Re: [PATCH 1/2] mm/MADV_COLLAPSE: set EAGAIN on unexpected page refcount

[Hugh Dickins <hughd@google.com>]

On Tue, 24 Jan 2023, Zach O'Keefe wrote:

> During collapse, in a few places we check to see if a given small page
> has any unaccounted references.  If the refcount on the page doesn't
> match our expectations, it must be there is an unknown user concurrently
> interested in the page, and so it's not safe to move the contents
> elsewhere. However, the unaccounted pins are likely an ephemeral state.
> 
> In such a situation, make MADV_COLLAPSE set EAGAIN errno, indicating that
> collapse may succeed on retry.
> 
> Fixes: 7d8faaf15545 ("mm/madvise: introduce MADV_COLLAPSE sync hugepage collapse")
> Reported-by: Hugh Dickins <hughd@google.com>
> Signed-off-by: Zach O'Keefe <zokeefe@google.com>

This was
Reviewed-by: Yang Shi <shy828301@gmail.com>
and now I'll give it a nudge with
Acked-by: Hugh Dickins <hughd@google.com>
since it hasn't appeared in mm-unstable or linux-next yet:
I think its Cc:stable sibling 2/2, already in 6.2-rc, got all the attention.

Thanks!
Hugh

> 
> ---
>  mm/khugepaged.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index e23619bfecc4..fa38cae240b9 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -2712,6 +2712,7 @@ static int madvise_collapse_errno(enum scan_result r)
>  	case SCAN_CGROUP_CHARGE_FAIL:
>  		return -EBUSY;
>  	/* Resource temporary unavailable - trying again might succeed */
> +	case SCAN_PAGE_COUNT:
>  	case SCAN_PAGE_LOCK:
>  	case SCAN_PAGE_LRU:
>  	case SCAN_DEL_PAGE_LRU:
> -- 
> 2.39.1.405.gd4c25cc71f-goog