On Thu, 14 Apr 2011, Andrew Morton wrote: > > The page allocator will improperly return a page from ZONE_NORMAL even > > when __GFP_DMA is passed if CONFIG_ZONE_DMA is disabled. The caller > > expects DMA memory, perhaps for ISA devices with 16-bit address > > registers, and may get higher memory resulting in undefined behavior. > > > > This patch causes the page allocator to return NULL in such circumstances > > with a warning emitted to the kernel log on the first occurrence. > > > > Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx> > > --- > > mm/page_alloc.c | 4 ++++ > > 1 files changed, 4 insertions(+), 0 deletions(-) > > > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > > --- a/mm/page_alloc.c > > +++ b/mm/page_alloc.c > > @@ -2225,6 +2225,10 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order, > > > > if (should_fail_alloc_page(gfp_mask, order)) > > return NULL; > > +#ifndef CONFIG_ZONE_DMA > > + if (WARN_ON_ONCE(gfp_mask & __GFP_DMA)) > > + return NULL; > > +#endif > > Worried. We have a large number of drivers which use GFP_DMA and I bet > some of them didn't really need to set it, and can use DMA32 memory. > They will now break. > They were already broken for CONFIG_ZONE_DMA=n since passing __GFP_DMA never guaranteed the memory came from ZONE_DMA32 (and would return ZONE_NORMAL memory even if CONFIG_ZONE_DMA32=n). > What is drivers/pci/intel-iommu.c doing with GFP_DMA btw? > Looks like if no identity mapping is needed for that device (using a non-identity mapping instead), then there is no lowmem restriction on its allocation; otherwise, we use lowmem is used when the device's mask specifically requires it. Adding Alex and David to the cc. > How commonly are people disabling ZONE_DMA? > Probably not that much, it's usually def_bool y everywhere and just implicitly on. I'm changing that for x86 on a CONFIG_EXPERT kernel so that we can avoid a ZONE_DMA entirely; that prevents reserving memory we don't need with the lowmem reserve by default, we can eliminate the DMA slab caches, etc. Google has been running with CONFIG_ZONE_DMA=n for a couple years because we simply don't need it. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>