Re: [Qemu-devel] [PATCH] Ensure migrate_cancel does not block doing I/O

[Daniel P. Berrange]

On Fri, Aug 26, 2011 at 11:59:28AM +0100, Daniel P. Berrange wrote:
> From: "Daniel P. Berrange" <address@hidden>
> 
> There are two common cases where migrate_cancel is intended to be
> used
> 
>   1. When migration is not converging due to an overactive
>      guest and insufficient network bandwidth
>   2. When migration is stuck due a network outage, waiting
>      for the TCP transmit timeout to occurr & return an I/O
>      error for send()
> 
> In the second case, if you attempt to use 'migrate_cancel' it
> will also get stuck. This can be seen by attempting to migrate
> to a QEMU which has been SIGSTOP'd
> 
>   $ ./x86_64-softmmu/qemu-system-x86_64 -cdrom ~/boot.iso -m 600 \
>        -monitor stdio -vnc :2 -incoming tcp:localhost:9000
>    QEMU 0.14.1 monitor - type 'help' for more information
>    (qemu)
>    <Ctrl-Z>
>    [1]+  Stopped
> 
> And in another shell
> 
>   $ ./x86_64-softmmu/qemu-system-x86_64 -cdrom ~/boot.iso -m 600 \
>         -monitor stdio -vnc :1
>    QEMU 0.14.1 monitor - type 'help' for more information
>    (qemu) migrate -d tcp:localhost:9000
>    (qemu) info migrate
>    Migration status: active
>    transferred ram: 416 kbytes
>    remaining ram: 621624 kbytes
>    total ram: 623040 kbytes
>    (qemu) migrate_cancel
> 
> This last command will never return, until the first QEMU is
> resumed. Looking at the stack trace in GDB you see
> 
>  #0  0x0000003a8320e4c2 in __libc_send (fd=10, buf=0x1bc7c70, n=19777, 
> flags=0)
>     at ../sysdeps/unix/sysv/linux/x86_64/send.c:28
>  #1  0x000000000048fb1e in socket_write (s=<optimized out>, buf=<optimized 
> out>, size=<optimized out>)
>     at migration-tcp.c:39
>  #2  0x000000000048eba4 in migrate_fd_put_buffer (opaque=0x1b76ad0, 
> data=0x1bc7c70, size=19777)
>     at migration.c:324
>  #3  0x000000000048e442 in buffered_flush (s=0x1b76b90) at buffered_file.c:87
>  #4  0x000000000048e4cf in buffered_close (opaque=0x1b76b90) at 
> buffered_file.c:177
>  #5  0x0000000000496d57 in qemu_fclose (f=0x1bbfc10) at savevm.c:479
>  #6  0x000000000048f4ca in migrate_fd_cleanup (s=0x1b76ad0) at migration.c:291
>  #7  0x000000000048f035 in do_migrate_cancel (mon=<optimized out>, 
> qdict=<optimized out>,
>     ret_data=<optimized out>) at migration.c:136[snip]
>  [snip]
> 
> The migration_fd_cleanup method is where the problem really starts.
> Specifically it does
> 
>     if (s->file) {
>         DPRINTF("closing file\n");
>         if (qemu_fclose(s->file) != 0) {
>             ret = -1;
>         }
>         s->file = NULL;
>     }
> 
>     if (s->fd != -1)
>         close(s->fd);
> 
> And gets stuck in the qemu_fclose() bit because that method (rightly) tries
> to flush all outstanding buffers before closing. Unfortunately while this is
> desirable when migration ends successfully, it is undesirable when we are
> failing/cancelling migration.
> 
> It is hard to tell qemu_fclose() that it shouldn't flush buffers directly,
> so the alternative is to ensure that this method fails quickly when it
> attempts I/O. This is easily achieved, simply by closing 's->fd' before
> calling qemu_fclose.
> 
> Signed-off-by: Daniel P. Berrange <address@hidden>
> ---
>  migration.c |    7 +++++++
>  1 files changed, 7 insertions(+), 0 deletions(-)
> 
> diff --git a/migration.c b/migration.c
> index f5959b4..a432c3b 100644
> --- a/migration.c
> +++ b/migration.c
> @@ -286,6 +286,13 @@ int migrate_fd_cleanup(FdMigrationState *s)
>  
>      qemu_set_fd_handler2(s->fd, NULL, NULL, NULL, NULL);
>  
> +    if ((s->state == MIG_STATE_ERROR ||
> +         s->state == MIG_STATE_CANCELLED) &&
> +        s->fd != -1) {
> +        close(s->fd);
> +        s->fd = -1;
> +    }
> +
>      if (s->file) {
>          DPRINTF("closing file\n");
>          if (qemu_fclose(s->file) != 0) {

This approach results in 'socket_write' doing a send(-1) which
gets back a EBADF errno, causing the flush to abort.  An alternative
approach is to make the migrate_fd_put_buffer short-circuit the
send() call by checking the migration state thus:

diff --git a/migration.c b/migration.c
index f5959b4..6448d0b 100644
--- a/migration.c
+++ b/migration.c
@@ -319,6 +319,11 @@ ssize_t migrate_fd_put_buffer(void *opaque, const void 
*data, size_t size)
     FdMigrationState *s = opaque;
     ssize_t ret;
 
+    if (s->state == MIG_STATE_ERROR ||
+        s->state == MIG_STATE_CANCELLED) {
+        return -EIO;
+    }
+
     do {
         ret = s->write(s, data, size);
     } while (ret == -1 && ((s->get_error(s)) == EINTR));


I think I slightly prefer this second option, since it avoids the EBADF
scenario. Other opinions ?

Regards,
Daniel
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