***Problem observed: On x86_64, when crash is triggered and entering into kdump kernel, page allocation failure can always be seen. --------------------------------- DMA: preallocated 128 KiB GFP_KERNEL pool for atomic allocations swapper/0: page allocation failure: order:5, mode:0xcc1(GFP_KERNEL|GFP_DMA), nodemask=(null),cpuset=/,mems_allowed=0 CPU: 0 PID: 1 Comm: swapper/0 Call Trace: dump_stack+0x7f/0xa1 warn_alloc.cold+0x72/0xd6 ...... __alloc_pages+0x24d/0x2c0 ...... dma_atomic_pool_init+0xdb/0x176 do_one_initcall+0x67/0x320 ? rcu_read_lock_sched_held+0x3f/0x80 kernel_init_freeable+0x290/0x2dc ? rest_init+0x24f/0x24f kernel_init+0xa/0x111 ret_from_fork+0x22/0x30 Mem-Info: ------------------------------------ ***Root cause: In the current kernel, it assumes that DMA zone must have managed pages and try to request pages if CONFIG_ZONE_DMA is enabled. While this is not always true. E.g in kdump kernel of x86_64, only low 1M is presented and locked down at very early stage of boot, so that this low 1M won't be added into buddy allocator to become managed pages of DMA zone. This exception will always cause page allocation failure if page is requested from DMA zone. ***Investigation: This failure happens since below commit merged into linus's tree. 1a6a9044b967 x86/setup: Remove CONFIG_X86_RESERVE_LOW and reservelow= options 23721c8e92f7 x86/crash: Remove crash_reserve_low_1M() f1d4d47c5851 x86/setup: Always reserve the first 1M of RAM 7c321eb2b843 x86/kdump: Remove the backup region handling 6f599d84231f x86/kdump: Always reserve the low 1M when the crashkernel option is specified Before them, on x86_64, the low 640K area will be reused by kdump kernel. So in kdump kernel, the content of low 640K area is copied into a backup region for dumping before jumping into kdump. Then except of those firmware reserved region in [0, 640K], the left area will be added into buddy allocator to become available managed pages of DMA zone. However, after above commits applied, in kdump kernel of x86_64, the low 1M is reserved by memblock, but not released to buddy allocator. So any later page allocation requested from DMA zone will fail. This low 1M lock down is needed because AMD SME encrypts memory making the old backup region mechanims impossible when switching into kdump kernel. And Intel engineer mentioned their TDX (Trusted domain extensions) which is under development in kernel also needs lock down the low 1M. So we can't simply revert above commits to fix the page allocation failure from DMA zone as someone suggested. ***Solution: Currently, only DMA atomic pool and dma-kmalloc will initialize and request page allocation with GFP_DMA during bootup. So only initialize them when DMA zone has available managed pages, otherwise just skip the initialization. From testing and code, this doesn't matter. In kdump kernel of x86_64, the page allocation failure disappear. ***Further thinking On x86_64, it consistently takes [0, 16M] into ZONE_DMA, and (16M, 4G] into ZONE_DMA32 by default. The zone DMA covering low 16M is used to take care of antique ISA devices. In fact, on 64bit system, it rarely need ZONE_DMA (which is low 16M) to support almost extinct ISA devices. However, some components treat DMA as a generic concept, e.g kmalloc-dma, slab allocator initializes it for later any DMA related buffer allocation, but not limited to ISA DMA. On arm64, even though both CONFIG_ZONE_DMA and CONFIG_ZONE_DMA32 are enabled, it makes ZONE_DMA covers the low 4G area, and ZONE_DMA32 empty. Unless on specific platforms (e.g. 30-bit on Raspberry Pi 4), then zone DMA covers the 1st 1G area, zone DMA32 covers the rest of the 32-bit addressable memory. I am wondering if we can also change the size of DMA and DMA32 ZONE as dynamically adjusted, just as arm64 is doing? On x86_64, we can make zone DMA covers the 32-bit addressable memory, and empty zone DMA32 by default. Once ISA_DMA_API is enabled, we go back to make zone DMA covers low 16M area, zone DMA32 covers the rest of 32-bit addressable memory. (I am not familiar with ISA_DMA_API, will it require 24-bit addressable memory when enabled?) Change history: v2 post: https://lore.kernel.org/all/20210810094835.13402-1-bhe@xxxxxxxxxx/T/#u v1 post: https://lore.kernel.org/all/20210624052010.5676-1-bhe@xxxxxxxxxx/T/#u v2->v2 RESEND: John pinged to push the repost of this patchset. So fix one typo of suject of patch 3/5; Fix a building error caused by mix declaration in patch 5/5. Both of them are found by John from his testing. v1->v2: Change to check if managed DMA zone exists. If DMA zone has managed pages, go further to request page from DMA zone to initialize. Otherwise, just skip to initialize stuffs which need pages from DMA zone. Baoquan He (5): docs: kernel-parameters: Update to reflect the current default size of atomic pool dma-pool: allow user to disable atomic pool mm_zone: add function to check if managed dma zone exists dma/pool: create dma atomic pool only if dma zone has managed pages mm/slub: do not create dma-kmalloc if no managed pages in DMA zone .../admin-guide/kernel-parameters.txt | 5 ++++- include/linux/mmzone.h | 21 +++++++++++++++++++ kernel/dma/pool.c | 11 ++++++---- mm/page_alloc.c | 11 ++++++++++ mm/slab_common.c | 9 ++++++++ 5 files changed, 52 insertions(+), 5 deletions(-) -- 2.17.2
