Re: [PATCH] mm, compaction: Make sure we isolate a valid PFN

[Anshuman Khandual]

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>