Re: [PATCH 09/31] mm/pagewalkers: ACTION_AGAIN if pte_offset_map_lock() fails

[SeongJae Park]

Hello Hugh,

On Sun, 21 May 2023 22:00:15 -0700 (PDT) Hugh Dickins <hughd@xxxxxxxxxx> wrote:

> Simple walk_page_range() users should set ACTION_AGAIN to retry when
> pte_offset_map_lock() fails.
> 
> No need to check pmd_trans_unstable(): that was precisely to avoid the
> possiblity of calling pte_offset_map() on a racily removed or inserted
> THP entry, but such cases are now safely handled inside it.  Likewise
> there is no need to check pmd_none() or pmd_bad() before calling it.
> 
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>

For below mm/damon part,

Reviewed-by: SeongJae Park <sj@xxxxxxxxxx>


Thanks,
SJ

> ---
>  fs/proc/task_mmu.c | 32 ++++++++++++++++----------------
>  mm/damon/vaddr.c   | 12 ++++++++----
>  mm/mempolicy.c     |  7 ++++---
>  mm/mincore.c       |  9 ++++-----
>  mm/mlock.c         |  4 ++++
>  5 files changed, 36 insertions(+), 28 deletions(-)
> 
[...]
> diff --git a/mm/damon/vaddr.c b/mm/damon/vaddr.c
> index 1fec16d7263e..b8762ff15c3c 100644
> --- a/mm/damon/vaddr.c
> +++ b/mm/damon/vaddr.c
> @@ -318,9 +318,11 @@ static int damon_mkold_pmd_entry(pmd_t *pmd, unsigned long addr,
>  		spin_unlock(ptl);
>  	}
>  
> -	if (pmd_none(*pmd) || unlikely(pmd_bad(*pmd)))
> -		return 0;
>  	pte = pte_offset_map_lock(walk->mm, pmd, addr, &ptl);
> +	if (!pte) {
> +		walk->action = ACTION_AGAIN;
> +		return 0;
> +	}
>  	if (!pte_present(*pte))
>  		goto out;
>  	damon_ptep_mkold(pte, walk->mm, addr);
> @@ -464,9 +466,11 @@ static int damon_young_pmd_entry(pmd_t *pmd, unsigned long addr,
>  regular_page:
>  #endif	/* CONFIG_TRANSPARENT_HUGEPAGE */
>  
> -	if (pmd_none(*pmd) || unlikely(pmd_bad(*pmd)))
> -		return -EINVAL;
>  	pte = pte_offset_map_lock(walk->mm, pmd, addr, &ptl);
> +	if (!pte) {
> +		walk->action = ACTION_AGAIN;
> +		return 0;
> +	}
>  	if (!pte_present(*pte))
>  		goto out;
>  	folio = damon_get_folio(pte_pfn(*pte));
[...]