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

[Linus Torvalds]

On Sat, Mar 26, 2022 at 10:19 PM Muchun Song <songmuchun@xxxxxxxxxxxxx> 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.

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

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.

                     Linus