MAXPHYS bump for FreeBSD 13

[Alexander Motin]

On Fri, 13 Nov 2020 21:09:37 +0200 Konstantin Belousov wrote:
> To put the specific numbers, for struct buf it means increase by 1792
> bytes. For bio it does not, because it does not embed vm_page_t[] into
> the structure.
>
> Worse, typical struct buf addend for excess vm_page pointers is going
> to be unused, because normal size of the UFS block is 32K.  It is
> going to be only used by clusters and physbufs.
>
> So I object against bumping this value without reworking buffers
> handling of b_pages[].  Most straightforward approach is stop using
> MAXPHYS to size this array, and use external array for clusters.
> Pbufs can embed large array.

I am not very familiar with struct buf usage, so I'd appreciate some
help there.

Quickly looking on pbuf, it seems trivial to allocate external b_pages
array of any size in pbuf_init, that should easily satisfy all of pbuf
descendants.  Cluster and vnode/swap pagers code are pbuf descendants
also.  Vnode pager I guess may only need replacement for
nitems(bp->b_pages) in few places.

Could you or somebody help with vfs/ffs code, where I suppose the
smaller page lists are used?

-- 
Alexander Motin