Re: [PATCH v3 3/3] docs: core-api: add memory allocation guide

[Jonathan Corbet <corbet@lwn.net>]

Sorry for being so slow to get to this...it fell into a dark crack in my
rickety email folder hierarchy.  I do have one question...

On Fri, 17 Aug 2018 17:47:16 +0300
Mike Rapoport <rppt@linux.vnet.ibm.com> wrote:

> +    ``GFP_HIGHUSER_MOVABLE`` does not require that allocated memory
> +    will be directly accessible by the kernel or the hardware and
> +    implies that the data is movable.
> +
> +    ``GFP_HIGHUSER`` means that the allocated memory is not movable,
> +    but it is not required to be directly accessible by the kernel or
> +    the hardware. An example may be a hardware allocation that maps
> +    data directly into userspace but has no addressing limitations.
> +
> +    ``GFP_USER`` means that the allocated memory is not movable and it
> +    must be directly accessible by the kernel or the hardware. It is
> +    typically used by hardware for buffers that are mapped to
> +    userspace (e.g. graphics) that hardware still must DMA to.

I realize that this is copied from elsewhere, but still...as I understand
it, the "HIGH" part means that the allocation can be satisfied from high
memory, nothing more.  So...it's irrelevant on 64-bit machines to start
with, right?  And it has nothing to do with DMA, I would think.  That would
be handled by the DMA infrastructure and, perhaps, the DMA* zones.  Right?

I ask because high memory is an artifact of how things are laid out on
32-bit systems; hardware can often DMA quite easily into memory that the
kernel sees as "high".  So, to me, this description seems kind of
confusing; I wouldn't mention hardware at all.  But maybe I'm missing
something?

Thanks,

jon