radix_tree: exceptional entries and indices

A patchset to extend tmpfs to MAX_LFS_FILESIZE by abandoning its
peculiar swap vector, instead keeping a file's swap entries in the same
radix tree as its struct page pointers: thus saving memory, and
simplifying its code and locking.

This patch:

The radix_tree is used by several subsystems for different purposes.  A
major use is to store the struct page pointers of a file's pagecache for
memory management.  But what if mm wanted to store something other than
page pointers there too?

The low bit of a radix_tree entry is already used to denote an indirect
pointer, for internal use, and the unlikely radix_tree_deref_retry()
case.

Define the next bit as denoting an exceptional entry, and supply inline
functions radix_tree_exception() to return non-0 in either unlikely
case, and radix_tree_exceptional_entry() to return non-0 in the second
case.

If a subsystem already uses radix_tree with that bit set, no problem: it
does not affect internal workings at all, but is defined for the
convenience of those storing well-aligned pointers in the radix_tree.

The radix_tree_gang_lookups have an implicit assumption that the caller
can deduce the offset of each entry returned e.g.  by the page->index of
a struct page.  But that may not be feasible for some kinds of item to
be stored there.

radix_tree_gang_lookup_slot() allow for an optional indices argument,
output array in which to return those offsets.  The same could be added
to other radix_tree_gang_lookups, but for now keep it to the only one
for which we need it.

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>

1 files changed, 2 insertions, 2 deletions

diff --git a/mm/filemap.c b/mm/filemap.c
index 867d40222ec7..b83aebfd0a00 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -840,7 +840,7 @@ unsigned find_get_pages(struct address_space *mapping, pgoff_t start,
         rcu_read_lock();
 restart:
         nr_found = radix_tree_gang_lookup_slot(&mapping->page_tree,
-                                (void ***)pages, start, nr_pages);
+                                (void ***)pages, NULL, start, nr_pages);
         ret = 0;
         for (i = 0; i < nr_found; i++) {
                 struct page *page;
@@ -903,7 +903,7 @@ unsigned find_get_pages_contig(struct address_space *mapping, pgoff_t index,
         rcu_read_lock();
 restart:
         nr_found = radix_tree_gang_lookup_slot(&mapping->page_tree,
-                                (void ***)pages, index, nr_pages);
+                                (void ***)pages, NULL, index, nr_pages);
         ret = 0;
         for (i = 0; i < nr_found; i++) {
                 struct page *page;