[PATCH] cgroup/dmem: Don't clobber pool in dmem_cgroup_calculate_protection

[Michal Koutný]

On Thu, Jan 16, 2025 at 09:20:08AM +0100, Friedrich Vock <friedrich.vock at gmx.de> wrote:
> These pools are allocated on-demand, so if a
> cgroup has not made any allocations for a specific device, there will be
> no pool corresponding to that device's memory.

Here I understand.

> Pools have a hierarchy of their own (that is, for a given cgroup's
> pool corresponding to some device, the "parent pool" refers to the
> parent cgroup's pool corresponding to the same device).
> 
> In dmem_cgroup_calculate_protection, we're trying to update the
> protection values for the entire pool hierarchy between
> limit_pool/test_pool (with the end goal of having accurate
> effective-protection values for test_pool).

If you check and bail out at start:
	if (!cgroup_is_descendant(test_pool->cs->css.cgroup, limit_pool->cs->css.cgroup))
		return;
...

> Since pools only store parent pointers to establish that hierarchy, to
> find child pools given only the parent pool, we iterate over the pools
> of all child cgroups and check if the parent pointer matches with our
> current "parent pool" pointer.
 
> The bug happens when some cgroup doesn't have any pool in the hierarchy
> we're iterating over (that is, we iterate over all pools but don't find
> any pool whose parent matches our current "parent pool" pointer).

...then the initial check ensures, you always find a pool that is
a descendant of limit_pool (at least the test_pool).
And there are pools for whole path between limit_pool and test_pool, or
am I mistaken here?

> The cgroup itself is part of the (cgroup) hierarchy, so the result of
> cgroup_is_descendant is obviously true - but because of how we
> allocate pools on-demand, it's still possible there is no pool that is
> part of the (pool) hierarchy we're iterating over.

Can there be a pool without cgroup?

Thanks,
Michal
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