RE: [External] Re: [PATCH 2/3] include/linux/gfp.h: use unsigned int in gfp_zone

[Huaisheng HS1 Ye <yehs1@lenovo.com>]

> On Mon, May 07, 2018 at 05:16:50PM +0000, Huaisheng HS1 Ye wrote:
> > I hope it couldn't cause problem, but based on my analyzation it has the
> potential to go wrong if users still use the flags as usual, which are __GFP_DMA,
> __GFP_DMA32 and __GFP_HIGHMEM.
> > Let me take an example with my testing platform, these logics are much
> abstract, an example will be helpful.
> >
> > There is a two sockets X86_64 server, No HIGHMEM and it has 16 + 16GB
> memories.
> > Its zone types shall be like this below,
> >
> > ZONE_DMA		0		0b0000
> > ZONE_DMA32		1		0b0001
> > ZONE_NORMAL		2		0b0010
> > (OPT_ZONE_HIGHMEM)	2		0b0010
> > ZONE_MOVABLE		3		0b0011
> > ZONE_DEVICE		4		0b0100 (virtual zone)
> > __MAX_NR_ZONES	5
> >
> > __GFP_DMA	= ZONE_DMA ^ ZONE_NORMAL= 0b0010
> > __GFP_DMA32	= ZONE_DMA32 ^ ZONE_NORMAL= 0b0011
> > __GFP_HIGHMEM = OPT_ZONE_HIGHMEM ^ ZONE_NORMAL = 0b0000
> > __GFP_MOVABLE	= ZONE_MOVABLE ^ ZONE_NORMAL |
> ___GFP_MOVABLE = 0b1001
> >
> > Eg.
> > If a driver uses flags like this below,
> > Step 1:
> > gfp_mask  |  __GFP_DMA32;
> > (0b 0000		|	0b 0011	= 0b 0011)
> > gfp_mask's low four bits shall equal to 0011, assuming no __GFP_MOVABLE
> >
> > Step 2:
> > gfp_mask  & ~__GFP_DMA;
> > (0b 0011	 & ~0b0010   = 0b0001)
> > gfp_mask's low four bits shall equal to 0001 now, then when it enter
> gfp_zone(),
> >
> > return ((__force int)flags & ___GFP_ZONE_MASK) ^ ZONE_NORMAL;
> > (0b0001 ^ 0b0010 = 0b0011)
> > You know 0011 means that ZONE_MOVABLE will be returned.
> > In this case, error can be found, because gfp_mask needs to get
> ZONE_DMA32 originally.
> > But with existing GFP_ZONE_TABLE/BAD, it is correct. Because the bits are
> way of 0x1, 0x2, 0x4, 0x8
> 
> Yes, I understand your point here.  My point was that this was already a bug;
> the caller shouldn't simply be clearing __GFP_DMA; they really mean to clear
> all of the GFP_ZONE bits so that they allocate from ZONE_NORMAL.  And for
> that, they should be using ~GFP_ZONEMASK
That is great, if they can follow this principle, I don't worry it. Maybe I am too cautious.

> 
> Unless they already know, of course.  For example, this one in
> arch/x86/mm/pgtable.c is fine:
> 
>         if (strcmp(arg, "nohigh") == 0)
>                 __userpte_alloc_gfp &= ~__GFP_HIGHMEM;
> 
> because it knows that __userpte_alloc_gfp can only have __GFP_HIGHMEM set.
> 
> But something like btrfs should almost certainly be using ~GFP_ZONEMASK.


> > > +#define __GFP_HIGHMEM  ((__force gfp_t)OPT_ZONE_HIGHMEM ^
> > > ZONE_NORMAL)
> > > -#define __GFP_MOVABLE  ((__force gfp_t)___GFP_MOVABLE)  /*
> > > ZONE_MOVABLE allowed */
> > > +#define __GFP_MOVABLE  ((__force gfp_t)ZONE_MOVABLE ^
> > > ZONE_NORMAL | \
> > > +					___GFP_MOVABLE)
> > >
> > > Then I think you can just make it:
> > >
> > > static inline enum zone_type gfp_zone(gfp_t flags)
> > > {
> > > 	return ((__force int)flags & ___GFP_ZONE_MASK) ^ ZONE_NORMAL;
> > > }
> > Sorry, I think it has risk in this way, let me introduce a failure case for
> example.
> >
> > Now suppose that, there is a flag should represent DMA flag with movable.
> > It should be like this below,
> > __GFP_DMA | __GFP_MOVABLE
> > (0b 0010       |   0b 1001   = 0b 1011)
> > Normally, gfp_zone shall return ZONE_DMA but with MOVABLE policy, right?
> 
> No, if you somehow end up with __GFP_MOVABLE | __GFP_DMA, it should give
> you ZONE_DMA.
Exactly, it should return ZONE_DMA, that's what I thought.

> 
> > But with your code, gfp_zone will return ZONE_DMA32 with MOVABLE
> >policy.
> > (0b 1011  ^  0b 0010 = 1001)
> 
> ___GFP_ZONE_MASK is 0x7, so it excludes __GFP_MOVABLE.
Sorry, I made a mistake here. I rewrite it as below.

((__GFP_DMA | __GFP_MOVABLE) & ___GFP_ZONE_MASK)
   ((0b 0010  |  0b 1001  = 0b 1011) & 0b 0111)	= 0b 0011

0b 0011 ^ 0b 0010 = 0b 0001
So ZONE_DMA32 will be returned, but what user needs is ZONE_DMA.

Thanks,
Huaisheng