[PATCH 2/2] mm/khugepaged: return EAGAIN for transient dirty pages in MADV_COLLAPSE

[Shivank Garg <shivankg@amd.com>]

When MADV_COLLAPSE encounters dirty file-backed pages, it currently
returns -EINVAL, this is misleading as EINVAL suggests invalid arguments,
whereas dirty pages are a transient condition that may resolve on retry.

Introduce SCAN_PAGE_DIRTY and map it to -EAGAIN. For khugepaged, this
is harmless as it will revisit the range after async writeback completes.

Signed-off-by: Shivank Garg <shivankg@amd.com>
---
 include/trace/events/huge_memory.h | 3 ++-
 mm/khugepaged.c                    | 4 +++-
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/include/trace/events/huge_memory.h b/include/trace/events/huge_memory.h
index dd94d14a2427..9014a9bbe64c 100644
--- a/include/trace/events/huge_memory.h
+++ b/include/trace/events/huge_memory.h
@@ -38,7 +38,8 @@
 	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_DIRTY,		"page_dirty")
 
 #undef EM
 #undef EMe
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index d08ed6eb9ce1..7df329c9c87d 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -60,6 +60,7 @@ enum scan_result {
 	SCAN_STORE_FAILED,
 	SCAN_COPY_MC,
 	SCAN_PAGE_FILLED,
+	SCAN_PAGE_DIRTY,
 };
 
 #define CREATE_TRACE_POINTS
@@ -1967,7 +1968,7 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 				 */
 				xas_unlock_irq(&xas);
 				filemap_flush(mapping);
-				result = SCAN_FAIL;
+				result = SCAN_PAGE_DIRTY;
 				goto xa_unlocked;
 			} else if (folio_test_writeback(folio)) {
 				xas_unlock_irq(&xas);
@@ -2747,6 +2748,7 @@ 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_DIRTY:
 		return -EAGAIN;
 	/*
 	 * Other: Trying again likely not to succeed / error intrinsic to
-- 
2.43.0