Re: [PATCH V2 2/2] mm/khugepaged: map dirty/writeback pages failures to EAGAIN

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

On 20/11/25 12:20 pm, Shivank Garg wrote:
> When collapse_file encounters dirty or writeback pages in file-backed
> mappings, it currently SCAN_FAIL which maps to -EINVAL. This is
> misleading as EINVAL suggests invalid arguments, whereas dirty/writeback
> pages represent transient conditions that may resolve on retry.
>
> Introduce SCAN_PAGE_NOT_CLEAN to cover both dirty and writeback states,
> mapping it to -EAGAIN. For MADV_COLLAPSE, this provides userspace with
> a clear signal that retry may succeed after writeback completes, making
> -EAGAIN semantically correct. For khugepaged, this is harmless as it
> will naturally revisit the range during periodic scans after async
> writeback completes.
>
> Signed-off-by: Shivank Garg <shivankg@amd.com>
> ---
>   include/trace/events/huge_memory.h | 3 ++-
>   mm/khugepaged.c                    | 8 +++++---
>   2 files changed, 7 insertions(+), 4 deletions(-)
>
> diff --git a/include/trace/events/huge_memory.h b/include/trace/events/huge_memory.h
> index 4cde53b45a85..1caf24b951e1 100644
> --- a/include/trace/events/huge_memory.h
> +++ b/include/trace/events/huge_memory.h
> @@ -37,7 +37,8 @@
>   	EM( SCAN_PAGE_HAS_PRIVATE,	"page_has_private")		\
>   	EM( SCAN_STORE_FAILED,		"store_failed")			\
>   	EM( SCAN_COPY_MC,		"copy_poisoned_page")		\
> -	EMe(SCAN_PAGE_FILLED,		"page_filled")
> +	EM( SCAN_PAGE_FILLED,		"page_filled")			\
> +	EMe(SCAN_PAGE_NOT_CLEAN,	"page_not_clean")
>   
>   #undef EM
>   #undef EMe
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 066a332c76ad..282b413d17e8 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -59,6 +59,7 @@ enum scan_result {
>   	SCAN_STORE_FAILED,
>   	SCAN_COPY_MC,
>   	SCAN_PAGE_FILLED,
> +	SCAN_PAGE_NOT_CLEAN,
>   };
>   
>   #define CREATE_TRACE_POINTS
> @@ -1968,11 +1969,11 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
>   				 */
>   				xas_unlock_irq(&xas);
>   				filemap_flush(mapping);
> -				result = SCAN_FAIL;
> +				result = SCAN_PAGE_NOT_CLEAN;
>   				goto xa_unlocked;
>   			} else if (folio_test_writeback(folio)) {
>   				xas_unlock_irq(&xas);
> -				result = SCAN_FAIL;
> +				result = SCAN_PAGE_NOT_CLEAN;
>   				goto xa_unlocked;
>   			} else if (folio_trylock(folio)) {
>   				folio_get(folio);
> @@ -2019,7 +2020,7 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
>   			 * folio is dirty because it hasn't been flushed
>   			 * since first write.
>   			 */
> -			result = SCAN_FAIL;
> +			result = SCAN_PAGE_NOT_CLEAN;
>   			goto out_unlock;
>   		}
>   
> @@ -2748,6 +2749,7 @@ 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_NOT_CLEAN:
>   		return -EAGAIN;
>   	/*
>   	 * Other: Trying again likely not to succeed / error intrinsic to

SCAN_PAGE_NOT_CLEAN is confusing - NOT_CLEAN literally means dirty, so why not SCAN_PAGE_DIRTY?
Or SCAN_PAGE_DIRTY_OR_UNDER_WRITEBACK? Since folio_test_writeback() is true as a result of
the folio being dirty, maybe just SCAN_PAGE_DIRTY can do.

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