[PATCH V2 1/2] mm/khugepaged: do synchronous writeback for MADV_COLLAPSE

[Shivank Garg <shivankg@amd.com>]

When MADV_COLLAPSE is called on file-backed mappings (e.g., executable
text sections), the pages may still be dirty from recent writes and
cause collapse to fail with -EINVAL. This is particularly problematic
for freshly copied executables on filesystems, where page cache folios
remain dirty until background writeback completes.

The current code in collapse_file() triggers async writeback via
filemap_flush() and expects khugepaged to revisit the page later.
However, MADV_COLLAPSE is a synchronous operation where userspace
expects immediate results.

Perform synchronous writeback in madvise_collapse() before attempting
collapse to avoid failing on first attempt.

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")
Suggested-by: David Hildenbrand <david@kernel.org>
Signed-off-by: Shivank Garg <shivankg@amd.com>
---
 mm/khugepaged.c | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 97d1b2824386..066a332c76ad 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -22,6 +22,7 @@
 #include <linux/dax.h>
 #include <linux/ksm.h>
 #include <linux/pgalloc.h>
+#include <linux/backing-dev.h>
 
 #include <asm/tlb.h>
 #include "internal.h"
@@ -2784,6 +2785,31 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
 	hstart = (start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
 	hend = end & HPAGE_PMD_MASK;
 
+	/*
+	 * For file-backed VMAs, perform synchronous writeback to ensure
+	 * dirty folios are flushed before attempting collapse. This avoids
+	 * failing on the first attempt when freshly-written executable text
+	 * is still dirty in the page cache.
+	 */
+	if (!vma_is_anonymous(vma) && vma->vm_file) {
+		struct address_space *mapping = vma->vm_file->f_mapping;
+
+		if (mapping_can_writeback(mapping)) {
+			pgoff_t pgoff_start = linear_page_index(vma, hstart);
+			pgoff_t pgoff_end = linear_page_index(vma, hend);
+			loff_t lstart = (loff_t)pgoff_start << PAGE_SHIFT;
+			loff_t lend = ((loff_t)pgoff_end << PAGE_SHIFT) - 1;
+
+			mmap_read_unlock(mm);
+			mmap_locked = false;
+
+			if (filemap_write_and_wait_range(mapping, lstart, lend)) {
+				last_fail = SCAN_FAIL;
+				goto out_maybelock;
+			}
+		}
+	}
+
 	for (addr = hstart; addr < hend; addr += HPAGE_PMD_SIZE) {
 		int result = SCAN_FAIL;
 
-- 
2.43.0