Re: [PATCH] mm/damon/vaddr: preserve migration weights above UINT_MAX

[SJ Park <sj@kernel.org>]

Hello Daemyung,

On Tue, 27 Oct 2026 12:54:31 +0900 Daemyung Kang <charsyam@gmail.com> wrote:

> DAMOS migration accepts an unsigned int weight for each destination,
> but sums the weights into an unsigned int.  If the total wraps to zero,
> no folio is migrated.  Other wrapped totals select the wrong node.

Thank you for finding the issue.

But, is it realistic to have the weight_total overflow?  Have you found the
issue on real usage?  Or, theoretical?

> 
> Sum weights into a u64 once per migration walk.  Keep the existing
> 32-bit divisor path for totals up to UINT_MAX and use a 64-bit remainder
> for larger totals.  Add KUnit coverage for the actual summing path and
> for destination selection at the overflow boundary.

Do we really need to support that user inputs?  Unless we have a very real use
case that require such huge weight values, I'd prefer not.

How about just clearly notify users those are invalid parameters?  We could do
so by adding weight_total overflow check to the central parameters validation
path: damos_commit_dests().

> 
> Fixes: 19c1dc15c859 ("mm/damon/vaddr: use damos->migrate_dests in migrate_{hot,cold}")

Should we Cc: stable?


Thanks,
SJ

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