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

Dan Williams
2021-01-05T04:19:03Z

Ping, this bug is still present on v5.11-rc2, need a resend?

On Wed, Dec 2, 2020 at 10:28 PM Dan Williams <dan.j.williams@intel.com> 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@vger.kernel.org>
> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: x86@kernel.org
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Co-debugged-by: Matthew Wilcox <willy@infradead.org>
> Tested-by: Yi Zhang <yi.zhang@redhat.com>
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> ---
> 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;