Re: [PATCH v3] arm64/mm: fix incorrect file_map_count for invalid pmd

[Denys Vlasenko]

On 11/21/22 19:18, Will Deacon wrote:
> On Mon, Nov 21, 2022 at 03:36:08PM +0800, 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_leaf() will return true even if the pmd is invalid due to
> > pmd_present_invalid() check. So 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.
> >
> > Add !pmd_present_invalid() check in pmd_user_accessible_page() to fix the
> > problem.
> >
> > Fixes: 42b2547137f5 ("arm64/mm: enable ARCH_SUPPORTS_PAGE_TABLE_CHECK")
> > Reported-by: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
> > Signed-off-by: Liu Shixin <liushixin2@xxxxxxxxxx>
> > Acked-by: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>
> > Acked-by: David Hildenbrand <david@xxxxxxxxxx>
> > Reviewed-by: Kefeng Wang <wangkefeng.wang@xxxxxxxxxx>
> > ---
> > v1->v2: Update comment and optimize the code by moving p?d_valid() at
> > 	first place suggested by Mark.
> > v2->v3: Replace pmd_valid() with pmd_present_invalid() suggested by Will.
> >
> >   arch/arm64/include/asm/pgtable.h | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> > index edf6625ce965..17afb09f386f 100644
> > --- a/arch/arm64/include/asm/pgtable.h
> > +++ b/arch/arm64/include/asm/pgtable.h
> > @@ -863,7 +863,7 @@ static inline bool pte_user_accessible_page(pte_t pte)
> >   
> >   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_present_invalid(pmd) && (pmd_user(pmd) || pmd_user_exec(pmd));
> >   }
>
> Acked-by: Will Deacon <will@xxxxxxxxxx>
>
> But please see my comment on v2 about pud_user_exec() for the PUD case.

Can you be more specific? Do you ask for pud_user_exec() to be defined
and used here? Or something else?

Until this patch lands, amd64 PAGE_TABLE_CHECK + THP remains broken...