Re: [PATCH 2/2] arm64/mm: fix incorrect file_map_count for invalid pmd/pud

[Mark Rutland <mark.rutland@arm.com>]

On Wed, Nov 16, 2022 at 10:08:27AM +0100, David Hildenbrand wrote:
> On 16.11.22 09:38, Liu Shixin 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.
> 
> 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/

... 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 

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

Anshuman?

Thanks,
Mark.