Re: [PATCH 3/7] mm/damon/lru_sort: cover all system rams

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] Single bounding box for sparse memory layouts causes soft lockups and algorithmic failure
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commit 692650636f4214ec9c2fa1c5d7fd43fe3bf049fe
Author: SeongJae Park <sj@kernel.org>

mm/damon/lru_sort: cover all system rams

This commit changes the default monitoring region from the biggest System RAM
resource to a single region that spans all system RAMs. This is intended to
cover the entire physical memory instead of just the largest block.

> diff --git a/mm/damon/lru_sort.c b/mm/damon/lru_sort.c
> index 7569e471160a..2eb559d913b6 100644
> --- a/mm/damon/lru_sort.c
> +++ b/mm/damon/lru_sort.c

[ ... ]

> @@ -326,7 +328,7 @@ static int damon_lru_sort_apply_parameters(void)
>  	if (err)
>  		goto out;
>  
> -	err = damon_set_region_biggest_system_ram_default(param_target,
> +	err = damon_set_region_system_rams_default(param_target,
>  					&monitor_region_start,
>  					&monitor_region_end,
>  					param_ctx->addr_unit,

Could this lead to a regression on systems with sparse memory layouts?

When creating a single bounding box from the first to the last physical
address, systems with sparse memory like CXL or distant NUMA nodes could end
up with a region containing terabytes of unpopulated physical address space.

Will this cause soft lockups during DAMOS operations?

If a scheme like PAGEOUT iterates linearly over this massive region in
PAGE_SIZE increments, iterating over these large memory holes might stall the
CPU and trigger the watchdog, since cond_resched() is typically placed after
the loop.

Does this also cause an algorithmic regression with the sampling probability?

DAMON's adaptive logic merges adjacent regions with identical access
frequencies. Since memory holes report 0 accesses, they might be aggressively
merged with interspersed RAM into maximum-sized regions. Because sampling
selects a single random address per region, the probability of actually
sampling the populated RAM might drop to near zero.

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Sashiko AI review · https://sashiko.dev/#/patchset/20260429041232.90257-1-sj@kernel.org?part=3