6.14-stable review patch. If anyone has any objections, please let me know.
------------------
From: Balbir Singh balbirs@nvidia.com
[ Upstream commit 7170130e4c72ce0caa0cb42a1627c635cc262821 ]
As Bert Karwatzki reported, the following recent commit causes a performance regression on AMD iGPU and dGPU systems:
7ffb791423c7 ("x86/kaslr: Reduce KASLR entropy on most x86 systems")
It exposed a bug with nokaslr and zone device interaction.
The root cause of the bug is that, the GPU driver registers a zone device private memory region. When KASLR is disabled or the above commit is applied, the direct_map_physmem_end is set to much higher than 10 TiB typically to the 64TiB address. When zone device private memory is added to the system via add_pages(), it bumps up the max_pfn to the same value. This causes dma_addressing_limited() to return true, since the device cannot address memory all the way up to max_pfn.
This caused a regression for games played on the iGPU, as it resulted in the DMA32 zone being used for GPU allocations.
Fix this by not bumping up max_pfn on x86 systems, when pgmap is passed into add_pages(). The presence of pgmap is used to determine if device private memory is being added via add_pages().
More details:
devm_request_mem_region() and request_free_mem_region() request for device private memory. iomem_resource is passed as the base resource with start and end parameters. iomem_resource's end depends on several factors, including the platform and virtualization. On x86 for example on bare metal, this value is set to boot_cpu_data.x86_phys_bits. boot_cpu_data.x86_phys_bits can change depending on support for MKTME. By default it is set to the same as log2(direct_map_physmem_end) which is 46 to 52 bits depending on the number of levels in the page table. The allocation routines used iomem_resource's end and direct_map_physmem_end to figure out where to allocate the region.
[ arch/powerpc is also impacted by this problem, but this patch does not fix the issue for PowerPC. ]
Testing:
1. Tested on a virtual machine with test_hmm for zone device inseration
2. A previous version of this patch was tested by Bert, please see: https://lore.kernel.org/lkml/d87680bab997fdc9fb4e638983132af235d9a03a.camel@...
[ mingo: Clarified the comments and the changelog. ]
Reported-by: Bert Karwatzki spasswolf@web.de Tested-by: Bert Karwatzki spasswolf@web.de Fixes: 7ffb791423c7 ("x86/kaslr: Reduce KASLR entropy on most x86 systems") Signed-off-by: Balbir Singh balbirs@nvidia.com Signed-off-by: Ingo Molnar mingo@kernel.org Cc: Brian Gerst brgerst@gmail.com Cc: Juergen Gross jgross@suse.com Cc: H. Peter Anvin hpa@zytor.com Cc: Linus Torvalds torvalds@linux-foundation.org Cc: Andrew Morton akpm@linux-foundation.org Cc: Christoph Hellwig hch@lst.de Cc: Pierre-Eric Pelloux-Prayer pierre-eric.pelloux-prayer@amd.com Cc: Alex Deucher alexander.deucher@amd.com Cc: Christian König christian.koenig@amd.com Cc: David Airlie airlied@gmail.com Cc: Simona Vetter simona@ffwll.ch Link: https://lore.kernel.org/r/20250401000752.249348-1-balbirs@nvidia.com Signed-off-by: Sasha Levin sashal@kernel.org --- arch/x86/mm/init_64.c | 15 ++++++++++++--- 1 file changed, 12 insertions(+), 3 deletions(-)
diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c index 01ea7c6df3036..17c89dad4f7ff 100644 --- a/arch/x86/mm/init_64.c +++ b/arch/x86/mm/init_64.c @@ -967,9 +967,18 @@ int add_pages(int nid, unsigned long start_pfn, unsigned long nr_pages, ret = __add_pages(nid, start_pfn, nr_pages, params); WARN_ON_ONCE(ret);
- /* update max_pfn, max_low_pfn and high_memory */ - update_end_of_memory_vars(start_pfn << PAGE_SHIFT, - nr_pages << PAGE_SHIFT); + /* + * Special case: add_pages() is called by memremap_pages() for adding device + * private pages. Do not bump up max_pfn in the device private path, + * because max_pfn changes affect dma_addressing_limited(). + * + * dma_addressing_limited() returning true when max_pfn is the device's + * addressable memory can force device drivers to use bounce buffers + * and impact their performance negatively: + */ + if (!params->pgmap) + /* update max_pfn, max_low_pfn and high_memory */ + update_end_of_memory_vars(start_pfn << PAGE_SHIFT, nr_pages << PAGE_SHIFT);
return ret; }