Re: [PATCH] mm/khugepaged: return -EAGAIN for SCAN_PAGE_HAS_PRIVATE in MADV_COLLAPSE

[Dev Jain <dev.jain@arm.com>]

On 29/04/26 7:34 pm, Vineet Agarwal wrote:
> MADV_COLLAPSE uses errno values to provide actionable feedback to
> userspace. Temporary resource constraints are mapped to -EAGAIN so the
> caller may retry, while intrinsic failures of the specified range are
> mapped to -EINVAL.
> 
> collapse_file() returns SCAN_PAGE_HAS_PRIVATE when
> filemap_release_folio() fails while isolating file-backed folios for
> collapse. This currently falls through the default case in
> madvise_collapse_errno() and is reported to userspace as -EINVAL.
> 
> However, filemap_release_folio() failure commonly reflects temporary
> folio state rather than a permanently uncollapsible range.
> 
> For example, ext4 returns false when a folio still has dirty
> journalled data, btrfs returns false for dirty or writeback folios
> before extent state release, and NFS may return false while reclaiming
> filesystem-private folio state.
> 
> In such cases, retrying MADV_COLLAPSE after writeback, reclaim or
> journal progress may succeed. This matches the existing -EAGAIN
> handling for SCAN_PAGE_DIRTY_OR_WRITEBACK and other transient collapse
> failures more closely than -EINVAL.
> 
> Therefore, map SCAN_PAGE_HAS_PRIVATE to -EAGAIN so userspace receives
> retryable feedback for this temporary failure path.
> 
> Signed-off-by: Vineet Agarwal <agarwal.vineet2006@gmail.com>
> ---
>  mm/khugepaged.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index b8452dbdb043..58f55115d7d8 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -2808,6 +2808,7 @@ static int madvise_collapse_errno(enum scan_result r)
>  	case SCAN_PAGE_LRU:
>  	case SCAN_DEL_PAGE_LRU:
>  	case SCAN_PAGE_FILLED:
> +	case SCAN_PAGE_HAS_PRIVATE:
>  	case SCAN_PAGE_DIRTY_OR_WRITEBACK:
>  		return -EAGAIN;
>  	/*

https://lore.kernel.org/all/4ab54ae0-2607-443d-8698-788d8e951bdd@arm.com/

I had mentioned at that time, other error codes may fit the -EAGAIN
(or something else) definition as well, for example, probably SCAN_TRUNCATED.

Could you please audit other error codes as well?

Anyhow to your patch here:

Reviewed-by: Dev Jain <dev.jain@arm.com>