ Containers mailing list Containers@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/containers