[REGRESSION] x86/hugetlb: AMD F15h VA alignment offset breaks MAP_HUGETLB alignment

[Karsten Desler]

Hi,

I found a reproducible hugetlb regression on an AMD Family 15h system.

On some boots, mmap(MAP_HUGETLB) returns a virtual address that is not aligned
to the hugepage size. The mapping is nevertheless installed as a hugetlb VMA.
When the process exits, the kernel later BUGs in __unmap_hugepage_range().

6.18.33 x86_64, AMD opteron 6238, 2M hugepages

Example bad mapping captured from /proc/$pid/maps:

  7fc67f604000-7fc67f804000 rw-p 00000000 00:0f 12340 /anon_hugepage (deleted)

The address has offset 0x4000 within a 2 MiB hugepage.

smaps confirms it is really hugetlb:

  KernelPageSize:     2048 kB
  MMUPageSize:        2048 kB
  Private_Hugetlb:    2048 kB
  VmFlags: rd wr mr mw me de ht

Minimal reproducer:

  echo 1000 > /proc/sys/vm/nr_hugepages

  mmap(NULL, 1229824, PROT_READ|PROT_WRITE,
       MAP_PRIVATE|MAP_ANONYMOUS|MAP_POPULATE|MAP_HUGETLB, -1, 0)

On bad boots this returns e.g.:

  mmap returned 0x7fc67f604000 aligned=no offset=16384

and exiting the process triggers:

  Kernel BUG at __unmap_hugepage_range+0x5ef/0x640
  RIP: __unmap_hugepage_range+0x5ef/0x640
  Fixing recursive fault but reboot is needed!

The following is AI work, sorry if that's total BS but at the very least,
I can reproduce the kernelBUG and booting with
  align_va_addr=off
works around the issue.

This is boot-dependent. Some boots work, some fail. The reason appears
to be the per-boot AMD F15h VA alignment offset.

The old x86 hugetlb path in arch/x86/mm/hugetlbpage.c only set:

  info.align_mask = PAGE_MASK & ~huge_page_mask(h);

It did not add the AMD F15h align offset.

After the v6.13-rc1 hugetlb mmap rework, hugetlb mappings go through
arch_get_unmapped_area*(), and x86 currently does:

  if (filp) {
          info.align_mask = get_align_mask(filp);
          info.align_offset += get_align_bits();
  }

For hugetlb, get_align_mask(filp) correctly returns the hugepage alignment
mask, but get_align_bits() can still return the AMD F15h per-boot offset,
e.g. 0x4000. That produces a non-hugepage-aligned hugetlb VMA.

Likely introduced by the v6.13-rc1 series:

  1317a5e7f7b1 arch/x86: teach arch_get_unmapped_area_vmflags to handle hugetlb mappings
  7bd3f1e1a9ae mm: make hugetlb mappings go through mm_get_unmapped_area_vmflags
  cc92882ee218 mm: drop hugetlb_get_unmapped_area{_*} functions

AI suggests passing filp to get_align_bits and doing
if (filp && is_file_hugepages(filp))
	return 0;

Best regards,
 Karsten