linux-kernel - Re: [PATCH v2 1/7] kernel/cgroup: Add "dmem" memory accounting cgroup

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Message-ID: <uj6railxyazpu6ocl2ygo6lw4lavbsgg26oq57pxxqe5uzxw42@fhnqvq3tia6n>
Date: Mon, 6 Jan 2025 18:48:31 +0100
From: Michal Koutný <mkoutny@...e.com>
To: Maarten Lankhorst <dev@...khorst.se>
Cc: linux-kernel@...r.kernel.org, intel-xe@...ts.freedesktop.org, 
	dri-devel@...ts.freedesktop.org, Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>, 
	Johannes Weiner <hannes@...xchg.org>, Andrew Morton <akpm@...ux-foundation.org>, 
	Friedrich Vock <friedrich.vock@....de>, Maxime Ripard <mripard@...nel.org>, cgroups@...r.kernel.org, 
	linux-mm@...ck.org, Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>
Subject: Re: [PATCH v2 1/7] kernel/cgroup: Add "dmem" memory accounting cgroup

Hello.

On Wed, Dec 04, 2024 at 02:44:01PM +0100, Maarten Lankhorst <dev@...khorst.se> wrote:
> +bool dmem_cgroup_state_evict_valuable(struct dmem_cgroup_pool_state *limit_pool,
> +				      struct dmem_cgroup_pool_state *test_pool,
> +				      bool ignore_low, bool *ret_hit_low)
> +{
> +	struct dmem_cgroup_pool_state *pool = test_pool;
> +	struct page_counter *climit, *ctest;
> +	u64 used, min, low;
> +
> +	/* Can always evict from current pool, despite limits */
> +	if (limit_pool == test_pool)
> +		return true;
> +

> +	if (limit_pool) {
> +		if (!parent_dmemcs(limit_pool->cs))
> +			return true;
> +
> +		for (pool = test_pool; pool && limit_pool != pool; pool = pool_parent(pool))
> +			{}
> +
> +		if (!pool)
> +			return false;
> +	} else {
> +		/*
> +		 * If there is no cgroup limiting memory usage, use the root
> +		 * cgroup instead for limit calculations.
> +		 */
> +		for (limit_pool = test_pool; pool_parent(limit_pool); limit_pool = pool_parent(limit_pool))
> +			{}
> +	}

I'm trying to understand the two branches above. If limit_pool is a root
one, eviction is granted and no protection is evaluated.

Then it checks that test_pool is below limit_pool (can this ever fail,
given the limit_pool must have been above when charging in test_pool?).
(OK, this may be called arbitrarily by modules.)

I think it could be simplified and corrected like this:

	/* Resolve NULL limit_pool */
	if (!limit_pool)
		for (limit_pool = test_pool; pool_parent(limit_pool); limit_pool = pool_parent(limit_pool));
	
	/* Check ancestry */
	if (!cgroup_is_descendant(test_pool->cs->css.cgroup, limit_pool->cs->css.cgroup))
		return false;

HTH,
Michal

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