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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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