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

From: Michal Hocko [mailto:mhocko@xxxxxxxxxx]
Sent: Thursday, May 24, 2018 8:19 PM> 
> > Let me try to reply your questions.
> > Exactly, GFP_ZONE_TABLE is too complicated. I think there are two advantages
> > from the series of patches.
> >
> > 1. XOR operation is simple and efficient, GFP_ZONE_TABLE/BAD need to do 
> > twice
> > shift operations, the first is for getting a zone_type and the second is for
> > checking the to be returned type is a correct or not. But with these patch 
> > XOR
> > operation just needs to use once. Because the bottom 3 bits of GFP bitmask 
> > have
> > been used to represent the encoded zone number, we can say there is no bad 
> > zone
> > number if all callers could use it without buggy way. Of course, the 
> > returned
> > zone type in gfp_zone needs to be no more than ZONE_MOVABLE.
> But you are losing the ability to check for wrong usage. And it seems
> that the sad reality is that the existing code do screw up.

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.

Ideally, before any user wants to modify the address zone modifier, they should 
clear it firstly, then ORing the GFP zone flag which comes from the zone they 
With these patches, we can loudly announce that, the bottom 3 bits of zone mask 
couldn't accept internal ORing operations.
The operations like __GFP_DMA | __GFP_DMA32 | __GFP_HIGHMEM is illegal. The 
current GFP_ZONE_TABLE is precisely the root of this problem, that is 
__GFP_DMA, __GFP_DMA32 and __GFP_HIGHMEM are formatted as 0x1, 0x2 and 0x4.

> > 2. GFP_ZONE_TABLE has limit with the amount of zone types. Current 
> > is 32 bits, in general, there are 4 zone types for most ofX86_64 platform, 
> > they
> > are ZONE_DMA, ZONE_DMA32, ZONE_NORMAL and ZONE_MOVABLE. If we want to 
> > expand the
> > amount of zone types to larger than 4, the zone shift should be 3.
> But we do not want to expand the number of zones IMHO. The existing zoo
> is quite a maint. pain.
> That being said. I am not saying that I am in love with GFP_ZONE_TABLE.
> It always makes my head explode when I look there but it seems to work
> with the current code and it is optimized for it. If you want to change
> this then you should make sure you describe reasons _why_ this is an
> improvement. And I would argue that "we can have more zones" is a
> relevant one.

Yes, GFP_ZONE_TABLE is too complicated. The patches have 4 advantages as below.

* The address zone modifiers have new operation method, that is, user should 
decide which zone is preferred at first, then give the encoded zone number to 
bottom 3 bits in GFP mask. That is much direct and clear than before.

* No bad zone combination, because user should choose just one address zone 
modifier always.
* Better performance and efficiency, current gfp_zone has to take shifting 
operation twice for GFP_ZONE_TABLE and GFP_ZONE_BAD. With these patches, 
gfp_zone() just needs one XOR.
* Up to 8 zones can be used. At least it isn't a disadvantage, right?

Huaisheng Ye

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