linux-kernel - Re: mapcount corruption regression

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Date:   Tue, 1 Dec 2020 18:28:45 -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>
Subject: Re: mapcount corruption regression

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.

Yi, do you see the same?