Re: [Xen-devel] [External] Re: [RFC PATCH v2 00/12] get rid of GFP_ZONE_TABLE/BAD

[Michal Hocko <mhocko@xxxxxxxxxx>]

On Wed 30-05-18 09:02:13, Huaisheng HS1 Ye wrote:
> From: owner-linux-mm@xxxxxxxxx [mailto:owner-linux-mm@xxxxxxxxx] 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

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