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     vm_map_entry_resize_free — vm map free space algorithm


     #include <sys/param.h>
     #include <vm/vm.h>
     #include <vm/vm_map.h>

     vm_map_entry_resize_free(vm_map_t map, vm_map_entry_t entry);


     This manual page describes the vm_map_entry fields used in the VM map
     free space algorithm, how to maintain consistency of these variables, and
     the vm_map_entry_resize_free() function.

     VM map entries are organized as both a doubly-linked list (prev and next
     pointers) and as a binary search tree (left and right pointers).  The
     search tree is organized as a Sleator and Tarjan splay tree, also known
     as a “self-adjusting tree”.

           struct vm_map_entry {
                   struct vm_map_entry *prev;
                   struct vm_map_entry *next;
                   struct vm_map_entry *left;
                   struct vm_map_entry *right;
                   vm_offset_t start;
                   vm_offset_t end;
                   vm_offset_t avail_ssize;
                   vm_size_t adj_free;
                   vm_size_t max_free;

     The free space algorithm adds two fields to struct vm_map_entry: adj_free
     and max_free.  The adj_free field is the amount of free address space
     adjacent to and immediately following (higher address) the map entry.
     This field is unused in the map header.  Note that adj_free depends on
     the linked list, not the splay tree and that adj_free can be computed as:

           entry->adj_free = (entry->next == &map->header ?
               map->max_offset : entry->next->start) - entry->end;

     The max_free field is the maximum amount of contiguous free space in the
     entry's subtree.  Note that max_free depends on the splay tree, not the
     linked list and that max_free is computed by taking the maximum of its
     own adj_free and the max_free of its left and right subtrees.  Again,
     max_free is unused in the map header.

     These fields allow for an O(log n) implementation of vm_map_findspace().
     Using max_free, we can immediately test for a sufficiently large free
     region in an entire subtree.  This makes it possible to find a first-fit
     free region of a given size in one pass down the tree, so O(log n)
     amortized using splay trees.

     When a free region changes size, we must update adj_free and max_free in
     the preceding map entry and propagate max_free up the tree.  This is
     handled in vm_map_entry_link() and vm_map_entry_unlink() for the cases of
     inserting and deleting an entry.  Note that vm_map_entry_link() updates
     both the new entry and the previous entry, and that vm_map_entry_unlink()
     updates the previous entry.  Also note that max_free is not actually
     propagated up the tree.  Instead, that entry is first splayed to the root
     and then the change is made there.  This is a common technique in splay
     trees and is also how map entries are linked and unlinked into the tree.

     The vm_map_entry_resize_free() function updates the free space variables
     in the given entry and propagates those values up the tree.  This
     function should be called whenever a map entry is resized in-place, that
     is, by modifying its start or end values.  Note that if you change end,
     then you should resize that entry, but if you change start, then you
     should resize the previous entry.  The map must be locked before calling
     this function, and again, propagating max_free is performed by splaying
     that entry to the root.


     Consider adding a map entry with vm_map_insert().

           ret = vm_map_insert(map, object, offset, start, end, prot,
               max_prot, cow);

     In this case, no further action is required to maintain consistency of
     the free space variables.  The vm_map_insert() function calls
     vm_map_entry_link() which updates both the new entry and the previous
     entry.  The same would be true for vm_map_delete() and for calling
     vm_map_entry_link() or vm_map_entry_unlink() directly.

     Now consider resizing an entry in-place without a call to
     vm_map_entry_link() or vm_map_entry_unlink().

           entry->start = new_start;
           if (entry->prev != &map->header)
                   vm_map_entry_resize_free(map, entry->prev);

     In this case, resetting start changes the amount of free space following
     the previous entry, so we use vm_map_entry_resize_free() to update the
     previous entry.

     Finally, suppose we change an entry's end address.

           entry->end = new_end;
           vm_map_entry_resize_free(map, entry);

     Here, we call vm_map_entry_resize_free() on the entry itself.


     vm_map(9), vm_map_findspace(9)

     Daniel D. Sleator and Robert E. Tarjan, "Self-Adjusting Binary Search
     Trees", JACM, vol. 32(3), pp. 652-686, July 1985.


     Splay trees were added to the VM map in FreeBSD 5.0, and the O(log n)
     tree-based free space algorithm was added in FreeBSD 5.3.


     The tree-based free space algorithm and this manual page were written by
     Mark W. Krentel ⟨⟩.