mm: let swap use exceptional entries

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>

diff --git a/include/linux/swapops.h b/include/linux/swapops.h
index cd42e30..2189d3f 100644 (file)
--- a/include/linux/swapops.h
+++ b/include/linux/swapops.h
@@ -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 */

diff --git a/mm/filemap.c b/mm/filemap.c
index b83aebf..76bfb64 100644 (file)
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -714,9 +714,12 @@ repeat:
                page = radix_tree_deref_slot(pagep);
                if (unlikely(!page))
                        goto out;
-               if (radix_tree_deref_retry(page))
+               if (radix_tree_exception(page)) {
+                       if (radix_tree_exceptional_entry(page))
+                               goto out;
+                       /* radix_tree_deref_retry(page) */
                        goto repeat;
-
+               }
                if (!page_cache_get_speculative(page))
                        goto repeat;
 
@@ -753,7 +756,7 @@ struct page *find_lock_page(struct address_space *mapping, pgoff_t offset)
 
 repeat:
        page = find_get_page(mapping, offset);
-       if (page) {
+       if (page && !radix_tree_exception(page)) {
                lock_page(page);
                /* Has the page been truncated? */
                if (unlikely(page->mapping != mapping)) {
@@ -849,11 +852,14 @@ repeat:
                if (unlikely(!page))
                        continue;
 
-               /*
-                * This can only trigger when the entry at index 0 moves out
-                * of or back to the root: none yet gotten, safe to restart.
-                */
-               if (radix_tree_deref_retry(page)) {
+               if (radix_tree_exception(page)) {
+                       if (radix_tree_exceptional_entry(page))
+                               continue;
+                       /*
+                        * radix_tree_deref_retry(page):
+                        * can only trigger when entry at index 0 moves out of
+                        * or back to root: none yet gotten, safe to restart.
+                        */
                        WARN_ON(start | i);
                        goto restart;
                }
@@ -912,12 +918,16 @@ repeat:
                if (unlikely(!page))
                        continue;
 
-               /*
-                * This can only trigger when the entry at index 0 moves out
-                * of or back to the root: none yet gotten, safe to restart.
-                */
-               if (radix_tree_deref_retry(page))
+               if (radix_tree_exception(page)) {
+                       if (radix_tree_exceptional_entry(page))
+                               break;
+                       /*
+                        * radix_tree_deref_retry(page):
+                        * can only trigger when entry at index 0 moves out of
+                        * or back to root: none yet gotten, safe to restart.
+                        */
                        goto restart;
+               }
 
                if (!page_cache_get_speculative(page))
                        goto repeat;
@@ -977,12 +987,15 @@ repeat:
                if (unlikely(!page))
                        continue;
 
-               /*
-                * This can only trigger when the entry at index 0 moves out
-                * of or back to the root: none yet gotten, safe to restart.
-                */
-               if (radix_tree_deref_retry(page))
+               if (radix_tree_exception(page)) {
+                       BUG_ON(radix_tree_exceptional_entry(page));
+                       /*
+                        * radix_tree_deref_retry(page):
+                        * can only trigger when entry at index 0 moves out of
+                        * or back to root: none yet gotten, safe to restart.
+                        */
                        goto restart;
+               }
 
                if (!page_cache_get_speculative(page))
                        goto repeat;

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 1b8c339..17bc224 100644 (file)
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1924,20 +1924,24 @@ static unsigned long read_swap_header(struct swap_info_struct *p,
 
        /*
         * Find out how many pages are allowed for a single swap
-        * device. There are two limiting factors: 1) the number of
-        * bits for the swap offset in the swp_entry_t type and
-        * 2) the number of bits in the a swap pte as defined by
-        * the different architectures. In order to find the
-        * largest possible bit mask a swap entry with swap type 0
+        * device. There are three limiting factors: 1) the number
+        * of bits for the swap offset in the swp_entry_t type, and
+        * 2) the number of bits in the swap pte as defined by the
+        * the different architectures, and 3) the number of free bits
+        * in an exceptional radix_tree entry. In order to find the
+        * largest possible bit mask, a swap entry with swap type 0
         * and swap offset ~0UL is created, encoded to a swap pte,
-        * decoded to a swp_entry_t again and finally the swap
+        * decoded to a swp_entry_t again, and finally the swap
         * offset is extracted. This will mask all the bits from
         * the initial ~0UL mask that can't be encoded in either
         * the swp_entry_t or the architecture definition of a
-        * swap pte.
+        * swap pte.  Then the same is done for a radix_tree entry.
         */
        maxpages = swp_offset(pte_to_swp_entry(
-                       swp_entry_to_pte(swp_entry(0, ~0UL)))) + 1;
+                       swp_entry_to_pte(swp_entry(0, ~0UL))));
+       maxpages = swp_offset(radix_to_swp_entry(
+                       swp_to_radix_entry(swp_entry(0, maxpages)))) + 1;
+
        if (maxpages > swap_header->info.last_page) {
                maxpages = swap_header->info.last_page + 1;
                /* p->max is an unsigned int: don't overflow it */