Re: [RFC PATCH v1.2 4/8] mm/damon/vaddr: respect folio end for DAMOS_STAT

[sashiko-bot@kernel.org]

> For applying DAMOS_STAT action to a region, DAMON virtual address space
> operation set (vaddr) calls walk_page_range[_vma]() for the region.  The
> pmd walk entry function, namely damon_va_stat_pmd_entry(), applies DAMOS
> filters to folios of addresses of the region in the pmd.  It starts from
> the walking address and advances the address by the size of the folio of
> the address until it goes out of the pmd or the region.
> 
> Let's suppose it is for the first pmd of the region, and the region
> start address is in the middle of a large folio.  Also, the next folios
> are small.  Then, some of the next folios could be skipped.  Fix the
> issue by advancing the address to exactly the start address of the next
> folio.
> 
> The user impact is that the DAMOS_STAT-based page level monitoring
> results become inaccurate.  Since the page level monitoring is supposed
> to provide relatively high precision, this is definitely a problem.  It
> is arguably not critical since it is only monitoring quality
> degradation.
> 
> The issue was discovered [1] by Sashiko.
> 
> [1] https://lore.kernel.org/20260514015053.149396-1-sj@kernel.org
> 
> Fixes: 63f39737d1e3 ("mm/damon/vaddr: support stat-purpose DAMOS filters")
> Cc: <stable@vger.kernel.org> # 6.18.x
> Signed-off-by: SJ Park <sj@kernel.org>

Sashiko has reviewed this patch and found no issues. It looks great!

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