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

[Shivank Garg <shivankg@amd.com>]

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
-- 
2.43.0