Re: [PATCH v2] mm/damon/vaddr: attempt per-vma lock during page table walk

[SeongJae Park <sj@kernel.org>]

On Wed, 13 May 2026 22:52:03 +0000 sashiko-bot@kernel.org wrote:

> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> - [High] `damos_va_migrate_pmd_entry` and `damos_va_stat_pmd_entry` incorrectly advance iteration by the full folio size, skipping unrelated PTEs for mTHP.
> --
> 
> commit 25c04a646a951de8c86c5c718af6cfc459b7b5e0
> Author: Kefeng Wang <wangkefeng.wang@huawei.com>
> 
> mm/damon/vaddr: attempt per-vma lock during page table walk
> 
> Currently, DAMON virtual address operations use mmap_read_lock
> during page table walks, which can cause unnecessary contention
> under high concurrency.
> 
> > diff --git a/mm/damon/vaddr.c b/mm/damon/vaddr.c
> > index 1b0ebe3b6951e..d271476035641 100644
> > --- a/mm/damon/vaddr.c
> > +++ b/mm/damon/vaddr.c
> [ ... ]
> > @@ -750,7 +773,6 @@ static unsigned long damos_va_migrate(struct damon_target *target,
> >  	struct mm_walk_ops walk_ops = {
> >  		.pmd_entry = damos_va_migrate_pmd_entry,
> >  		.pte_entry = NULL,
> > -		.walk_lock = PGWALK_RDLOCK,
> >  	};
> 
> This isn't a regression from this patch,

Yes, no blocker for this patch.

> but there appears to be a pre-existing
> issue in damos_va_migrate_pmd_entry() and damos_va_stat_pmd_entry() when
> handling large folios.
> 
> In the PTE iteration loops, when a large folio is found, the code sets
> nr = folio_nr_pages(folio) and updates pte += nr and addr += nr * PAGE_SIZE.
> 
> Could this skip unrelated PTEs if the iteration started on a tail page of this
> folio (e.g., halfway into an mTHP)?
> 
> Advancing by the full folio size could overshoot the end of the folio and skip
> unrelated PTEs mapped immediately after it within the same PMD.
> 
> Would it be better to subtract the offset within the folio to safely skip only
> the remaining pages of the current folio?

The user impact should be minor, but makes sense.  I will work on this.


Thanks,
SJ

> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/20260512151523.2092638-1-wangkefeng.wang@huawei.com?part=1