Re: mapcount corruption regression

[Dan Williams]

On Mon, Nov 30, 2020 at 6:24 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> 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.