linux-kernel - Re: mapcount corruption regression

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Date:   Wed, 2 Dec 2020 00:49:57 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     "Shutemov, Kirill" <kirill.shutemov@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Yi Zhang <yi.zhang@...hat.com>, Toshi Kani <toshi.kani@....com>
Subject: Re: mapcount corruption regression

On Tue, Dec 1, 2020 at 9:07 PM Dan Williams <dan.j.williams@...el.com> wrote:
>
> On Tue, Dec 1, 2020 at 7:43 PM Matthew Wilcox <willy@...radead.org> wrote:
> >
> > On Tue, Dec 01, 2020 at 06:28:45PM -0800, Dan Williams wrote:
> > > On Tue, Dec 1, 2020 at 12:49 PM Matthew Wilcox <willy@...radead.org> wrote:
> > > >
> > > > On Tue, Dec 01, 2020 at 12:42:39PM -0800, Dan Williams wrote:
> > > > > On Mon, Nov 30, 2020 at 6:24 PM Matthew Wilcox <willy@...radead.org> wrote:
> > > > > >
> > > > > > On Mon, Nov 30, 2020 at 05:20:25PM -0800, Dan Williams wrote:
> > > > > > > Kirill, Willy, compound page experts,
> > > > > > >
> > > > > > > I am seeking some debug ideas about the following splat:
> > > > > > >
> > > > > > > BUG: Bad page state in process lt-pmem-ns  pfn:121a12
> > > > > > > page:0000000051ef73f7 refcount:0 mapcount:-1024
> > > > > > > mapping:0000000000000000 index:0x0 pfn:0x121a12
> > > > > >
> > > > > > Mapcount of -1024 is the signature of:
> > > > > >
> > > > > > #define PG_guard        0x00000400
> > > > >
> > > > > Oh, thanks for that. I overlooked how mapcount is overloaded. Although
> > > > > in v5.10-rc4 that value is:
> > > > >
> > > > > #define PG_table        0x00000400
> > > >
> > > > Ah, I was looking at -next, where Roman renumbered it.
> > > >
> > > > I know UML had a problem where it was not clearing PG_table, but you
> > > > seem to be running on bare metal.  SuperH did too, but again, you're
> > > > not using SuperH.
> > > >
> > > > > >
> > > > > > (the bits are inverted, so this turns into 0xfffffbff which is reported
> > > > > > as -1024)
> > > > > >
> > > > > > I assume you have debug_pagealloc enabled?
> > > > >
> > > > > Added it, but no extra spew. I'll dig a bit more on how PG_table is
> > > > > not being cleared in this case.
> > > >
> > > > I only asked about debug_pagealloc because that sets PG_guard.  Since
> > > > the problem is actually PG_table, it's not relevant.
> > >
> > > As a shot in the dark I reverted:
> > >
> > >     b2b29d6d0119 mm: account PMD tables like PTE tables
> > >
> > > ...and the test passed.
> >
> > That's not really surprising ... you're still freeing PMD tables without
> > calling the destructor, which means that you're leaking ptlocks on
> > configs that can't embed the ptlock in the struct page.
>
> Ok, so potentially this new tracking is highlighting a long standing
> bug that was previously silent. That would explain the ambiguous
> bisect results.
>
> > I suppose it shows that you're leaking a PMD table rather than a PTE
> > table, so that might help track it down.  Checking for PG_table in
> > free_unref_page() and calling show_stack() will probably help more.
>
> Will do.

Thanks for the pointers Willy this fix below tests ok and looks
correct to me given the history:

diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
index dfd82f51ba66..7ed99314dcdf 100644
--- a/arch/x86/mm/pgtable.c
+++ b/arch/x86/mm/pgtable.c
@@ -829,6 +829,7 @@ 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;

In 2013 Kirill noticed that he missed a pmd page table free site:

    c283610e44ec x86, mm: do not leak page->ptl for pmd page tables

In 2018 Toshi added a new pmd page table free site without the destructor:

    28ee90fe6048 x86/mm: implement free pmd/pte page interfaces

In 2020 Willy adds PG_table accounting that flags the missing
pgtable_pmd_page_dtor()

Yi, I would appreciate a confirmation that the fix works for you.