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SubjectRe: [PATCH 1/6] ioprio: move io priority from task_struct to io_context
On Wed, Jan 23 2008, Andrew Morton wrote:
> > On Tue, 22 Jan 2008 10:49:16 +0100 Jens Axboe <jens.axboe@oracle.com> wrote:
> > This is where it belongs and then it doesn't take up space for a
> > process that doesn't do IO.
> >
> > ...
> >
> > struct io_context *get_io_context(gfp_t gfp_flags, int node)
> > {
> > - struct io_context *ret;
> > - ret = current_io_context(gfp_flags, node);
> > - if (likely(ret))
> > - atomic_inc(&ret->refcount);
> > + struct io_context *ret = NULL;
> > +
> > + do {
> > + ret = current_io_context(gfp_flags, node);
> > + if (unlikely(!ret))
> > + break;
> > + } while (!atomic_inc_not_zero(&ret->refcount));
>
> Looks weird. Could do with a comment. Or unweirding ;)
>
> What's going on here?

In the unlikely event that we find a task that is on its way to exiting.
This hunk should actually be a part of the cfq lockless stuff...

> > return ret;
> > }
> > EXPORT_SYMBOL(get_io_context);
> > diff --git a/fs/ioprio.c b/fs/ioprio.c
> > index e4e01bc..a760040 100644
> > --- a/fs/ioprio.c
> > +++ b/fs/ioprio.c
> > @@ -41,18 +41,29 @@ static int set_task_ioprio(struct task_struct *task, int ioprio)
> > return err;
> >
> > task_lock(task);
> > + do {
> > + ioc = task->io_context;
> > + /* see wmb() in current_io_context() */
> > + smp_read_barrier_depends();
> > + if (ioc)
> > + break;
> >
> > - task->ioprio = ioprio;
> > -
> > - ioc = task->io_context;
> > - /* see wmb() in current_io_context() */
> > - smp_read_barrier_depends();
> > + ioc = alloc_io_context(GFP_ATOMIC, -1);
> > + if (!ioc) {
> > + err = -ENOMEM;
> > + break;
> > + }
> > + task->io_context = ioc;
> > + ioc->task = task;
> > + } while (1);
>
> argh. Can't sit there in a loop retrying GFP_ATOMIC!

It's not, read the loop again!

> > - if (ioc)
> > + if (!err) {
> > + ioc->ioprio = ioprio;
> > ioc->ioprio_changed = 1;
> > + }
> >
> > task_unlock(task);
> > - return 0;
> > + return err;
> > }
> >
> > asmlinkage long sys_ioprio_set(int which, int who, int ioprio)
> >
> > ...
> >
> > void put_io_context(struct io_context *ioc);
> > void exit_io_context(void);
> > struct io_context *get_io_context(gfp_t gfp_flags, int node);
> > +struct io_context *alloc_io_context(gfp_t, int);
> > void copy_io_context(struct io_context **pdst, struct io_context **psrc);
> > void swap_io_context(struct io_context **ioc1, struct io_context **ioc2);
>
> The rest of the declarations around here nicely name their args.

A clear sign I didn't put those declarations there, but the inconsistent
style is surely not a good thing. Will fix that up.

> > +static int copy_io(struct task_struct *tsk)
> > +{
> > + struct io_context *ioc = current->io_context;
> > +
> > + if (!ioc)
> > + return 0;
> > +
> > + if (ioprio_valid(ioc->ioprio)) {
> > + tsk->io_context = alloc_io_context(GFP_KERNEL, -1);
> > + if (unlikely(!tsk->io_context))
> > + return -ENOMEM;
> > +
> > + tsk->io_context->task = tsk;
> > + tsk->io_context->ioprio = ioc->ioprio;
> > + }
> > +
> > + return 0;
> > +}
>
> Should this depend on CONFIG_BLOCK?

Good questions, checks... Looks like it would break, I'll do a
!CONFIG_BLOCK fixup round.

--
Jens Axboe



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