Re: [PATCH] fs: remove power of 2 and length boundary atomic write restrictions

[John Garry <john.g.garry@oracle.com>]

On 30/12/2025 09:01, Vitaliy Filippov wrote:
> I think that even with the 2^N requirement the user still has to look
> for boundaries.
> 1) NVMe disks may have NABO != 0 (atomic boundary offset). In this
> case 2^N aligned writes won't work at all.

We don't support NABO != 0

> 2) NABSPF is expressed in blocks in the NVMe spec and it's not
> restricted to 2^N, it can be for example 3 (3*4096 = 12 KB). The spec
> allows it. 2^N breaks this case too.

We could support NABSPF which is not a power-of-2, but we don't today.

If you can find some real HW which has NABSPF which is not a power-of-2, 
then it can be considered.

> And the user also has to look for the maximum atomic write size
> anyway, he can't just assume all writes are atomic out of the box,
> regardless of the 2^N requirement.
> So my idea is that the kernel's task is just to guarantee correctness
> of atomic writes. It anyway can't provide the user with atomic writes
> in all cases.

What good is that to a user?

Consider the user wants to atomic write a range of a file which is 
backed by disk blocks which straddle a boundary - in this case, the 
write would fail. What is the user supposed to do then? That API could 
have arbitrary failures, which effectively makes it a useless API.

As I said before, just don't use RWF_ATOMIC if you don't want to deal 
with these restrictions.