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[ARM] pgtable: swp pte layout documentation, definitions, and check 

Document the layout of our swp PTE entries, adding definitions for
the bit masks/shifts/sizes, and implement MAX_SWAPFILES_CHECK()
such that we fail to build if we are unable to properly encode the
swp type field.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
This commit is contained in:
Russell King 2009-07-05 11:30:15 +01:00 committed by Russell King
parent ba9b42e4ff
commit fb93a1c75e
1 changed files with 25 additions and 5 deletions
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30
arch/arm/include/asm/pgtable.h
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@ -384,16 +384,36 @@ static inline pte_t pte_modify(pte_t pte, pgprot_t newprot)
extern pgd_t swapper_pg_dir[PTRS_PER_PGD];
/* Encode and decode a swap entry.
/*
* Encode and decode a swap entry. Swap entries are stored in the Linux
* page tables as follows:
*
* We support up to 32GB of swap on 4k machines
* 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
*
* This gives us up to 127 swap files and 32GB per swap file. Note that
* the offset field is always non-zero.
*/
#define __swp_type(x) (((x).val >> 2) & 0x7f)
#define __swp_offset(x) ((x).val >> 9)
#define __swp_entry(type,offset) ((swp_entry_t) { ((type) << 2) | ((offset) << 9) })
#define __SWP_TYPE_SHIFT 2
#define __SWP_TYPE_BITS 7
#define __SWP_TYPE_MASK ((1 << __SWP_TYPE_BITS) - 1)
#define __SWP_OFFSET_SHIFT (__SWP_TYPE_BITS + __SWP_TYPE_SHIFT)
#define __swp_type(x) (((x).val >> __SWP_TYPE_SHIFT) & __SWP_TYPE_MASK)
#define __swp_offset(x) ((x).val >> __SWP_OFFSET_SHIFT)
#define __swp_entry(type,offset) ((swp_entry_t) { ((type) << __SWP_TYPE_SHIFT) | ((offset) << __SWP_OFFSET_SHIFT) })
#define __pte_to_swp_entry(pte) ((swp_entry_t) { pte_val(pte) })
#define __swp_entry_to_pte(swp) ((pte_t) { (swp).val })
/*
* It is an error for the kernel to have more swap files than we can
* encode in the PTEs. This ensures that we know when MAX_SWAPFILES
* is increased beyond what we presently support.
*/
#define MAX_SWAPFILES_CHECK() BUILD_BUG_ON(MAX_SWAPFILES_SHIFT > __SWP_TYPE_BITS)
/* Needs to be defined here and not in linux/mm.h, as it is arch dependent */
/* FIXME: this is not correct */
#define kern_addr_valid(addr) (1)