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SubjectRe: [PATCH REBASED] mm, memcg: Make scan aggression always exclude protection
[Sorry for a late reply]

On Fri 22-03-19 16:03:07, Chris Down wrote:
[...]
> With this patch, memory.low and memory.min affect reclaim pressure in a
> more understandable and composable way. For example, from a user
> standpoint, "protected" memory now remains untouchable from a reclaim
> aggression standpoint, and users can also have more confidence that
> bursty workloads will still receive some amount of guaranteed
> protection.

Maybe I am missing something so correct me if I am wrong but the new
calculation actually means that we always allow to scan even min
protected memcgs right?

Because ...

[...]

> +static inline unsigned long mem_cgroup_protection(struct mem_cgroup *memcg,
> + bool in_low_reclaim)
> {
> - if (mem_cgroup_disabled()) {
> - *min = 0;
> - *low = 0;
> - return;
> - }
> + if (mem_cgroup_disabled())
> + return 0;
> +
> + if (in_low_reclaim)
> + return READ_ONCE(memcg->memory.emin);
>
> - *min = READ_ONCE(memcg->memory.emin);
> - *low = READ_ONCE(memcg->memory.elow);
> + return max(READ_ONCE(memcg->memory.emin),
> + READ_ONCE(memcg->memory.elow));
> }
[...]
> + unsigned long cgroup_size = mem_cgroup_size(memcg);
> +
> + /* Avoid TOCTOU with earlier protection check */
> + cgroup_size = max(cgroup_size, protection);
> +
> + scan = lruvec_size - lruvec_size * protection /
> + cgroup_size;
>
[...]
> - scan = clamp(scan, SWAP_CLUSTER_MAX, lruvec_size);
> + scan = max(scan, SWAP_CLUSTER_MAX);

here the zero or sub SWAP_CLUSTER_MAX scan target gets extended to
SWAP_CLUSTER_MAX. Unless I am missing something this is not correct
because min protection should be a guarantee even in in_low_reclaim
mode.

> } else {
> scan = lruvec_size;
> }
> --
> 2.21.0

--
Michal Hocko
SUSE Labs

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