From: Jonathan Corbet <corbet@lwn.net>
To: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Michal Hocko <mhocko@suse.com>,
Randy Dunlap <rdunlap@infradead.org>,
Matthew Wilcox <willy@infradead.org>,
Vlastimil Babka <vbabka@suse.cz>,
linux-mm@kvack.org, linux-doc@vger.kernel.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 3/3] docs: core-api: add memory allocation guide
Date: Tue, 11 Sep 2018 11:55:55 -0600 [thread overview]
Message-ID: <20180911115555.5fce5631@lwn.net> (raw)
In-Reply-To: <1534517236-16762-4-git-send-email-rppt@linux.vnet.ibm.com>
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
next prev parent reply other threads:[~2018-09-11 17:55 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-08-17 14:47 [PATCH v3 0/3] docs/core-api: add memory allocation guide Mike Rapoport
2018-08-17 14:47 ` [PATCH v3 1/3] docs: core-api/gfp_mask-from-fs-io: add a label for cross-referencing Mike Rapoport
2018-08-17 14:47 ` [PATCH v3 2/3] docs: core-api/mm-api: add a lable for GFP flags section Mike Rapoport
2018-08-17 14:47 ` [PATCH v3 3/3] docs: core-api: add memory allocation guide Mike Rapoport
2018-09-11 17:55 ` Jonathan Corbet [this message]
2018-09-12 10:33 ` Mike Rapoport
2018-09-13 22:41 ` Jonathan Corbet
2018-09-03 5:12 ` [PATCH v3 0/3] docs/core-api: " Mike Rapoport
2018-09-11 16:24 ` Mike Rapoport
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