Re: [RFC PATCH] mm/damon/reclaim: allow disabling watermarks to force reclaim of cold pages

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:
- [Low] mm/damon/reclaim: missing kernel-parameters.txt entry for ignore_wmarks
- [Low] mm/damon/reclaim: inaccurate ignore_wmarks parameter apply documentation
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commit 8e407c81e469d1e743666b3331a460c372ffea07
Author: Jinjie Ruan <ruanjinjie@huawei.com>

mm/damon/reclaim: allow disabling watermarks to force reclaim of cold pages

This commit adds the ignore_wmarks module parameter to DAMON_RECLAIM so
that the scheme can use DAMOS_WMARK_NONE and reclaim cold pages even when
free memory is plentiful.

> diff --git a/Documentation/admin-guide/mm/damon/reclaim.rst b/Documentation/admin-guide/mm/damon/reclaim.rst
> index ec7e3e32b4ac6..ef57f60a91059 100644
> --- a/Documentation/admin-guide/mm/damon/reclaim.rst
> +++ b/Documentation/admin-guide/mm/damon/reclaim.rst
> @@ -159,6 +159,23 @@ this auto-tuning feature.
>  
>  Disabled by default.
>  
> +ignore_wmarks
> +-------------
> +
> +Ignore the watermarks-based scheme activation condition.
> +
> +If set to ``Y``, DAMON_RECLAIM runs its scheme regardless of the system's
> +free memory rate, so the reclaim is bounded only by ``min_age`` and the
> +quota.  This is useful when cold pages should be reclaimed even while the
> +system still has free memory, e.g. to squeeze the cold anonymous pages of a
> +just-parked virtual machine out immediately.
> +
> +By default, ``N``, so the scheme is activated based on the system's free
> +memory rate, as described for the watermarks below.
> +
> +Changes take effect when the scheme is rebuilt, i.e. on the next ``enabled``
> +``0 -> 1`` transition, like the other DAMON_RECLAIM parameters.

[Severity: Low]
Is this description of when changes take effect accurate?

In mm/damon/reclaim.c, ignore_wmarks is applied in
damon_reclaim_apply_parameters() and committed via damon_commit_ctx().
Because damon_reclaim_commit_inputs_store() invokes
damon_reclaim_commit_inputs_fn() -> damon_reclaim_apply_parameters():

    static int damon_reclaim_commit_inputs_fn(void *arg)
    {
        return damon_reclaim_apply_parameters();
    }

writing Y to commit_inputs also applies ignore_wmarks changes online while
DAMON_RECLAIM is already running, without requiring an enabled 0 -> 1
transition.

Could the documentation note that changes can also be applied online via
commit_inputs?

>  wmarks_interval
>  ---------------
>  
> diff --git a/mm/damon/reclaim.c b/mm/damon/reclaim.c
> index 014b0779ea6dd..effd963f3c783 100644
> --- a/mm/damon/reclaim.c
> +++ b/mm/damon/reclaim.c
> @@ -112,6 +112,23 @@ static struct damos_watermarks damon_reclaim_wmarks = {
>  };
>  DEFINE_DAMON_MODULES_WMARKS_PARAMS(damon_reclaim_wmarks);
>  
> +/*
> + * Ignore the watermarks-based scheme activation condition.
> + *
> + * With the default ``N``, DAMON_RECLAIM runs its scheme only while the
> + * system's free memory rate is inside [wmarks_low, wmarks_mid].
> + * Reclaiming cold pages itself raises the free rate, so the scheme
> + * deactivates as soon as memory looks plentiful again.  For a VMM parking
> + * an idle sandbox, cold anonymous pages should be reclaimed immediately
> + * even while the host still has plenty of free memory; setting this ``Y``
> + * makes the scheme ignore the watermarks, so the reclaim is bounded only
> + * by min_age and the quota.
> + */
> +static bool ignore_wmarks __read_mostly;
> +module_param(ignore_wmarks, bool, 0600);
> +MODULE_PARM_DESC(ignore_wmarks,
> +	"Ignore watermarks: reclaim cold pages even while free memory is plentiful (default: false)");

[Severity: Low]
Since mm/damon/reclaim.c defines MODULE_PARAM_PREFIX as "damon_reclaim.",
registering ignore_wmarks via module_param() exposes
damon_reclaim.ignore_wmarks as a kernel command-line parameter at boot.

Should this patch also document damon_reclaim.ignore_wmarks in
Documentation/admin-guide/kernel-parameters.txt?

[ ... ]

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261020133233.3920818-1-ruanjinjie@huawei.com?part=1