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dan.j.williams@intel.com wrote: > [ add Gregory and linux-mm ] > > [ full context for new Cc: ] > Balbir Singh wrote: >> Commit 7ffb791423c7 ("x86/kaslr: Reduce KASLR entropy on most x86 systems") >> exposed a bug with nokaslr and zone device >> interaction, as seen on a system with an AMD iGPU and dGPU (see [1]). >> The root cause of the issue 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 bug, 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 [2] >> >> Link: https://lore.kernel.org/lkml/20250310112206.4168-1-spasswolf@web.de/ [1] >> Link: https://lore.kernel.org/lkml/d87680bab997fdc9fb4e638983132af235d9a03a.camel@web.de/ [2] >> Fixes: 7ffb791423c7 ("x86/kaslr: Reduce KASLR entropy on most x86 systems") >> >> Cc: "Christian König" >> Cc: Ingo Molnar >> Cc: Kees Cook >> Cc: Bjorn Helgaas >> Cc: Linus Torvalds >> Cc: Peter Zijlstra >> Cc: Andy Lutomirski >> Cc: Alex Deucher >> Cc: Bert Karwatzki >> Cc: Madhavan Srinivasan >> Cc: Nicholas Piggin >> >> >> Signed-off-by: Balbir Singh >> --- >> I've left powerpc out of this regression change due to the time required >> to setup and test via qemu. I wanted to address the regression quickly >> >> >> 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 dce60767124f..cc60b57473a4 100644 >> --- a/arch/x86/mm/init_64.c >> +++ b/arch/x86/mm/init_64.c >> @@ -970,9 +970,18 @@ 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); >> + /* >> + * add_pages() is called by memremap_pages() for adding device private >> + * pages. Do not bump up max_pfn in the device private path. 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 >> + */ >> + 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); > > The comment says that this adjustment is only for the device-private > case, but it applies to all driver-managed device memory. > > Why not actually do what the comment says and limit this to > DEVICE_PRIVATE? I.e.: > > diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c > index 0e4270e20fad..4cc8175f9ffd 100644 > --- a/arch/x86/mm/init_64.c > +++ b/arch/x86/mm/init_64.c > @@ -989,7 +989,7 @@ int add_pages(int nid, unsigned long start_pfn, unsigned long nr_pages, > * addressable memory can force device drivers to use bounce buffers > * and impact their performance negatively: > */ > - if (!params->pgmap) > + if (!params->pgmap || params->pgmap->type != MEMORY_DEVICE_PRIVATE) > /* update max_pfn, max_low_pfn and high_memory */ > update_end_of_memory_vars(start_pfn << PAGE_SHIFT, nr_pages << PAGE_SHIFT); > At that time when I audited the code, I did notice that max_pfn was already set to the upper end of physical memory (even if it's not hotunplugged, because those regions were parsed and added to memblock_add() and I noticed that all hotplug regions changing max_pfn were coming from the device private). I agree we should check for pgmap->type, so definitely the right fix Reviewed-by: Balbir Singh