linux-kernel - Re: mapcount corruption regression

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Message-ID: <CAPcyv4j7wtjOSg8vL5q0PPjWdaknY-PC7m9x-Q1R_YL5dhE+bQ@mail.gmail.com>
Date:   Tue, 1 Dec 2020 12:42:39 -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>
Subject: Re: mapcount corruption regression

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

>
> (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.