On Thu 07-07-16 17:19:11, Dave Hansen wrote:
>
> From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
>
> This erratum can result in Accessed/Dirty getting set by the hardware
> when we do not expect them to be (on !Present PTEs).
>
> Instead of trying to fix them up after this happens, we just
> allow the bits to get set and try to ignore them. We do this by
> shifting the layout of the bits we use for swap offset/type in
> our 64-bit PTEs.
>
> It looks like this:
>
> bitnrs: | ... | 11| 10| 9|8|7|6|5| 4| 3|2|1|0|
> names: | ... |SW3|SW2|SW1|G|L|D|A|CD|WT|U|W|P|
> before: | OFFSET (9-63) |0|X|X| TYPE(1-5) |0|
> after: | OFFSET (14-63) | TYPE (9-13) |0|X|X|X| X| X|X|X|0|
>
> Note that D was already a don't care (X) even before. We just
> move TYPE up and turn its old spot (which could be hit by the
> A bit) into all don't cares.
>
> We take 5 bits away from the offset, but that still leaves us
> with 50 bits which lets us index into a 62-bit swapfile (4 EiB).
> I think that's probably fine for the moment. We could
> theoretically reclaim 5 of the bits (1, 2, 3, 4, 7) but it
> doesn't gain us anything.
>
> Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Yes, this seems like a safest option. Feel free to add
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
>
> b/arch/x86/include/asm/pgtable_64.h | 26 ++++++++++++++++++++------
> 1 file changed, 20 insertions(+), 6 deletions(-)
>
> diff -puN arch/x86/include/asm/pgtable_64.h~knl-strays-10-move-swp-pte-bits arch/x86/include/asm/pgtable_64.h
> --- a/arch/x86/include/asm/pgtable_64.h~knl-strays-10-move-swp-pte-bits 2016-07-07 17:17:43.556746185 -0700
> +++ b/arch/x86/include/asm/pgtable_64.h 2016-07-07 17:17:43.559746319 -0700
> @@ -140,18 +140,32 @@ static inline int pgd_large(pgd_t pgd) {
> #define pte_offset_map(dir, address) pte_offset_kernel((dir), (address))
> #define pte_unmap(pte) ((void)(pte))/* NOP */
>
> -/* Encode and de-code a swap entry */
> +/*
> + * Encode and de-code a swap entry
> + *
> + * | ... | 11| 10| 9|8|7|6|5| 4| 3|2|1|0| <- bit number
> + * | ... |SW3|SW2|SW1|G|L|D|A|CD|WT|U|W|P| <- bit names
> + * | OFFSET (14->63) | TYPE (10-13) |0|X|X|X| X| X|X|X|0| <- swp entry
> + *
> + * G (8) is aliased and used as a PROT_NONE indicator for
> + * !present ptes. We need to start storing swap entries above
> + * there. We also need to avoid using A and D because of an
> + * erratum where they can be incorrectly set by hardware on
> + * non-present PTEs.
> + */
> +#define SWP_TYPE_FIRST_BIT (_PAGE_BIT_PROTNONE + 1)
> #define SWP_TYPE_BITS 5
> -#define SWP_OFFSET_SHIFT (_PAGE_BIT_PROTNONE + 1)
> +/* Place the offset above the type: */
> +#define SWP_OFFSET_FIRST_BIT (SWP_TYPE_FIRST_BIT + SWP_TYPE_BITS + 1)
>
> #define MAX_SWAPFILES_CHECK() BUILD_BUG_ON(MAX_SWAPFILES_SHIFT > SWP_TYPE_BITS)
>
> -#define __swp_type(x) (((x).val >> (_PAGE_BIT_PRESENT + 1)) \
> +#define __swp_type(x) (((x).val >> (SWP_TYPE_FIRST_BIT)) \
> & ((1U << SWP_TYPE_BITS) - 1))
> -#define __swp_offset(x) ((x).val >> SWP_OFFSET_SHIFT)
> +#define __swp_offset(x) ((x).val >> SWP_OFFSET_FIRST_BIT)
> #define __swp_entry(type, offset) ((swp_entry_t) { \
> - ((type) << (_PAGE_BIT_PRESENT + 1)) \
> - | ((offset) << SWP_OFFSET_SHIFT) })
> + ((type) << (SWP_TYPE_FIRST_BIT)) \
> + | ((offset) << SWP_OFFSET_FIRST_BIT) })
> #define __pte_to_swp_entry(pte) ((swp_entry_t) { pte_val((pte)) })
> #define __swp_entry_to_pte(x) ((pte_t) { .pte = (x).val })
>
> _
--
Michal Hocko
SUSE Labs
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