Re: [PATCH] fs: remove power of 2 and length boundary atomic write restrictions
From: John Garry <john.g.garry@oracle.com>
Date: 2025-12-30 07:54:37
Also in:
linux-fsdevel, linux-nvme
On 24/12/2025 11:53, Vitaliy Filippov wrote:
generic_atomic_write_valid() returns EINVAL for non-power-of-2 and for non-length-aligned writes. This check is used for block devices, ext4 and xfs, but neither ext4 nor xfs rely on power of 2 restrictions. For block devices, neither NVMe nor SCSI specification doesn't require length alignment and 2^N length. Both specifications only require to respect the atomic write boundary if it's set (NABSPF/NABO for NVMe and ATOMIC BOUNDARY for SCSI).
NVMe subsystem already checks writes against this boundary; SCSI uses an explicit atomic write command so the write is checked by the drive itself.
Yes, they do check it - this is a safeguard against being sent something which cannot be atomically written. But we should not be sending something to the driver or disk which cannot be atomically written. So we are providing protection against kernel bugs. The user should not be concerned about atomic boundaries. They should not encounter a scenario where they try a write which crosses a boundary (and cannot be atomically written). Hence the power-of-2 and alignment rule to avoid this.
quoted hunk ↗ jump to hunk
Signed-off-by: Vitaliy Filippov <redacted> --- fs/read_write.c | 8 -------- 1 file changed, 8 deletions(-)diff --git a/fs/read_write.c b/fs/read_write.c index 833bae068770..5467d710108d 100644 --- a/fs/read_write.c +++ b/fs/read_write.c@@ -1802,17 +1802,9 @@ int generic_file_rw_checks(struct file *file_in, struct file *file_out) int generic_atomic_write_valid(struct kiocb *iocb, struct iov_iter *iter) { - size_t len = iov_iter_count(iter); - if (!iter_is_ubuf(iter)) return -EINVAL; - if (!is_power_of_2(len)) - return -EINVAL; - - if (!IS_ALIGNED(iocb->ki_pos, len)) - return -EINVAL; - if (!(iocb->ki_flags & IOCB_DIRECT)) return -EOPNOTSUPP;