Re: [PATCH] x86/mm: Fix leak of pmd ptlock

[Dan Williams]

Might I tempt an x86/mm maintainer to ack this, or a x86-tip
maintainer to apply it outright?

On Wed, Dec 2, 2020 at 10:28 PM Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
>
> Commit 28ee90fe6048 ("x86/mm: implement free pmd/pte page interfaces")
> introduced a new location where a pmd was released, but neglected to run
> the pmd page destructor. In fact, this happened previously for a
> different pmd release path and was fixed by commit:
>
> c283610e44ec ("x86, mm: do not leak page->ptl for pmd page tables").
>
> This issue was hidden until recently because the failure mode is silent,
> but commit:
>
> b2b29d6d0119 ("mm: account PMD tables like PTE tables")
>
> ...turns the failure mode into this signature:
>
>  BUG: Bad page state in process lt-pmem-ns  pfn:15943d
>  page:000000007262ed7b refcount:0 mapcount:-1024 mapping:0000000000000000 index:0x0 pfn:0x15943d
>  flags: 0xaffff800000000()
>  raw: 00affff800000000 dead000000000100 0000000000000000 0000000000000000
>  raw: 0000000000000000 ffff913a029bcc08 00000000fffffbff 0000000000000000
>  page dumped because: nonzero mapcount
>  [..]
>   dump_stack+0x8b/0xb0
>   bad_page.cold+0x63/0x94
>   free_pcp_prepare+0x224/0x270
>   free_unref_page+0x18/0xd0
>   pud_free_pmd_page+0x146/0x160
>   ioremap_pud_range+0xe3/0x350
>   ioremap_page_range+0x108/0x160
>   __ioremap_caller.constprop.0+0x174/0x2b0
>   ? memremap+0x7a/0x110
>   memremap+0x7a/0x110
>   devm_memremap+0x53/0xa0
>   pmem_attach_disk+0x4ed/0x530 [nd_pmem]
>   ? __devm_release_region+0x52/0x80
>   nvdimm_bus_probe+0x85/0x210 [libnvdimm]
>
> Given this is a repeat occurrence it seemed prudent to look for other
> places where this destructor might be missing and whether a better
> helper is needed. try_to_free_pmd_page() looks like a candidate, but
> testing with setting up and tearing down pmd mappings via the dax unit
> tests is thus far not triggering the failure. As for a better helper
> pmd_free() is close, but it is a messy fit due to requiring an @mm arg.
> Also, ___pmd_free_tlb() wants to call paravirt_tlb_remove_table()
> instead of free_page(), so open-coded pgtable_pmd_page_dtor() seems the
> best way forward for now.
>
> Fixes: 28ee90fe6048 ("x86/mm: implement free pmd/pte page interfaces")
> Cc: <stable@xxxxxxxxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxxxx>
> Cc: x86@xxxxxxxxxx
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Co-debugged-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Tested-by: Yi Zhang <yi.zhang@xxxxxxxxxx>
> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> ---
>  arch/x86/mm/pgtable.c |    2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
> index dfd82f51ba66..f6a9e2e36642 100644
> --- a/arch/x86/mm/pgtable.c
> +++ b/arch/x86/mm/pgtable.c
> @@ -829,6 +829,8 @@ int pud_free_pmd_page(pud_t *pud, unsigned long addr)
>         }
>
>         free_page((unsigned long)pmd_sv);
> +
> +       pgtable_pmd_page_dtor(virt_to_page(pmd));
>         free_page((unsigned long)pmd);
>
>         return 1;
>