Re: mapcount corruption regression

[Matthew Wilcox]

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@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

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.