Date:   Tue, 7 Dec 2021 11:16:31 +0800
From:   Baoquan He <bhe@redhat.com>
To:     linux-kernel@vger.kernel.org, tglx@linutronix.de, mingo@redhat.com,
        bp@alien8.de, dave.hansen@linux.intel.com, luto@kernel.org,
        peterz@infradead.org
Cc:     linux-mm@kvack.org, akpm@linux-foundation.org, hch@lst.de,
        robin.murphy@arm.com, cl@linux.com, penberg@kernel.org,
        rientjes@google.com, iamjoonsoo.kim@lge.com, vbabka@suse.cz,
        m.szyprowski@samsung.com, John.p.donnelly@oracle.com,
        kexec@lists.infradead.org, rppt@linux.ibm.com
Subject: Re: [PATCH RESEND v2 0/5] Avoid requesting page from DMA zone when
 no managed pages
Message-ID: <20211207031631.GA5604@MiWiFi-R3L-srv>
References: <20211207030750.30824-1-bhe@redhat.com>
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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
>