Re: [PATCH v3 1/4] mm/vmalloc: warn on invalid vmalloc gfp flags

[Nathan Chancellor <nathan@kernel.org>]

Hi Vishal,

On Mon, Nov 17, 2025 at 09:35:27AM -0800, Vishal Moola (Oracle) wrote:
> Vmalloc explicitly supports a list of flags, but we never enforce them.
> vmalloc has been trying to handle unsupported flags by clearing and
> setting flags wherever necessary. This is messy and makes the code
> harder to understand, when we could simply check for a supported input
> immediately instead.
> 
> Define a helper mask and function telling callers they have passed in
> invalid flags, and clear those unsupported vmalloc flags.
> 
> Suggested-by: Christoph Hellwig <hch@infradead.org>
> Signed-off-by: Vishal Moola (Oracle) <vishal.moola@gmail.com>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> ---
>  mm/vmalloc.c | 26 ++++++++++++++++++++++++++
>  1 file changed, 26 insertions(+)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 0832f944544c..5dc467c6cab4 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -3911,6 +3911,28 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
>  	return NULL;
>  }
>  
> +/*
> + * See __vmalloc_node_range() for a clear list of supported vmalloc flags.
> + * This gfp lists all flags currently passed through vmalloc. Currently,
> + * __GFP_ZERO is used by BPF and __GFP_NORETRY is used by percpu. Both drm
> + * and BPF also use GFP_USER. Additionally, various users pass
> + * GFP_KERNEL_ACCOUNT.
> + */
> +#define GFP_VMALLOC_SUPPORTED (GFP_KERNEL | GFP_ATOMIC | GFP_NOWAIT |\
> +				__GFP_NOFAIL |  __GFP_ZERO | __GFP_NORETRY |\
> +				GFP_NOFS | GFP_NOIO | GFP_KERNEL_ACCOUNT |\
> +				GFP_USER)
> +
> +static gfp_t vmalloc_fix_flags(gfp_t flags)
> +{
> +	gfp_t invalid_mask = flags & ~GFP_VMALLOC_SUPPORTED;
> +
> +	flags &= GFP_VMALLOC_SUPPORTED;
> +	WARN(1, "Unexpected gfp: %#x (%pGg). Fixing up to gfp: %#x (%pGg). Fix your code!\n",
> +			invalid_mask, &invalid_mask, flags, &flags);
> +	return flags;
> +}

I am seeing this warning trigger when starting a VM on one of my arm64
boxes.

  [ 6345.145795] ------------[ cut here ]------------
  [ 6345.145803] Unexpected gfp: 0x2 (__GFP_HIGHMEM). Fixing up to gfp: 0x400dc0 (GFP_KERNEL_ACCOUNT|__GFP_ZERO). Fix your code!
  [ 6345.145819] WARNING: mm/vmalloc.c:3940 at vmalloc_fix_flags+0x60/0x90, CPU#32: qemu-system-aar/4325
  [ 6345.176990] Modules linked in: ...
  [ 6345.254421] CPU: 32 UID: 1000 PID: 4325 Comm: qemu-system-aar Not tainted 6.18.0-rc6-next-20251118-00002-g2331e73a4769 #1 PREEMPT(voluntary)
  [ 6345.267101] Hardware name: To be filled by O.E.M Ampere Altra Developer Platform/Ampere Altra Developer Platform, BIOS TianoCore 2.10.100.02 (SYS: 2.10.20
  [ 6345.280907] pstate: 60400009 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
  [ 6345.287856] pc : vmalloc_fix_flags+0x60/0x90
  [ 6345.292115] lr : vmalloc_fix_flags+0x58/0x90
  [ 6345.296374] sp : ffff80008d5b3a10
  [ 6345.299676] x29: ffff80008d5b3a20 x28: ffff07ff8a148000 x27: 0000000000000000
  [ 6345.306800] x26: 0000000000000000 x25: 0000000000000000 x24: 0000000000000000
  [ 6345.313923] x23: 000000000000000b x22: 000000000000ae01 x21: 0000000000000028
  [ 6345.321047] x20: ffffc682ea643fbc x19: 0000000000001040 x18: ffff800083465050
  [ 6345.328170] x17: 00000000d949c370 x16: 00000000d949c370 x15: 0000000000000004
  [ 6345.335294] x14: 0000000000000002 x13: 0000000000000002 x12: 0000000000000000
  [ 6345.342417] x11: 0000000000000001 x10: ffffc682ec577744 x9 : bf6a46a6bf3b7900
  [ 6345.349541] x8 : bf6a46a6bf3b7900 x7 : 65646f632072756f x6 : 7920786946202e29
  [ 6345.356664] x5 : ffff081f6fcdc678 x4 : 0000000000000000 x3 : ffff80008d5b3688
  [ 6345.363788] x2 : 0000000000000021 x1 : 0000000000000000 x0 : 000000000000006f
  [ 6345.370911] Call trace:
  [ 6345.373346]  vmalloc_fix_flags+0x60/0x90 (P)
  [ 6345.377606]  __vmalloc_noprof+0xa0/0xb0
  [ 6345.381431]  kvm_arch_alloc_vm+0x64/0x70
  [ 6345.385344]  kvm_dev_ioctl+0x9c/0x58c
  [ 6345.388997]  __arm64_sys_ioctl+0xb0/0x100
  [ 6345.392995]  invoke_syscall+0x84/0xf4
  [ 6345.396648]  el0_svc_common.llvm.4390888008543260363+0x90/0xf4
  [ 6345.402469]  do_el0_svc+0x2c/0x3c
  [ 6345.405773]  el0_svc+0x54/0x2a8
  [ 6345.408905]  el0t_64_sync_handler+0x88/0x134
  [ 6345.413164]  el0t_64_sync+0x1b8/0x1bc
  [ 6345.416815] ---[ end trace 0000000000000000 ]---

where kvm_arch_alloc_vm() from arch/arm64/kvm/arm.c is

  struct kvm *kvm_arch_alloc_vm(void)
  {
      size_t sz = sizeof(struct kvm);

      if (!has_vhe())
          return kzalloc(sz, GFP_KERNEL_ACCOUNT);

      return __vmalloc(sz, GFP_KERNEL_ACCOUNT | __GFP_HIGHMEM | __GFP_ZERO);
  }

Should __GFP_HIGHMEM be dropped from the call to __vmalloc? It looks
like it was added by commit 115bae923ac8 ("KVM: arm64: Add memcg
accounting to KVM allocations") back in 5.16.

Cheers,
Nathan