Re: [PATCH v6 1/3] memory: implement memory_block_advise/probe_max_size

[Gregory Price]

On Tue, Nov 12, 2024 at 01:28:03PM -0800, Dan Williams wrote:
> Gregory Price wrote:
> > Hotplug memory sources may have opinions on what the memblock size
> > should be - usually for alignment purposes.  For example, CXL memory
> > extents can be 256MB with a matching alignment. If this size/alignment
> > is smaller than the block size, it can result in stranded capacity.
> > 
> > Implement memory_block_advise_max_size for use prior to allocator init,
> > for software to advise the system on the max block size.
> > 
> > Implement memory_block_probe_max_size for use by arch init code to
> > calculate the best block size. Use of advice is architecture defined.
> > 
> > The probe value can never change after first probe. Calls to advise
> > after probe will return -EBUSY to aid debugging.
> > 
> > On systems without hotplug, always return -ENODEV and 0 respectively.
> 
> Should the advice just succeed when the result does not matter?
> 

I figure at some point during __init the value will be probed and subsequent
calls will be ignored. I'd rather fail explicitly in that case to assist
debugging - otherwise it might be a little maddening to discover your callsite
is too late in the process.

> Otherwise, it depends on the caller to not care based on config.
> 
> I do not feel that strongly about it, so either way:
> 
> Acked-by: Dan Williams <dan.j.williams@xxxxxxxxx>