Re: mapcount corruption regression

[Dan Williams]

On Tue, Dec 1, 2020 at 7:43 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx> 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@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.
> >
> > 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.