From 7170130e4c72ce0caa0cb42a1627c635cc262821 Mon Sep 17 00:00:00 2001 From: Balbir Singh Date: Tue, 1 Apr 2025 11:07:52 +1100 Subject: [PATCH] x86/mm/init: Handle the special case of device private pages in add_pages(), to not increase max_pfn and trigger dma_addressing_limited() bounce buffers MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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@web.de/ [ mingo: Clarified the comments and the changelog. ] Reported-by: Bert Karwatzki Tested-by: Bert Karwatzki Fixes: 7ffb791423c7 ("x86/kaslr: Reduce KASLR entropy on most x86 systems") Signed-off-by: Balbir Singh Signed-off-by: Ingo Molnar Cc: Brian Gerst Cc: Juergen Gross Cc: H. Peter Anvin Cc: Linus Torvalds Cc: Andrew Morton Cc: Christoph Hellwig Cc: Pierre-Eric Pelloux-Prayer Cc: Alex Deucher Cc: Christian König Cc: David Airlie Cc: Simona Vetter Link: https://lore.kernel.org/r/20250401000752.249348-1-balbirs@nvidia.com --- 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 519aa53114fa..821a0b53b21c 100644 --- a/arch/x86/mm/init_64.c +++ b/arch/x86/mm/init_64.c @@ -959,9 +959,18 @@ 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; }