Re: [PATCH RESEND v2 0/5] Avoid requesting page from DMA zone when no managed pages

Baoquan He
2021-12-07T03:16:52Z

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@redhat.com/T/#u
>
> v1 post:
> https://lore.kernel.org/all/20210624052010.5676-1-bhe@redhat.com/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
>