[PATCH 2/6] fs/ntfs3: keep page cache data when converting resident $DATA

[Kamil Chojnowski via B4 Relay]

From: Kamil Chojnowski <contact@xxxxxxxxx>

attr_make_nonresident() fills folio 0 from the resident copy of $DATA
and marks it dirty.  If the folio is uptodate, it holds the newest data:
stores through a shared mmap dirty the folio and reach the resident copy
only at writeback.  Overwriting the folio loses them:

  pwrite 300 x 'A'; mmap and overwrite with 'B'; pwrite 2000 x 'C'
  -> the file reads back 'A' x 300 + 'C' x 2000

Fill the folio from the resident copy only when it is not uptodate;
otherwise keep it and zero it past the old size.

This depends on the previous patch, which passes the locked folio 0 to
attr_make_nonresident().

Fixes: be71b5cba2e6 ("fs/ntfs3: Add attrib operations")
Cc: stable@xxxxxxxxxxxxxxx
Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Kamil Chojnowski <contact@xxxxxxxxx>
---
 fs/ntfs3/attrib.c | 47 ++++++++++++++++++++++++++++++-----------------
 1 file changed, 30 insertions(+), 17 deletions(-)

diff --git a/fs/ntfs3/attrib.c b/fs/ntfs3/attrib.c
index 080edebe529e..b21c913050f7 100644
--- a/fs/ntfs3/attrib.c
+++ b/fs/ntfs3/attrib.c
@@ -329,27 +329,40 @@ int attr_make_nonresident(struct ntfs_inode *ni, struct ATTRIB *attr,
 			err = ntfs_sb_write_run(sbi, run, 0, data, rsize, 0);
 			if (err)
 				goto out2;
-		} else if (folio) {
-			folio_fill_tail(folio, 0, data, rsize);
-			folio_mark_uptodate(folio);
-			folio_mark_dirty(folio);
 		} else if (!page) {
 			struct address_space *mapping = ni->vfs_inode.i_mapping;
+			struct folio *f = folio;
+
+			if (!f) {
+				WARN_ONCE(1, "ntfs3: r=%llx: folio 0 locked under ni_lock",
+					  (u64)ni->mi.rno);
+				f = __filemap_get_folio(
+					mapping, 0,
+					FGP_LOCK | FGP_ACCESSED | FGP_CREAT,
+					mapping_gfp_mask(mapping));
+				if (IS_ERR(f)) {
+					err = PTR_ERR(f);
+					goto out2;
+				}
+			}
 
-			WARN_ONCE(1, "ntfs3: r=%llx: folio 0 locked under ni_lock",
-				  (u64)ni->mi.rno);
-			folio = __filemap_get_folio(
-				mapping, 0, FGP_LOCK | FGP_ACCESSED | FGP_CREAT,
-				mapping_gfp_mask(mapping));
-			if (IS_ERR(folio)) {
-				err = PTR_ERR(folio);
-				goto out2;
+			if (folio_test_uptodate(f)) {
+				/*
+				 * The page cache copy is the newest one: it may
+				 * be dirty through mmap and not yet written back
+				 * to the resident attribute.  Keep it.
+				 */
+				folio_zero_segment(f, rsize, folio_size(f));
+			} else {
+				folio_fill_tail(f, 0, data, rsize);
+				folio_mark_uptodate(f);
+			}
+			folio_mark_dirty(f);
+
+			if (!folio) {
+				folio_unlock(f);
+				folio_put(f);
 			}
-			folio_fill_tail(folio, 0, data, rsize);
-			folio_mark_uptodate(folio);
-			folio_mark_dirty(folio);
-			folio_unlock(folio);
-			folio_put(folio);
 		}
 	}
 

-- 
2.56.0