The patch titled Subject: mm_zone: add function to check if managed dma zone exists has been added to the -mm tree. Its filename is mm_zone-add-function-to-check-if-managed-dma-zone-exists.patch

This patch should soon appear at https://ozlabs.org/~akpm/mmots/broken-out/mm_zone-add-function-to-check-if-m... and later at https://ozlabs.org/~akpm/mmotm/broken-out/mm_zone-add-function-to-check-if-m...

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------------------------------------------------------ From: Baoquan He bhe@redhat.com Subject: mm_zone: add function to check if managed dma zone exists

Patch series "Handle warning of allocation failure on DMA zone w/o managed pages", v4.

**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.

This patch (of 3):

In some places of the current kernel, it assumes that dma zone must have managed 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 there's no managed pages at all in DMA zone. This exception will always cause page allocation failure if page is requested from DMA zone.

Here add function has_managed_dma() and the relevant helper functions to check if there's DMA zone with managed pages. It will be used in later patches.

Link: https://lkml.kernel.org/r/20211223094435.248523-1-bhe@redhat.com Link: https://lkml.kernel.org/r/20211223094435.248523-2-bhe@redhat.com Fixes: 6f599d84231f ("x86/kdump: Always reserve the low 1M when the crashkernel option is specified") Signed-off-by: Baoquan He bhe@redhat.com Reviewed-by: David Hildenbrand david@redhat.com Acked-by: John Donnelly john.p.donnelly@oracle.com Cc: Christoph Hellwig hch@lst.de Cc: Christoph Lameter cl@linux.com Cc: Hyeonggon Yoo 42.hyeyoo@gmail.com Cc: Pekka Enberg penberg@kernel.org Cc: David Rientjes rientjes@google.com Cc: Joonsoo Kim iamjoonsoo.kim@lge.com Cc: Vlastimil Babka vbabka@suse.cz Cc: David Laight David.Laight@ACULAB.COM Cc: Borislav Petkov bp@alien8.de Cc: Marek Szyprowski m.szyprowski@samsung.com Cc: Robin Murphy robin.murphy@arm.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

include/linux/mmzone.h | 9 +++++++++ mm/page_alloc.c | 15 +++++++++++++++ 2 files changed, 24 insertions(+)

--- a/include/linux/mmzone.h~mm_zone-add-function-to-check-if-managed-dma-zone-exists +++ a/include/linux/mmzone.h @@ -1046,6 +1046,15 @@ static inline int is_highmem_idx(enum zo #endif }

+#ifdef CONFIG_ZONE_DMA +bool has_managed_dma(void); +#else +static inline bool has_managed_dma(void) +{ + return false; +} +#endif + /** * is_highmem - helper function to quickly check if a struct zone is a * highmem zone or not. This is an attempt to keep references --- a/mm/page_alloc.c~mm_zone-add-function-to-check-if-managed-dma-zone-exists +++ a/mm/page_alloc.c @@ -9510,3 +9510,18 @@ bool take_page_off_buddy(struct page *pa return ret; } #endif + +#ifdef CONFIG_ZONE_DMA +bool has_managed_dma(void) +{ + struct pglist_data *pgdat; + + for_each_online_pgdat(pgdat) { + struct zone *zone = &pgdat->node_zones[ZONE_DMA]; + + if (managed_zone(zone)) + return true; + } + return false; +} +#endif /* CONFIG_ZONE_DMA */ _

Patches currently in -mm which might be from bhe@redhat.com are

mm_zone-add-function-to-check-if-managed-dma-zone-exists.patch dma-pool-create-dma-atomic-pool-only-if-dma-zone-has-managed-pages.patch mm-page_allocc-do-not-warn-allocation-failure-on-zone-dma-if-no-managed-pages.patch