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Date:   Wed, 30 May 2018 11:12:06 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Huaisheng HS1 Ye <yehs1@lenovo.com>
Cc:     "akpm@linux-foundation.org" <akpm@linux-foundation.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
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        NingTing Cheng <chengnt@lenovo.com>,
        Ocean HY1 He <hehy1@lenovo.com>,
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        "linux-btrfs@vger.kernel.org" <linux-btrfs@vger.kernel.org>,
        Christoph Hellwig <hch@lst.de>
Subject: Re: [External]  Re: [RFC PATCH v2 00/12] get rid of
 GFP_ZONE_TABLE/BAD
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On Wed 30-05-18 09:02:13, Huaisheng HS1 Ye wrote:
> From: owner-linux-mm@kvack.org [mailto:owner-linux-mm@kvack.org] On Behalf Of Michal Hocko
> Sent: Monday, May 28, 2018 9:38 PM
> > > In my opinion, originally there shouldn't be such many wrong
> > > combinations of these bottom 3 bits. For any user, whether or
> > > driver and fs, they should make a decision that which zone is they
> > > preferred. Matthew's idea is great, because with it the user must
> > > offer an unambiguous flag to gfp zone bits.
> > 
> > Well, I would argue that those shouldn't really care about any zones at
> > all. All they should carea bout is whether they really need a low mem
> > zone (aka directly accessible to the kernel), highmem or they are the
> > allocation is generally movable. Mixing zones into the picture just
> > makes the whole thing more complicated and error prone.
> 
> Dear Michal,
> 
> I don't quite understand that. I think those, mostly drivers, need to
> get the correct zone they want. ZONE_DMA32 is an example, if drivers can be
> satisfied with a low mem zone, why they mark the gfp flags as
> 'GFP_KERNEL|__GFP_DMA32'?
> GFP_KERNEL is enough to make sure a directly accessible low mem, but it is
> obvious that they want to get a DMA accessible zone below 4G.

They want a specific pfn range. Not a _zone_. Zone is an MM abstraction
to manage memory. And not a great one as the time has shown. We have
moved away from the per-zone reclaim because it just turned out to be
problematic. Leaking this abstraction to users was a mistake IMHO. It
was surely convenient but we can clearly see it was just confusing and
many users just got it wrong.

I do agree with Christoph in other email that the proper way for DMA
users is to use the existing DMA API which is more towards what they
need. Set a restriction on dma-able memory ranges.
-- 
Michal Hocko
SUSE Labs