[Devel] Re: [RFC][PATCH][cryo] Save/restore state of unnamed pipes
Matt Helsley
matthltc at us.ibm.com
Tue Jun 17 16:31:12 PDT 2008
On Tue, 2008-06-17 at 17:30 -0500, Serge E. Hallyn wrote:
> Quoting sukadev at us.ibm.com (sukadev at us.ibm.com):
> >
> > >From fd13986de32af31621b1badbcf7bfb5626648e0e Mon Sep 17 00:00:00 2001
> > From: Sukadev Bhattiprolu <sukadev at linux.vnet.ibm.com>
> > 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 at us.ibm.com>
> > ---
> > 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:
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. :(
Cheers,
-Matt
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