kernel: add CLONE_IO to specifically request sharing of IO contexts

syslets (or other threads/processes that want io context sharing) can
set this to enforce sharing of io context.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

2 files changed, 11 insertions, 4 deletions

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 80837e7d527e..2d0546e884ea 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -27,6 +27,7 @@
 #define CLONE_NEWUSER           0x10000000      /* New user namespace */
 #define CLONE_NEWPID            0x20000000      /* New pid namespace */
 #define CLONE_NEWNET            0x40000000      /* New network namespace */
+#define CLONE_IO                0x80000000      /* Clone io context */
 
 /*
  * Scheduling policies
diff --git a/kernel/fork.c b/kernel/fork.c
index 1987c57abb08..314f5101d2b0 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -792,15 +792,21 @@ out:
         return error;
 }
 
-static int copy_io(struct task_struct *tsk)
+static int copy_io(unsigned long clone_flags, struct task_struct *tsk)
 {
 #ifdef CONFIG_BLOCK
         struct io_context *ioc = current->io_context;
 
         if (!ioc)
                 return 0;
-
-        if (ioprio_valid(ioc->ioprio)) {
+        /*
+         * Share io context with parent, if CLONE_IO is set
+         */
+        if (clone_flags & CLONE_IO) {
+                tsk->io_context = ioc_task_link(ioc);
+                if (unlikely(!tsk->io_context))
+                        return -ENOMEM;
+        } else if (ioprio_valid(ioc->ioprio)) {
                 tsk->io_context = alloc_io_context(GFP_KERNEL, -1);
                 if (unlikely(!tsk->io_context))
                         return -ENOMEM;
@@ -1176,7 +1182,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
                 goto bad_fork_cleanup_mm;
         if ((retval = copy_namespaces(clone_flags, p)))
                 goto bad_fork_cleanup_keys;
-        if ((retval = copy_io(p)))
+        if ((retval = copy_io(clone_flags, p)))
                 goto bad_fork_cleanup_namespaces;
         retval = copy_thread(0, clone_flags, stack_start, stack_size, p, regs);
         if (retval)