[PATCH] f2fs: fix to zero post-EOF data when extending file size

[PATCH] f2fs: fix to zero post-EOF data when extending file size

[Chao Yu]

generic/794  4s ... - output mismatch (see /share/git/fstests/results//generic/794.out.bad)
    --- tests/generic/794.out   2026-06-12 08:46:32.766426241 +0800
    +++ /share/git/fstests/results//generic/794.out.bad 2026-07-05 18:32:55.000000000 +0800
    @@ -1,4 +1,16 @@
     QA output created by 794
     append_write
    +FAIL: non-zero data in gap [4080,4096) after shutdown+remount
    +000000 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a  >ZZZZZZZZZZZZZZZZ<
    +*
    +001000
     truncate_up
    ...
    (Run 'diff -u /share/git/fstests/tests/generic/794.out /share/git/fstests/results//generic/794.out.bad'  to see the entire diff)
Ran: generic/794
Failures: generic/794
Failed 1 of 1 tests

Steps of generic/794:
1. write 4096 bytes to file w/ 0x5a
2. use fiemap to get PBA of first block in file
3. truncate file to 4080
4. umount; write 4096 bytes to file w/ 0x5a directly via PBA; mount
5. extend filesize via
   a) append 4096 from offset 4096, or
   b) truncate 8192, or
   c) fallocate 4096 from offset 4096
6. verify the gap is zeroed in memory [4080,4096)
7. sync range 4096 from offset 4096; shutdown -f (flush meta before shutdown)
8. umount; mount; verify [4080,4096) is zeroed or not.

When extending file size (e.g. via truncate, fallocate, or write) across an
unaligned EOF boundary, we need to ensure that post-EOF data in the partial
page is zeroed out in pagecache and marked dirty, meanwhile, tagging the inode
with FI_ZERO_POST_EOF, so that following checkpoint() and fsync() can persist
the page contain zeroed data before committing inode w/ updated i_size.

This help to prevent stale disk data beyond the previous EOF from being exposed
after remounting or crash recovery.

Since f2fs is a LFS filesystem, we only support direct write via PBA in pinfile,
and pinfile has section-aligned filesize, so in Android, there should no problem,
but for other usage in different environment, let's fix this w/ fsync_mode=strict
mount option.

Cc: stable@kernel.org
Signed-off-by: Chao Yu <chao@kernel.org>
---
 fs/f2fs/checkpoint.c |  4 ++++
 fs/f2fs/f2fs.h       |  1 +
 fs/f2fs/file.c       | 35 ++++++++++++++++++++++++++++++++---
 3 files changed, 37 insertions(+), 3 deletions(-)

diff --git a/fs/f2fs/checkpoint.c b/fs/f2fs/checkpoint.c
index 064f5b537423..6f486d3a94c7 100644
--- a/fs/f2fs/checkpoint.c
+++ b/fs/f2fs/checkpoint.c
@@ -1359,6 +1359,10 @@ static int f2fs_sync_inode_meta(struct f2fs_sb_info *sbi)
 		inode = igrab(&fi->vfs_inode);
 		spin_unlock(&sbi->inode_lock[DIRTY_META]);
 		if (inode) {
+			if (is_inode_flag_set(inode, FI_ZERO_POST_EOF)) {
+				filemap_write_and_wait(inode->i_mapping);
+				clear_inode_flag(inode, FI_ZERO_POST_EOF);
+			}
 			sync_inode_metadata(inode, 0);
 
 			/* it's on eviction */
diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
index 8e2fb0bda467..41be09dad43c 100644
--- a/fs/f2fs/f2fs.h
+++ b/fs/f2fs/f2fs.h
@@ -939,6 +939,7 @@ enum {
 	FI_ATOMIC_REPLACE,	/* indicate atomic replace */
 	FI_OPENED_FILE,		/* indicate file has been opened */
 	FI_DONATE_FINISHED,	/* indicate page donation of file has been finished */
+	FI_ZERO_POST_EOF,	/* indicate unaligned EOF gap was zeroed in pagecache */
 	FI_MAX,			/* max flag, never be used */
 };
 
diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
index 11cc8d79c235..2e9d585073ec 100644
--- a/fs/f2fs/file.c
+++ b/fs/f2fs/file.c
@@ -36,21 +36,43 @@
 #include <trace/events/f2fs.h>
 #include <uapi/linux/f2fs.h>
 
+static int fill_zero(struct inode *inode, pgoff_t index,
+					loff_t start, loff_t len);
+
 static void f2fs_zero_post_eof_page(struct inode *inode,
 					loff_t new_size, bool lock)
 {
 	loff_t old_size = i_size_read(inode);
+	unsigned int offset;
 
 	if (old_size >= new_size)
 		return;
 
-	if (mapping_empty(inode->i_mapping))
-		return;
-
 	if (lock)
 		filemap_invalidate_lock(inode->i_mapping);
+
 	/* zero or drop pages only in range of [old_size, new_size] */
 	truncate_inode_pages_range(inode->i_mapping, old_size, new_size);
+
+	/*
+	 * When expanding an unaligned EOF size, zero post-EOF data in
+	 * pagecache and set FI_ZERO_POST_EOF, so following checkpointing
+	 * or fsync can persist correct data to disk before committing
+	 * inode w/ updated i_size.
+	 */
+	if (F2FS_OPTION(F2FS_I_SB(inode)).fsync_mode != FSYNC_MODE_STRICT)
+		goto out_unlock;
+
+	offset = old_size & (PAGE_SIZE - 1);
+	if (offset) {
+		unsigned int len = min_t(loff_t, PAGE_SIZE - offset,
+							new_size - old_size);
+		pgoff_t index = old_size >> PAGE_SHIFT;
+
+		fill_zero(inode, index, offset, len);
+		set_inode_flag(inode, FI_ZERO_POST_EOF);
+	}
+out_unlock:
 	if (lock)
 		filemap_invalidate_unlock(inode->i_mapping);
 }
@@ -304,6 +326,13 @@ static int f2fs_do_sync_file(struct file *file, loff_t start, loff_t end,
 	if (S_ISDIR(inode->i_mode))
 		goto go_write;
 
+	if (is_inode_flag_set(inode, FI_ZERO_POST_EOF)) {
+		ret = filemap_write_and_wait(inode->i_mapping);
+		if (ret)
+			return ret;
+		clear_inode_flag(inode, FI_ZERO_POST_EOF);
+	}
+
 	/* if fdatasync is triggered, let's do in-place-update */
 	if (datasync || get_dirty_pages(inode) <= SM_I(sbi)->min_fsync_blocks)
 		set_inode_flag(inode, FI_NEED_IPU);
-- 
2.49.0