On 11/16/22 21:16, Mark Rutland wrote:

On Wed, Nov 16, 2022 at 10:08:27AM +0100, David Hildenbrand wrote:

quoted

On 16.11.22 09:38, Liu Shixin wrote:

quoted

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.

I assume that's because of the pmd_present_invalid() check.

... I wonder why that behavior was chosen. Sounds error-prone to me.

That seems to be down to commit:

  b65399f6111b03df ("arm64/mm: Change THP helpers to comply with generic MM semantics")

... apparently because Andrea Arcangelli said this was necessary in:

  https://lore.kernel.org/lkml/20181017020930.GN30832@redhat.com/ (local)

... but that does see to contradict what's said in:

  Documentation/mm/arch_pgtable_helpers.rst

... which just says:

  pmd_present  Tests a valid mapped PMD

It should be as follows instead, will update. Not sure about PUD level though,
where anon THP is not supported (AFAIK).

+---------------------------+--------------------------------------------------+
| pmd_present               | Tests if pmd_page() points to valid memory page  |
+---------------------------+--------------------------------------------------+

... and it's not clear to me why this *only* applies to the PMD level.

Anshuman?

Because THP is supported at PMD level. As Andrea had explained earlier, pmd_present()
should return positive if pmd_page() on the entry points to valid memory irrespective
of whether the entry is valid/mapped or not. That is the semantics expected in generic
THP during PMD split, collapse, migration etc and other memory code walking past such
PMD entries. That was my understanding.

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