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mm: let swap use exceptional entries

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If swap entries are to be stored along with struct page pointers in a
radix tree, they need to be distinguished as exceptional entries.

Most of the handling of swap entries in radix tree will be contained in
shmem.c, but a few functions in filemap.c's common code need to check
for their appearance: find_get_page(), find_lock_page(),
find_get_pages() and find_get_pages_contig().

So as not to slow their fast paths, tuck those checks inside the
existing checks for unlikely radix_tree_deref_slot(); except for
find_lock_page(), where it is an added test.  And make it a BUG in
find_get_pages_tag(), which is not applied to tmpfs files.

A part of the reason for eliminating shmem_readpage() earlier, was to
minimize the places where common code would need to allow for swap
entries.

The swp_entry_t known to swapfile.c must be massaged into a slightly
different form when stored in the radix tree, just as it gets massaged
into a pte_t when stored in page tables.

In an i386 kernel this limits its information (type and page offset) to
30 bits: given 32 "types" of swapfile and 4kB pagesize, that's a maximum
swapfile size of 128GB.  Which is less than the 512GB we previously
allowed with X86_PAE (where the swap entry can occupy the entire upper
32 bits of a pte_t), but not a new limitation on 32-bit without PAE; and
there's not a new limitation on 64-bit (where swap filesize is already
limited to 16TB by a 32-bit page offset).  Thirty areas of 128GB is
probably still enough swap for a 64GB 32-bit machine.

Provide swp_to_radix_entry() and radix_to_swp_entry() conversions, and
enforce filesize limit in read_swap_header(), just as for ptes.

Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Hugh Dickins
2011-08-03 16:21:19 -07:00
committed by Linus Torvalds
parent 6328650bb4
commit a2c16d6cb0
3 changed files with 66 additions and 26 deletions
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23
include/linux/swapops.h
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@@ -1,3 +1,8 @@
#ifndef _LINUX_SWAPOPS_H
#define _LINUX_SWAPOPS_H
#include <linux/radix-tree.h>
/*
* swapcache pages are stored in the swapper_space radix tree. We want to
* get good packing density in that tree, so the index should be dense in
@@ -76,6 +81,22 @@ static inline pte_t swp_entry_to_pte(swp_entry_t entry)
return __swp_entry_to_pte(arch_entry);
}
static inline swp_entry_t radix_to_swp_entry(void *arg)
{
swp_entry_t entry;
entry.val = (unsigned long)arg >> RADIX_TREE_EXCEPTIONAL_SHIFT;
return entry;
}
static inline void *swp_to_radix_entry(swp_entry_t entry)
{
unsigned long value;
value = entry.val << RADIX_TREE_EXCEPTIONAL_SHIFT;
return (void *)(value | RADIX_TREE_EXCEPTIONAL_ENTRY);
}
#ifdef CONFIG_MIGRATION
static inline swp_entry_t make_migration_entry(struct page *page, int write)
{
@@ -169,3 +190,5 @@ static inline int non_swap_entry(swp_entry_t entry)
return 0;
}
#endif
#endif /* _LINUX_SWAPOPS_H */