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.
