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

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Message-Id: <20180912103305.GC6719@rapoport-lnx>
Date:   Wed, 12 Sep 2018 13:33:06 +0300
From:   Mike Rapoport <rppt@...ux.vnet.ibm.com>
To:     Jonathan Corbet <corbet@....net>
Cc:     Michal Hocko <mhocko@...e.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 3/3] docs: core-api: add memory allocation guide

On Tue, Sep 11, 2018 at 11:55:55AM -0600, Jonathan Corbet wrote:
> 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@...ux.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?

Well, I've amended the original text from gfp.h in attempt to make it more
"user friendly". The GFP_HIGHUSER became really confusing :)
I think that we can drop mentions of hardware from GFP_HIGHUSER_MOVABLE and
GFP_USER, but it makes sense to leave the example in the GFP_HIGHUSER
description.

How about:

    ``GFP_HIGHUSER_MOVABLE`` does not require that allocated memory
    will be directly accessible by the kernel 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. 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

 
> Thanks,
> 
> jon
> 

-- 
Sincerely yours,
Mike.