On 05/24/2019 09:01 PM, Suzuki K Poulose wrote:
> When we have holes in a normal memory zone, we could endup having
> cached_migrate_pfns which may not necessarily be valid, under heavy memory
> pressure with swapping enabled ( via __reset_isolation_suitable(), triggered
> by kswapd).
>
> Later if we fail to find a page via fast_isolate_freepages(), we may
> end up using the migrate_pfn we started the search with, as valid
> page. This could lead to accessing NULL pointer derefernces like below,
> due to an invalid mem_section pointer.
>
> Unable to handle kernel NULL pointer dereference at virtual address 0000000000000008 [47/1825]
> Mem abort info:
> ESR = 0x96000004
> Exception class = DABT (current EL), IL = 32 bits
> SET = 0, FnV = 0
> EA = 0, S1PTW = 0
> Data abort info:
> ISV = 0, ISS = 0x00000004
> CM = 0, WnR = 0
> user pgtable: 4k pages, 48-bit VAs, pgdp = 0000000082f94ae9
> [0000000000000008] pgd=0000000000000000
> Internal error: Oops: 96000004 [#1] SMP
> ...
> CPU: 10 PID: 6080 Comm: qemu-system-aar Not tainted 510-rc1+ #6
> Hardware name: AmpereComputing(R) OSPREY EV-883832-X3-0001/OSPREY, BIOS 4819 09/25/2018
> pstate: 60000005 (nZCv daif -PAN -UAO)
> pc : set_pfnblock_flags_mask+0x58/0xe8
> lr : compaction_alloc+0x300/0x950
> [...]
> Process qemu-system-aar (pid: 6080, stack limit = 0x0000000095070da5)
> Call trace:
> set_pfnblock_flags_mask+0x58/0xe8
> compaction_alloc+0x300/0x950
> migrate_pages+0x1a4/0xbb0
> compact_zone+0x750/0xde8
> compact_zone_order+0xd8/0x118
> try_to_compact_pages+0xb4/0x290
> __alloc_pages_direct_compact+0x84/0x1e0
> __alloc_pages_nodemask+0x5e0/0xe18
> alloc_pages_vma+0x1cc/0x210
> do_huge_pmd_anonymous_page+0x108/0x7c8
> __handle_mm_fault+0xdd4/0x1190
> handle_mm_fault+0x114/0x1c0
> __get_user_pages+0x198/0x3c0
> get_user_pages_unlocked+0xb4/0x1d8
> __gfn_to_pfn_memslot+0x12c/0x3b8
> gfn_to_pfn_prot+0x4c/0x60
> kvm_handle_guest_abort+0x4b0/0xcd8
> handle_exit+0x140/0x1b8
> kvm_arch_vcpu_ioctl_run+0x260/0x768
> kvm_vcpu_ioctl+0x490/0x898
> do_vfs_ioctl+0xc4/0x898
> ksys_ioctl+0x8c/0xa0
> __arm64_sys_ioctl+0x28/0x38
> el0_svc_common+0x74/0x118
> el0_svc_handler+0x38/0x78
> el0_svc+0x8/0xc
> Code: f8607840 f100001f 8b011401 9a801020 (f9400400)
> ---[ end trace af6a35219325a9b6 ]---
>
> The issue was reported on an arm64 server with 128GB with holes in the zone
> (e.g, [32GB@4GB, 96GB@544GB]), with a swap device enabled, while running 100 KVM
> guest instances.
>
> This patch fixes the issue by ensuring that the page belongs to a valid PFN
> when we fallback to using the lower limit of the scan range upon failure in
> fast_isolate_freepages().
>
> Fixes: 5a811889de10f1eb ("mm, compaction: use free lists to quickly locate a migration target")
> Reported-by: Marc Zyngier <marc.zyngier@xxxxxxx>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
Reviewed-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
