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

[Dev Jain]

On 10/11/25 5:02 pm, Shivank Garg wrote:

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.

Taking a cursory look at enum scan_result, I am sure there are other error
codes as well which redirect to -EINVAL when they actually should to something
else...

Signed-off-by: Shivank Garg <shiva...@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);

Now that you are at it, it would make sense to redirect this 
folio_test_writeback()
branch also to -EAGAIN, and call the common error code 
SCAN_PAGE_UNDER_WRITEBACK.

@@ -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