Re: [PATCH 03/19] mm/damon/paddr: set samples in apply_probes() if requested

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] Address advancement using `folio_size()` from an unaligned address within a large folio overshoots the next folio.
--

--- Patch [3]: [PATCH 03/19] mm/damon/paddr: set samples in apply_probes() if requested ---
commit 60a9acbe070a2be34c313d6be62fe7e4acb7ec54
Author: SJ Park <sj@kernel.org>

mm/damon/paddr: set samples in apply_probes() if requested

This commit updates damon_pa_apply_probes() to respect the set_samples
parameter. It adds logic to randomize the sampling address within the
region boundaries using damon_rand() when requested.

> diff --git a/mm/damon/paddr.c b/mm/damon/paddr.c
> index b13bf7c6eade3..ff4fcca949660 100644
> --- a/mm/damon/paddr.c
> +++ b/mm/damon/paddr.c
> @@ -178,6 +178,10 @@ static void damon_pa_apply_probes(struct damon_ctx *ctx, bool set_samples)
>  			phys_addr_t pa;
>  			struct folio *folio;
>  
> +			if (set_samples)
> +				r->sampling_addr = damon_rand(ctx, r->ar.start,
> +						r->ar.end);
> +
>  			pa = damon_pa_phys_addr(r->sampling_addr,
>  					ctx->addr_unit);
>  			folio = damon_get_folio(PHYS_PFN(pa));

[Severity: High]
This is a pre-existing issue in this file, but can the address advancement
logic skip over pages during region iteration?

Looking at iteration functions like damon_pa_pageout(), 
damon_pa_de_activate(), damon_pa_migrate(), and damon_pa_stat() in 
mm/damon/paddr.c, they all use a pattern for advancing the address
similar to this:

mm/damon/paddr.c:damon_pa_pageout() {
    ...
    folio = damon_get_folio(PHYS_PFN(addr));
    if (!folio) {
        addr += PAGE_SIZE;
        continue;
    }
    ...
    addr += folio_size(folio);
    ...
}

If the initial iteration address is not aligned to the folio boundary (for
example, after a region split or hole), wouldn't adding the full folio size
to this unaligned address cause it to overshoot the end of the current folio?

This seems like it would skip over the beginning of the next physical folio,
causing those pages to incorrectly escape reclaim, migration, or statistics
gathering.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260710134651.18084-1-sj@kernel.org?part=3