Re: [PATCH v2 3/3] iommu/io-pgtable-arm-v7s: Request DMA32 memory, and improve debugging

[Nicolas Boichat]

On Thu, Nov 22, 2018 at 2:02 AM Michal Hocko <mhocko@xxxxxxxxxx> wrote:
>
> On Wed 21-11-18 16:46:38, Will Deacon wrote:
> > On Sun, Nov 11, 2018 at 05:03:41PM +0800, Nicolas Boichat wrote:
> > > For level 1/2 pages, ensure GFP_DMA32 is used if CONFIG_ZONE_DMA32
> > > is defined (e.g. on arm64 platforms).
> > >
> > > For level 2 pages, allocate a slab cache in SLAB_CACHE_DMA32.
> > >
> > > Also, print an error when the physical address does not fit in
> > > 32-bit, to make debugging easier in the future.
> > >
> > > Fixes: ad67f5a6545f ("arm64: replace ZONE_DMA with ZONE_DMA32")
> > > Signed-off-by: Nicolas Boichat <drinkcat@xxxxxxxxxxxx>
> > > ---
> > >
> > > Changes since v1:
> > >  - Changed approach to use SLAB_CACHE_DMA32 added by the previous
> > >    commit.
> > >  - Use DMA or DMA32 depending on the architecture (DMA for arm,
> > >    DMA32 for arm64).
> > >
> > > drivers/iommu/io-pgtable-arm-v7s.c | 20 ++++++++++++++++----
> > >  1 file changed, 16 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/drivers/iommu/io-pgtable-arm-v7s.c b/drivers/iommu/io-pgtable-arm-v7s.c
> > > index 445c3bde04800c..996f7b6d00b44a 100644
> > > --- a/drivers/iommu/io-pgtable-arm-v7s.c
> > > +++ b/drivers/iommu/io-pgtable-arm-v7s.c
> > > @@ -161,6 +161,14 @@
> > >
> > >  #define ARM_V7S_TCR_PD1                    BIT(5)
> > >
> > > +#ifdef CONFIG_ZONE_DMA32
> > > +#define ARM_V7S_TABLE_GFP_DMA GFP_DMA32
> > > +#define ARM_V7S_TABLE_SLAB_CACHE SLAB_CACHE_DMA32
> > > +#else
> > > +#define ARM_V7S_TABLE_GFP_DMA GFP_DMA
> > > +#define ARM_V7S_TABLE_SLAB_CACHE SLAB_CACHE_DMA
> > > +#endif
> >
> > It's a bit grotty that GFP_DMA32 doesn't just map to GFP_DMA on 32-bit
> > architectures, since then we wouldn't need this #ifdeffery afaict.
>
> But GFP_DMA32 should map to GFP_KERNEL on 32b, no? Or what exactly is
> going on in here?

GFP_DMA32 will fail due to check_slab_flags (aka GFP_SLAB_BUG_MASK
before patch 1/3 of this series)... But yes, it may be neater if there
was transparent remapping of GFP_DMA32/SLAB_CACHE_DMA32 to
GFP_DMA/SLAB_CACHE_DMA on 32-bit arch...

> --
> Michal Hocko
> SUSE Labs