[PATCH AUTOSEL 6.18-6.6] fuse: Invalidate the page cache after FOPEN_DIRECT_IO write

[Sasha Levin <sashal@kernel.org>]

From: Bernd Schubert <bschubert@ddn.com>

[ Upstream commit b359af8275a982a458e8df6c6beab1415be1f795 ]

generic_file_direct_write() also does this and has a large
comment about.

Reproducer here is xfstest's generic/209, which is exactly to
have competing DIO write and cached IO read.

Signed-off-by: Bernd Schubert <bschubert@ddn.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---

LLM Generated explanations, may be completely bogus:

## Analysis Summary

### The Bug

This commit fixes a **data coherency bug** in FUSE when using
`FOPEN_DIRECT_IO`. The problem occurs when:
1. A direct I/O write completes on a FUSE filesystem
2. A concurrent buffered read has populated the page cache with (now
   stale) data
3. Without invalidation, the cached read returns **stale data**

The VFS layer handles this in `generic_file_direct_write()`
(mm/filemap.c:4172-4224) by calling
`kiocb_invalidate_post_direct_write()` after direct writes. The comment
in VFS explicitly mentions this is needed to "invalidate clean pages
which might have been cached by non-direct readahead."

### The Fix

The fix adds the same post-write invalidation pattern to FUSE's
`fuse_direct_io()` function:
- After a successful write (`res > 0 && write && fopen_direct_io`)
- Call `invalidate_inode_pages2_range(mapping, idx_from, idx_to)`
- This ensures stale read-ahead pages are invalidated

**Code size:** +9 lines, single function, single file

### Stable Kernel Criteria Assessment

| Criterion | Assessment |
|-----------|------------|
| Obviously correct | ✅ Yes - follows established VFS pattern |
| Fixes real bug | ✅ Yes - stale data reads (data integrity) |
| Has reproducer | ✅ Yes - xfstest generic/209 |
| Small and contained | ✅ Yes - 9 lines in one function |
| No new features | ✅ Correct - pure bug fix |
| Cc: stable tag | ❌ Not present |
| Fixes: tag | ❌ Not present |

### Dependencies and Backport Concerns

**Critical dependency:** This commit requires commit `80e4f25262f9f`
("fuse: invalidate page cache pages before direct write") which
introduced the `fopen_direct_io` variable and `idx_from`/`idx_to`
calculations. That commit was merged in **v6.6-rc1**.

**Backportable to:**
- stable/linux-6.6.y ✅
- stable/linux-6.11.y ✅
- stable/linux-6.12.y ✅

**NOT backportable to:**
- stable/linux-6.1.y ❌ (missing prerequisite code)
- Earlier LTS kernels ❌

### Risk Assessment

**LOW RISK:**
- Very small change (+9 lines)
- Uses existing, well-tested API (`invalidate_inode_pages2_range`)
- Follows the same pattern as the VFS layer
- Error return from invalidation is silently ignored (same as VFS
  behavior - "if this invalidation fails, tough, the write still
  worked...")
- Only affects FUSE filesystems using `FOPEN_DIRECT_IO` with concurrent
  cached reads

### User Impact

- **Severity:** Medium-High - stale data reads are a data integrity
  issue
- **Affected users:** FUSE filesystem users enabling `FOPEN_DIRECT_IO`
  (including some high-performance storage systems)
- **Reproducer:** Clear, well-known xfstest (generic/209)

### Verdict

Despite lacking explicit stable tags, this is a legitimate data
integrity fix. The bug causes **real user-visible corruption** (stale
data reads), the fix is small and surgical, follows an established VFS
pattern, and has low regression risk. The lack of `Cc: stable` doesn't
disqualify it - many important fixes arrive without explicit tags.

For kernels 6.6+, this should be backported. For earlier kernels, the
prerequisite code doesn't exist, so backporting would require additional
work.

**YES**

 fs/fuse/file.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index c5c82b3807911..bb4ecfd469a5e 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -1681,6 +1681,15 @@ ssize_t fuse_direct_io(struct fuse_io_priv *io, struct iov_iter *iter,
 	if (res > 0)
 		*ppos = pos;
 
+	if (res > 0 && write && fopen_direct_io) {
+		/*
+		 * As in generic_file_direct_write(), invalidate after the
+		 * write, to invalidate read-ahead cache that may have competed
+		 * with the write.
+		 */
+		invalidate_inode_pages2_range(mapping, idx_from, idx_to);
+	}
+
 	return res > 0 ? res : err;
 }
 EXPORT_SYMBOL_GPL(fuse_direct_io);
-- 
2.51.0