[PATCH 1/2] mm/khugepaged: do synchronous writeback for MADV_COLLAPSE

[Shivank Garg]

When MADV_COLLAPSE is called on file-backed mappings (e.g., executable
text sections), the pages may still be dirty from recent writes. The
current code triggers an async flush via filemap_flush() and returns
SCAN_FAIL, requiring userspace to retry the operation.

This is problematic for userspace that wants to collapse text pages into
THPs to reduce ITLB pressure. The first madvise() call always fails with
EINVAL, and only subsequent calls succeed after writeback completes.

For direct MADV_COLLAPSE calls (!cc->is_khugepaged), perform a synchronous
writeback using filemap_write_and_wait_range() before scanning the folios.
This ensures that folios are clean on the first attempt.

Reported-by: Branden Moore <Branden.Moore@xxxxxxx>
Closes: https://lore.kernel.org/all/4e26fe5e-7374-467c-a333-9dd48f85d7cc@xxxxxxx
Fixes: 34488399fa08 ("mm/madvise: add file and shmem support to MADV_COLLAPSE")
Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
Signed-off-by: Shivank Garg <shivankg@xxxxxxx>
---
Applies cleanly on:
6.18-rc5
mm-stable:e9a6fb0bc


 mm/khugepaged.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index abe54f0043c7..d08ed6eb9ce1 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -21,6 +21,7 @@
 #include <linux/shmem_fs.h>
 #include <linux/dax.h>
 #include <linux/ksm.h>
+#include <linux/backing-dev.h>
 
 #include <asm/tlb.h>
 #include <asm/pgalloc.h>
@@ -1845,6 +1846,7 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 	struct page *dst;
 	struct folio *folio, *tmp, *new_folio;
 	pgoff_t index = 0, end = start + HPAGE_PMD_NR;
+	loff_t range_start, range_end;
 	LIST_HEAD(pagelist);
 	XA_STATE_ORDER(xas, &mapping->i_pages, start, HPAGE_PMD_ORDER);
 	int nr_none = 0, result = SCAN_SUCCEED;
@@ -1853,6 +1855,21 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 	VM_BUG_ON(!IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && !is_shmem);
 	VM_BUG_ON(start & (HPAGE_PMD_NR - 1));
 
+	/*
+	 * For MADV_COLLAPSE on regular files, do a synchronous writeback
+	 * to ensure dirty folios are flushed before we attempt collapse.
+	 * This is a best-effort approach to avoid failing on the first
+	 * attempt when freshly-written executable text is still dirty.
+	 */
+	if (!is_shmem && cc && !cc->is_khugepaged && mapping_can_writeback(mapping)) {
+		range_start = (loff_t)start << PAGE_SHIFT;
+		range_end = ((loff_t)end << PAGE_SHIFT) - 1;
+		if (filemap_write_and_wait_range(mapping, range_start, range_end)) {
+			result = SCAN_FAIL;
+			goto out;
+		}
+	}
+
 	result = alloc_charge_folio(&new_folio, mm, cc);
 	if (result != SCAN_SUCCEED)
 		goto out;

base-commit: e9a6fb0bcdd7609be6969112f3fbfcce3b1d4a7c
-- 
2.43.0