Sorry, forgot adding x86 and x86/mm maintainers On 12/07/21 at 11:07am, Baoquan He wrote: > ***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 >
