ext4: drain in-flight DIO before buffered write fallback

Summary

In the Linux kernel, the following vulnerability has been resolved: ext4: drain in-flight DIO before buffered write fallback generic/746 started failing intermittently on ext3 (no-extent inodes). The test triggers 'Page cache invalidation failure on direct I/O' warnings and subsequent fsync returns -EIO. Adding a 50ms delay between ext4_buffered_write_iter() and filemap_write_and_wait_range() in ext4_dio_write_iter() makes the race almost always reproducible. On no-extent inodes, DIO writes to holes cannot use unwritten extents, so ext4_iomap_alloc() leaves m_flags=0 and ext4_map_blocks() returns 0. The iomap layer then returns -ENOTBLK, causing fallback to buffered I/O. The fallback path in ext4_dio_write_iter() calls ext4_buffered_write_iter() which dirties pages, then does flush and invalidate. However, there's an unprotected window between ext4_buffered_write_iter() returning (with inode lock released) and the subsequent flush+invalidate. Concurrent async DIO completions from other threads can run kiocb_invalidate_post_direct_write() during this window. If pages have been re-dirtied, post-invalidation finds dirty pages and triggers the warning, setting -EIO in the error sequence. Consider a file with two 4k extents: [hole][written]. Thread A does DIO to the written extent, while thread B does DIO spanning both: kworker A (4k DIO, allocated block) kworker B (8k DIO, fallback) ----------------------------------- ---------------------------- inode_lock_shared() inode_lock_shared() iomap_dio_rw(): iomap_dio_rw(): kiocb_invalidate_pages -> clean iomap_begin -> -ENOTBLK submit_bio (async) dio->size = 0 inode_unlock_shared() inode_unlock_shared() [bio pending in block layer] /* fallback: lock released */ ext4_buffered_write_iter() inode_lock(exclusive) generic_perform_write() -> dirty pages [0, 8k] inode_unlock(exclusive) /* pages dirty, no lock */ [bio completes] filemap_write_and_wait_range() iomap_dio_complete() -> flush dirty pages kiocb_invalidate_post_direct_write() invalidate_mapping_pages() invalidate_inode_pages2_range() -> finds dirty page! -> dio_warn_stale_pagecache() -> errseq_set(-EIO) This issue can be triggered through normal I/O paths, not just intentionally overlapping DIO writes from userspace. For example, generic/746 uses a loop device where multiple kworkers issue concurrent I/O to the backing file. Additionally, when block_size < folio_size, non-overlapping DIO writes that share a large folio can also trigger the race. Add inode_dio_wait() in ext4_buffered_write_iter() before ext4_write_checks() to drain all in-flight DIO. This ensures that all DIO clears existing pages before submitting IO (via kiocb_invalidate_pages()), all BIO waits for all DIO to complete (via inode_dio_wait()), and ext4_write_checks() observes the inode size after all completed DIO so that ext4_block_zero_eof() does not race with in-flight DIO, thus eliminating the race.