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    Subject[ 05/46] ARM: 7488/1: mm: use 5 bits for swapfile type encoding
    Date
    From: Greg KH <gregkh@linuxfoundation.org>

    3.0-stable review patch. If anyone has any objections, please let me know.

    ------------------

    From: Will Deacon <will.deacon@arm.com>

    commit f5f2025ef3e2cdb593707cbf87378761f17befbe upstream.

    Page migration encodes the pfn in the offset field of a swp_entry_t.
    For LPAE, we support physical addresses of up to 36 bits (due to
    sparsemem limitations with the size of page flags), requiring 24 bits
    to represent a pfn. A further 3 bits are used to encode a swp_entry into
    a pte, leaving 5 bits for the type field. Furthermore, the core code
    defines MAX_SWAPFILES_SHIFT as 5, so the additional type bit does not
    get used.

    This patch reduces the width of the type field to 5 bits, allowing us
    to create up to 31 swapfiles of 64GB each.

    Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
    Signed-off-by: Will Deacon <will.deacon@arm.com>
    Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    arch/arm/include/asm/pgtable.h | 6 +++---
    1 file changed, 3 insertions(+), 3 deletions(-)

    --- a/arch/arm/include/asm/pgtable.h
    +++ b/arch/arm/include/asm/pgtable.h
    @@ -418,13 +418,13 @@ static inline pte_t pte_modify(pte_t pte
    *
    * 3 3 2 2 2 2 2 2 2 2 2 2 1 1 1 1 1 1 1 1 1 1
    * 1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0
    - * <--------------- offset --------------------> <- type --> 0 0 0
    + * <--------------- offset ----------------------> < type -> 0 0 0
    *
    - * This gives us up to 63 swap files and 32GB per swap file. Note that
    + * This gives us up to 31 swap files and 64GB per swap file. Note that
    * the offset field is always non-zero.
    */
    #define __SWP_TYPE_SHIFT 3
    -#define __SWP_TYPE_BITS 6
    +#define __SWP_TYPE_BITS 5
    #define __SWP_TYPE_MASK ((1 << __SWP_TYPE_BITS) - 1)
    #define __SWP_OFFSET_SHIFT (__SWP_TYPE_BITS + __SWP_TYPE_SHIFT)




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