While qualifying NFSD's NFSD_IO_DIRECT write path with byte-level data verification, it was found that the ITER_BVEC payloads nfsd submits expose silent data corruption in two bio-based block drivers and two defects in nfsd itself. Example problematic payloads is the first fragment starts mid-page because the RPC header precedes it in the receive buffer, and fragment lengths need not be sector multiples. bio_iov_bvec_set() passes such an array to the queue as-is; nothing below it validates per-bvec sector alignment.
Patches 1-2 fix silent corruption (write completes successfully, data lands wrong) and are stable candidates: - brd re-derives each segment's device position from bio->bi_iter.bi_sector, which bio_advance_iter_single() advances by whole sectors only, so a sub-sector segment length skews everything that follows. - zram hardwires is_partial_io() to false on 4K-page kernels, sending sub-page bvecs down a whole-page path that ignores bv_offset/bv_len entirely, and has the same sector-cursor skew. Both are verified with a synthetic-bio reproducer (stamped pattern, write, read back, compare) across mid-page and page-aligned geometries. Patches 3-4 fix nfsd: the filecache never fetches DIO alignment attributes on the supplied-file acquire branch, so every WRITE to a file created via NFSv4 OPEN(CREATE) is refused direct I/O for the file's cached lifetime; and nfsd's statx-based DIO gate is weaker than bio_split_io_at()'s split-time checks, so an admitted iterator can still be rejected by the block layer -- retry the segment buffered instead of failing a valid WRITE with NFS4ERR_INVAL. One open question for the iomap/block maintainers: should ITER_BVEC direct I/O with sub-sector bvec boundaries be validated or bounced centrally rather than trusted to every driver's iteration? An audit of in-tree bio-based drivers found the same bi_sector-derived position pattern in dm-io, dm-log-writes, dm-writecache (pmem path) and dm-integrity -- unreachable through nfsd today only because dm queues advertise dma_alignment >= 511, which nfsd's alignment gate refuses. Tested with the reproducer matrix on brd, zram and nvme-loop at 4K and 16K page size (aarch64) and 4K (x86_64), plus 30-connection NFS write rigs comparing source against export byte-for-byte: clean with the fixes, corrupting or erroring without them. Mike Snitzer (3): brd: iterate the bio by byte position, not bi_sector zram: handle sub-page bvec segments without corrupting data nfsd: fall back to buffered I/O when a direct write gets -EINVAL David Flynn (1): nfsd: fetch direct I/O alignment for files handed to the filecache drivers/block/brd.c | 29 +++++++++++++++++++++++------ drivers/block/zram/zram_drv.c | 36 +++++++++++++++++------------------ fs/nfsd/filecache.c | 4 ++-- fs/nfsd/vfs.c | 29 +++++++++++++++++++++++++++++ 4 files changed, 77 insertions(+), 31 deletions(-) -- 2.52.0
