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From:   Huaisheng HS1 Ye <yehs1@lenovo.com>
To:     Matthew Wilcox <willy@infradead.org>,
        "dsterba@suse.cz" <dsterba@suse.cz>
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Subject: RE: [External]  Re: [PATCH 2/3] include/linux/gfp.h: use unsigned int
 in gfp_zone
Thread-Topic: [External]  Re: [PATCH 2/3] include/linux/gfp.h: use unsigned
 int in gfp_zone
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Date:   Wed, 9 May 2018 14:57:07 +0000
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> On Mon, May 07, 2018 at 11:44:10AM -0700, Matthew Wilcox wrote:
> > 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 muc=
h 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	=3D ZONE_DMA ^ ZONE_NORMAL=3D 0b0010
> > > __GFP_DMA32	=3D ZONE_DMA32 ^ ZONE_NORMAL=3D 0b0011
> > > __GFP_HIGHMEM =3D OPT_ZONE_HIGHMEM ^ ZONE_NORMAL =3D 0b0000
> > > __GFP_MOVABLE	=3D ZONE_MOVABLE ^ ZONE_NORMAL | ___GFP_MOVABLE =3D 0b1=
001
> > >
> > > Eg.
> > > If a driver uses flags like this below,
> > > Step 1:
> > > gfp_mask  |  __GFP_DMA32;
> > > (0b 0000		|	0b 0011	=3D 0b 0011)
> > > gfp_mask's low four bits shall equal to 0011, assuming no __GFP_MOVAB=
LE
> > >
> > > Step 2:
> > > gfp_mask  & ~__GFP_DMA;
> > > (0b 0011	 & ~0b0010   =3D 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 =3D 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 f=
or
> > that, they should be using ~GFP_ZONEMASK
> >
> > Unless they already know, of course.  For example, this one in
> > arch/x86/mm/pgtable.c is fine:
> >
> >         if (strcmp(arg, "nohigh") =3D=3D 0)
> >                 __userpte_alloc_gfp &=3D ~__GFP_HIGHMEM;
> >
> > because it knows that __userpte_alloc_gfp can only have __GFP_HIGHMEM s=
et.
> >
> > But something like btrfs should almost certainly be using ~GFP_ZONEMASK=
.
>=20
> Agreed, the direct use of __GFP_DMA32 was added in 3ba7ab220e8918176c6f
> to substitute GFP_NOFS, so the allocation flags are less restrictive but
> still acceptable for allocation from slab.
>=20
> The requirement from btrfs is to avoid highmem, the 'must be acceptable
> for slab' requirement is more MM internal and should have been hidden
> under some opaque flag mask. There was no strong need for that at the
> time.

Hi Matthew,

Should we add an error detection in gfp_zone? How about this?

@@ -377,6 +377,8 @@ static inline enum zone_type gfp_zone(gfp_t flags)
                z =3D OPT_ZONE_HIGHMEM +
                        !!((__force unsigned int)flags & ___GFP_MOVABLE);
        }
+
+       VM_BUG_ON(z > ZONE_MOVABLE);
        return z;
 }


Sincerely,
Huaisheng Ye