[PATCH V3 1/2] 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 returns 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_DIRTY_OR_WRITEBACK 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.
For khugepaged, this is harmless as it will naturally revisit the range
during periodic scans after async writeback completes.

Reported-by: Branden Moore <Branden.Moore@amd.com>
Closes: https://lore.kernel.org/all/4e26fe5e-7374-467c-a333-9dd48f85d7cc@amd.com
Fixes: 34488399fa08 ("mm/madvise: add file and shmem support to MADV_COLLAPSE")
Reviewed-by: Dev Jain <dev.jain@arm.com>
Reviewed-by: Lance Yang <lance.yang@linux.dev>
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..4e41bff31888 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_DIRTY_OR_WRITEBACK, "page_dirty_or_writeback")
 
 #undef EM
 #undef EMe
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 97d1b2824386..219dfa2e523c 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -58,6 +58,7 @@ enum scan_result {
 	SCAN_STORE_FAILED,
 	SCAN_COPY_MC,
 	SCAN_PAGE_FILLED,
+	SCAN_PAGE_DIRTY_OR_WRITEBACK,
 };
 
 #define CREATE_TRACE_POINTS
@@ -1967,11 +1968,11 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 				 */
 				xas_unlock_irq(&xas);
 				filemap_flush(mapping);
-				result = SCAN_FAIL;
+				result = SCAN_PAGE_DIRTY_OR_WRITEBACK;
 				goto xa_unlocked;
 			} else if (folio_test_writeback(folio)) {
 				xas_unlock_irq(&xas);
-				result = SCAN_FAIL;
+				result = SCAN_PAGE_DIRTY_OR_WRITEBACK;
 				goto xa_unlocked;
 			} else if (folio_trylock(folio)) {
 				folio_get(folio);
@@ -2018,7 +2019,7 @@ 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_DIRTY_OR_WRITEBACK;
 			goto out_unlock;
 		}
 
@@ -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_OR_WRITEBACK:
 		return -EAGAIN;
 	/*
 	 * Other: Trying again likely not to succeed / error intrinsic to
-- 
2.43.0