On Thu, 13 Mar 2025 11:52:18 -0400
Gregory Price <gourry@xxxxxxxxxx> wrote:
> On Thu, Mar 13, 2025 at 03:31:38PM +0900, Rakie Kim wrote:
> > > Is this correct? If kobject_init_and_add fails, from other examples we
> > > need only free the mempolicy_kobj - because it failed to initialize and
> > > therefore should not have any references. I think this causes an
> > > underflow.
> >
> > Regarding the reordering of mempolicy_kobj allocation:
> > 1) In kobject_init_and_add(), kobject_init() is always called, which
>
> Quite right, mea culpa.
>
> >
> > 2) The release function for mempolicy_kobj is responsible for freeing
> > associated memory:
> >
> > static void mempolicy_kobj_release(struct kobject *kobj)
> > {
> > ...
> > kfree(ngrp->nattrs);
> > kfree(ngrp);
> > kfree(kobj);
> > }
> >
>
> I see what you're trying to do now after looking at the free-ordering
> at little closer.
>
> Lets do the following:
>
> 1) allocate node_attrs and mempolicy_kobj up front and keep your
> reordering, this lets us clean up allocations on failure before
> kobject_init is called
>
> 2) after this remove all the other code and just let
> mempolicy_kobj_release clean up node_attrs
>
> 3) Add a (%d) to the error message to differentiate failures
Given how unlikely (and noisy) a memory allocation failure is,
maybe just drop the printing at all in those paths - allowing
early returns.
The lifetime rules around node_attrs in here are making readability
poor. It is implicitly owned by the mempolicy_kobj, but no direct association.
Maybe just encapsulating the kobject in a structure that contains
this as a [] array at the end. Then we end up with single allocation of
stuff that is effectively one thing.
>
> This is a little bit cleaner and is a bit less code. (Not built or
> tested, just a recommendation).
>
> I'd recommend submitting this patch by itself to mm-stable, since the
> remainder of the patch line changes functionality and this fixes a bug
> in LTS kernels.
>
> ~Gregory
>
> ---
>
>
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 530e71fe9147..05a410db08b4 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -3541,38 +3541,34 @@ static int __init mempolicy_sysfs_init(void)
> int err;
> static struct kobject *mempolicy_kobj;
>
> - mempolicy_kobj = kzalloc(sizeof(*mempolicy_kobj), GFP_KERNEL);
> - if (!mempolicy_kobj) {
> + node_attrs = kcalloc(nr_node_ids, sizeof(struct iw_node_attr *),
> + GFP_KERNEL);
> + if (!node_attrs) {
> err = -ENOMEM;
> goto err_out;
> }
>
> - node_attrs = kcalloc(nr_node_ids, sizeof(struct iw_node_attr *),
> - GFP_KERNEL);
> - if (!node_attrs) {
> + mempolicy_kobj = kzalloc(sizeof(*mempolicy_kobj), GFP_KERNEL);
> + if (!mempolicy_kobj) {
> err = -ENOMEM;
> - goto mempol_out;
> + kfree(node_attrs);
> + goto err_out;
> }
>
> err = kobject_init_and_add(mempolicy_kobj, &mempolicy_ktype, mm_kobj,
> "mempolicy");
> if (err)
> - goto node_out;
> + goto mempol_out;
>
> err = add_weighted_interleave_group(mempolicy_kobj);
> - if (err) {
> - pr_err("mempolicy sysfs structure failed to initialize\n");
> - kobject_put(mempolicy_kobj);
> - return err;
> - }
> + if (err)
> + goto mempol_out;
>
> - return err;
> -node_out:
> - kfree(node_attrs);
> + return 0;
> mempol_out:
> - kfree(mempolicy_kobj);
> + kobject_put(mempolicy_kobj);
> err_out:
> - pr_err("failed to add mempolicy kobject to the system\n");
> + pr_err("mempolicy sysfs structure failed to initialize (%d)\n", err);
> return err;
> }
>
>
