Re: [Update] Regression in 4.18 - 32-bit PowerPC crashes on boot - bisected to commit 1d40a5ea01d5

[Kirill A. Shutemov]

On Fri, Jun 29, 2018 at 02:01:46PM -0700, Linus Torvalds wrote:
> On Fri, Jun 29, 2018 at 1:42 PM Larry Finger <Larry.Finger@xxxxxxxxxxxx> wrote:
> >
> > I have more information regarding this BUG. Line 700 of page-flags.h is the
> > macro PAGE_TYPE_OPS(Table, table). For further debugging, I manually expanded
> > the macro, and found that the bug line is VM_BUG_ON_PAGE(!PageTable(page), page)
> > in routine __ClearPageTable(), which is called from pgtable_page_dtor() in
> > include/linux/mm.h. I also added a printk call to PageTable() that logs
> > page->page_type. The routine was called twice. The first had page_type of
> > 0xfffffbff, which would have been expected for a . The second call had
> > 0xffffffff, which led to the BUG.
> 
> So it looks to me like the tear-down of the page tables first found a
> page that is indeed a page table, and cleared the page table bit
> (well, it set it - the bits are reversed).
> 
> Then it took an exception (that "interrupt: 700") and that causes
> do_exit() again, and it tries to free the same page table - and now
> it's no longer marked as a page table, because it already went through
> the __ClearPageTable() dance once.
> 
> So on the second path through, it catches that "the bit already said
> it wasn't a page table" and does the BUG.
> 
> But the real question is what the problem was the *first* time around.

+Aneesh.

Looks like pgtable_page_dtor() gets called in __pte_free_tlb() path twice.
Once in __pte_free_tlb() itself and the second time in pgtable_free().

Would this help?

diff --git a/arch/powerpc/include/asm/book3s/32/pgalloc.h b/arch/powerpc/include/asm/book3s/32/pgalloc.h
index 6a6673907e45..e7a2f0e6b695 100644
--- a/arch/powerpc/include/asm/book3s/32/pgalloc.h
+++ b/arch/powerpc/include/asm/book3s/32/pgalloc.h
@@ -137,7 +137,6 @@ static inline void pgtable_free_tlb(struct mmu_gather *tlb,
 static inline void __pte_free_tlb(struct mmu_gather *tlb, pgtable_t table,
 				  unsigned long address)
 {
-	pgtable_page_dtor(table);
 	pgtable_free_tlb(tlb, page_address(table), 0);
 }
 #endif /* _ASM_POWERPC_BOOK3S_32_PGALLOC_H */
diff --git a/arch/powerpc/include/asm/nohash/32/pgalloc.h b/arch/powerpc/include/asm/nohash/32/pgalloc.h
index 1707781d2f20..30a13b80fd58 100644
--- a/arch/powerpc/include/asm/nohash/32/pgalloc.h
+++ b/arch/powerpc/include/asm/nohash/32/pgalloc.h
@@ -139,7 +139,6 @@ static inline void __pte_free_tlb(struct mmu_gather *tlb, pgtable_t table,
 				  unsigned long address)
 {
 	tlb_flush_pgtable(tlb, address);
-	pgtable_page_dtor(table);
 	pgtable_free_tlb(tlb, page_address(table), 0);
 }
 #endif /* _ASM_POWERPC_PGALLOC_32_H */
-- 
 Kirill A. Shutemov