On Wed, Nov 16, 2022 at 2:51 AM Liu Shixin [off-list ref] wrote:

The page table check trigger BUG_ON() unexpectedly when split hugepage:

 ------------[ cut here ]------------
 kernel BUG at mm/page_table_check.c:119!
 Internal error: Oops - BUG: 00000000f2000800 [#1] SMP
 Dumping ftrace buffer:
    (ftrace buffer empty)
 Modules linked in:
 CPU: 7 PID: 210 Comm: transhuge-stres Not tainted 6.1.0-rc3+ #748
 Hardware name: linux,dummy-virt (DT)
 pstate: 20000005 (nzCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
 pc : page_table_check_set.isra.0+0x398/0x468
 lr : page_table_check_set.isra.0+0x1c0/0x468
[...]
 Call trace:
  page_table_check_set.isra.0+0x398/0x468
  __page_table_check_pte_set+0x160/0x1c0
  __split_huge_pmd_locked+0x900/0x1648
  __split_huge_pmd+0x28c/0x3b8
  unmap_page_range+0x428/0x858
  unmap_single_vma+0xf4/0x1c8
  zap_page_range+0x2b0/0x410
  madvise_vma_behavior+0xc44/0xe78
  do_madvise+0x280/0x698
  __arm64_sys_madvise+0x90/0xe8
  invoke_syscall.constprop.0+0xdc/0x1d8
  do_el0_svc+0xf4/0x3f8
  el0_svc+0x58/0x120
  el0t_64_sync_handler+0xb8/0xc0
  el0t_64_sync+0x19c/0x1a0
[...]

On arm64, pmd_present() will return true even if the pmd is invalid.
in pmdp_invalidate() the file_map_count will not only decrease once but
also increase once. Then in set_pte_at(), the file_map_count increase
again, and so trigger BUG_ON() unexpectedly.

 static inline bool pmd_user_accessible_page(pmd_t pmd)
 {
-       return pmd_leaf(pmd) && (pmd_user(pmd) || pmd_user_exec(pmd));
+       return pmd_leaf(pmd) && (pmd_user(pmd) || pmd_user_exec(pmd)) && pmd_valid(pmd);
 }

 static inline bool pud_user_accessible_page(pud_t pud)
 {
-       return pud_leaf(pud) && pud_user(pud);
+       return pud_leaf(pud) && pud_user(pud) && pud_valid(pud);

This looks closer to x86 where the check is directly: pmd_val(pmd) &
_PAGE_PRESENT, without PTE_PROT_NONE that is part of pmd_present()

Acked-by: Pasha Tatashin <pasha.tatashin@soleen.com>

Thanks,
Pasha

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