Re: Regression with kernel 6.3 "kernel BUG at include/linux/swapops.h:472!"

David Hildenbrand
2023-05-13T13:22:00Z

> Yes, makes sense.
>
>> [1] Total is 1 Gbyte, and running
>> | dd if=/dev/zero bs=896M count=1 | pv --rate-limit=1k >/dev/null
>> might not be the best style but does the trick: Wait for pv to
>> count up to a minute, then ^C it. If the host is still okay after
>> that, it's considered "good".
>
> Thanks for bisecting and coming up with a testcase!
> The attached patch survives for me on my C3000 with 2GB RAM with this test:
> dd if=/dev/zero bs=1896M count=1 | pv
> (well, the OOM-killer might jump in, but even that is survived).
>
> Could you try the patch below?

Thanks for debugging! :)

>
> Helge
>
> -
>
> [PATCH] parisc: Fix encoding of swp_entry due to added SWP_EXCLUSIVE flag
>
> Fix the __swp_offset() and __swp_entry() macros due to commit 6d239fc78c0b
> ("parisc/mm: support __HAVE_ARCH_PTE_SWP_EXCLUSIVE") which introduced the
> SWP_EXCLUSIVE flag by reusing the _PAGE_ACCESSED flag.
>
> Reported-by: Christoph Biedl <linux-kernel.bfrz@manchmal.in-ulm.de>
> Fixes: 6d239fc78c0b ("parisc/mm: support __HAVE_ARCH_PTE_SWP_EXCLUSIVE")
>
> diff --git a/arch/parisc/include/asm/pgtable.h b/arch/parisc/include/asm/pgtable.h
> index e2950f5db7c9..522846be54b7 100644
> --- a/arch/parisc/include/asm/pgtable.h
> +++ b/arch/parisc/include/asm/pgtable.h
> @@ -413,12 +413,12 @@ extern void paging_init (void);
> * For the 64bit version, the offset is extended by 32bit.
> */
> #define __swp_type(x) ((x).val & 0x1f)
> -#define __swp_offset(x) ( (((x).val >> 6) & 0x7) | \
> - (((x).val >> 8) & ~0x7) )
> +#define __swp_offset(x) ( (((x).val >> 5) & 0x7) | \
> + (((x).val >> 10) << 3) )
> #define __swp_entry(type, offset) ((swp_entry_t) { \
> ((type) & 0x1f) | \
> - ((offset & 0x7) << 6) | \
> - ((offset & ~0x7) << 8) })
> + ((offset & 0x7) << 5) | \
> + ((offset & ~0x7) << 7) })
> #define __pte_to_swp_entry(pte) ((swp_entry_t) { pte_val(pte) })
> #define __swp_entry_to_pte(x) ((pte_t) { (x).val })

This fix makes it work like the layout I documented.

What I originally tried doing was reusing one of the spare bits instead of reworking
the layout. Apparently, I got the old layout wrong. :(

So if I understood the layout right this time, maybe we can just use one of the two
spare bits: _PAGE_HUGE (or alternatively, _PAGE_DIRTY_BIT)?

diff --git a/arch/parisc/include/asm/pgtable.h b/arch/parisc/include/asm/pgtable.h
index e2950f5db7c9..ee9f08cd5938 100644
--- a/arch/parisc/include/asm/pgtable.h
+++ b/arch/parisc/include/asm/pgtable.h
@@ -218,8 +218,8 @@ extern void __update_cache(pte_t pte);
#define _PAGE_KERNEL_RWX (_PAGE_KERNEL_EXEC | _PAGE_WRITE)
#define _PAGE_KERNEL (_PAGE_KERNEL_RO | _PAGE_WRITE)

-/* We borrow bit 23 to store the exclusive marker in swap PTEs. */
-#define _PAGE_SWP_EXCLUSIVE _PAGE_ACCESSED
+/* We borrow bit 21 to store the exclusive marker in swap PTEs. */
+#define _PAGE_SWP_EXCLUSIVE _PAGE_HUGE

/* The pgd/pmd contains a ptr (in phys addr space); since all pgds/pmds
* are page-aligned, we don't care about the PAGE_OFFSET bits, except
@@ -405,7 +405,7 @@ extern void paging_init (void);
*
* 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
* 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
- * <---------------- offset -----------------> P E <ofs> < type ->
+ * <---------------- offset ---------------> E P <ofs> 0 < type ->
*
* E is the exclusive marker that is not stored in swap entries.
* _PAGE_PRESENT (P) must be 0.
diff --git a/lib/maple_tree.c b/lib/maple_tree.c
index 110a36479dce..7510db355f48 100644

--
Thanks,

David / dhildenb