RFC ext4: Fix stale data read exposure problem with DIO read/page_mkwrite

Hello All,

Sorry for the delay on this patchset. I guess it's because there were lot of other context switches while working at it.

Please note that this is a RFC patch and also a WIP (due to a open problem listed below).
There is also another thread going on where making dioread_nolock as default mount opt [1] is being discussed. That approach should also solve the given race at hand. But since nothing is finalized yet, so I wanted to get this patch out for early review/discussion.

About patch
===========

Currently there is a small race window as pointed out by Jan [2] where, when ext4 tries to allocate a written block for mapped files and if DIO read is in progress, then this may result into stale data read exposure problem.

This patch tries to fix the mentioned issue by:
1. For non-delalloc path, page_mkwrite will use unwritten blocks by default for extent based files.

2. For delalloc path, we check if DIO is in progress during writeback. If yes, then we use unwritten blocks method to avoid this race.

Patch-1: This moves the inode_dio_begin() call before calling for filemap_write_and_wait_range.

Patch-2: This implementes the points (1) & (2) mentioned above.

Testing:
========
xfstests "-g auto" ran fine except one warn_on issue.

Below tests are giving kernel WARN_ON from "ext4_journalled_invalidatepage()", with 1024 blocksize, 4K pagesize & with "nodelalloc,data=journal" mount opt.
- generic/013, generic/269, generic/270

In case if someone has any pointers around this, I could dig more deeper into this.

References
==========
[1] https://www.spinics.net/lists/linux-ext4/msg69224.html
[2] https://lore.kernel.org/linux-ext4/[email protected]/

Ritesh Harjani (2):
iomap: direct-io: Move inode_dio_begin before
filemap_write_and_wait_range
ext4: Fix stale data read issue with DIO read & ext4_page_mkwrite path

 fs/ext4/inode.c      | 45 +++++++++++++++++++++++++++++++-------------
 fs/iomap/direct-io.c | 17 +++++++++++++----
 2 files changed, 45 insertions(+), 17 deletions(-)

2.21.0