Re: RFC: what to do about KASSERT() in allocuio()?

On Wed, May 13, 2026 at 7:50 PM Konstantin Belousov <kostikbel@gmail.com> wrote:
>
> On Wed, May 13, 2026 at 05:30:46PM -0700, Rick Macklem wrote:
> > Hi,
> >
> > The following KASSERT() is at the beginning of allocuio():
> > KASSERT(iovcnt <= UIO_MAXIOV,
> >     ("Requested %u iovecs exceed UIO_MAXIOV", iovcnt));
> >
> > This fails for the NFS server if it is configured for > 1Mbyte I/O
> > size, since the number of elements (mbufs) for the VOP_READ()
> > exceeds UIO_MAXIOV (1024). This shows up because ZFS
> > does a cloneuio() call which calls allocuio().
> >
> > Since UIO_MAXIOV is used is several places, including setting
> > the limit for copyinuio() and freebsd32_copyinuio(), I don't think
> > changing the value of UIO_MAXIOV is an appropriate fix.
> > (ie. This changes the APIs, etc.)
> >
> > Now, since all that the above check does it set a sanity limit
> > on how big the allocated uio can be, do you think it is
> > reasonable to change the above KASSERT() to:
> > KASSERT(iovcnt <= 4096,
> >     ("Requested %u iovecs exceed 4096", iovcnt));
> > which would allow a 4Mbyte NFS I/O to work.
> >
> > Note that copyinuio() and freebsd32_copyinuio() check the
> > iov length for < UIO_MAXIOV before calling allocuio(), so
> > those interfaces are not broken by this.
>
> I do not quite understand how changing the assert in allocuio() would
> change anything?  All callers of the function (copyinuio, cloneuio,
> and freebsd32_copyinuio) check iovcnt, and I do not see a call to
> allocuio() from the NFS server.
>
> Where specifically does the NFS server fail with too long iovec?
zfs_freebsd_write()->zfs_write()->zfs_uiocopy()->cloneuio()->allocuio()
(and zfs_freebsd_write() is ZFS's VOP_WRITE())

rick

>