Re: [PATCH 1/2] mm: kfence: fix missing objcg housekeeping for SLAB

[Muchun Song <songmuchun@bytedance.com>]

On Mon, Mar 28, 2022 at 5:08 AM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Sat, Mar 26, 2022 at 10:19 PM Muchun Song <songmuchun@bytedance.com> wrote:
> >
> > The objcg is not cleared and put for kfence object when it is freed, which
> > could lead to memory leak for struct obj_cgroup and wrong statistics of
> > NR_SLAB_RECLAIMABLE_B or NR_SLAB_UNRECLAIMABLE_B.  Since the last freed
> > object's objcg is not cleared, mem_cgroup_from_obj() could return the wrong
> > memcg when this kfence object, which is not charged to any objcgs, is
> > reallocated to other users.  A real word issue [1] is caused by this bug.
>
> Good that this looks sorted out.
>
> Patch 2/2 seems to still be up in the air. The patch not only causes
> build errors, but it looks really very odd to me.
>
> In particular, you do that loop with
>
>                 __SetPageSlab(&pages[i]);
>
> in kfence_init_pool(), but that is *not* where you set the
> MEMCG_DATA_OBJCGS, and instead do that virt_to_slab(addr) dance later.
>
> That looks very odd to me. I think the two should go hand-in-hand,
> since that __SetPageSlab() really is what makes it a slab thing, and I
> think it should go together with setting the slab state correctly.

Right. It is a little odd. I'll improve it in the next version.

>
> Finally, is there a syzbot report for that second problem?

No. The second bug does not trigger any oops, so it is hard to be seen.
It is just my code review.

>
> Anyway, should I apply this PATCH 1/2 now directly as the solution for
> the dentry issue, or should I wait for that second patch? They seem to
> be related only indirectly, in that the problems were both introduced
> by the same commit.
>

I think you could apply PATCH 1/2 now.  PATCH 2/2 is another issue not
related to dentry issue.

Thanks.