From:   Baoquan He <bhe@redhat.com>
To:     linux-kernel@vger.kernel.org
Cc:     linux-mm@kvack.org, akpm@linux-foundation.org, hch@lst.de,
        cl@linux.com, John.p.donnelly@oracle.com,
        kexec@lists.infradead.org, bhe@redhat.com, 42.hyeyoo@gmail.com,
        penberg@kernel.org, rientjes@google.com, iamjoonsoo.kim@lge.com,
        vbabka@suse.cz, David.Laight@ACULAB.COM, david@redhat.com,
        x86@kernel.org, bp@alien8.de
Subject: [PATCH v4 0/3] Handle warning of allocation failure on DMA zone w/o managed pages
Date:   Thu, 23 Dec 2021 17:44:32 +0800
Message-Id: <20211223094435.248523-1-bhe@redhat.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

**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.

At the beginning, if crashkernel is reserved, the low 1M need be locked
down because AMD SME encrypts memory making the old backup region
mechanims impossible when switching into kdump kernel.

Later, it was also observed that there are BIOSes corrupting memory
under 1M. To solve this, in commit f1d4d47c5851, the entire region of
low 1M is always reserved after the real mode trampoline is allocated.

Besides, recently, Intel engineer mentioned their TDX (Trusted domain
extensions) which is under development in kernel also needs to 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 initializ DMA atomic pool when DMA zone has available managed
pages, otherwise just skip the initialization.

For dma-kmalloc(), for the time being, let's mute the warning of
allocation failure if requesting pages from DMA zone while no manged
pages. Meanwhile, change code to use dma_alloc_xx/dma_map_xx API to
replace kmalloc(GFP_DMA), or do not use GFP_DMA when calling kmalloc()
if not necessary. Christoph is posting patches to fix those under
drivers/scsi/. Finally, we can remove the need of dma-kmalloc() as
people suggested.

Changelog:
v3->v4:
 - Split the old v3 into two separate patchset. The first two clean
   up/improvement patches in v3 have been sent out in a independent
   patchset. The fixes patchs are adapted and sent in this patchset.
 - Do not change dma-kmalloc(), mute the warning of allocation failure
   instead if it's requesting page from DMA zone which has no managed
   pages.

v2-Resend -> v3:
 - Re-implement has_managed_dma() according to David's suggestion.
 - Add Fixes tag and cc stable.

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.
 - Rewrite cover letter to add more information.

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.

v3:
https://lore.kernel.org/all/20211213122712.23805-1-bhe@redhat.com/T/#u

V2 RESEND post:
https://lore.kernel.org/all/20211207030750.30824-1-bhe@redhat.com/T/#u

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


Baoquan He (3):
  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/page_alloc.c: do not warn allocation failure on zone DMA if no
    managed pages

 include/linux/mmzone.h |  9 +++++++++
 kernel/dma/pool.c      |  4 ++--
 mm/page_alloc.c        | 18 +++++++++++++++++-
 3 files changed, 28 insertions(+), 3 deletions(-)

-- 
2.26.3