Btrfs: dummy extent buffers for tree mod log

The tree modification log needs two ways to create dummy extent buffers,
once by allocating a fresh one (to rebuild an old root) and once by
cloning an existing one (to make private rewind modifications) to it.

Signed-off-by: Jan Schmidt <list.btrfs@jan-o-sch.net>

1 files changed, 73 insertions, 7 deletions

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index 2fb52c26c677..3daed70a401a 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -3930,6 +3930,7 @@ static struct extent_buffer *__alloc_extent_buffer(struct extent_io_tree *tree,
         eb->start = start;
         eb->len = len;
         eb->tree = tree;
+        eb->bflags = 0;
         rwlock_init(&eb->lock);
         atomic_set(&eb->write_locks, 0);
         atomic_set(&eb->read_locks, 0);
@@ -3967,6 +3968,60 @@ static struct extent_buffer *__alloc_extent_buffer(struct extent_io_tree *tree,
         return eb;
 }
 
+struct extent_buffer *btrfs_clone_extent_buffer(struct extent_buffer *src)
+{
+        unsigned long i;
+        struct page *p;
+        struct extent_buffer *new;
+        unsigned long num_pages = num_extent_pages(src->start, src->len);
+
+        new = __alloc_extent_buffer(NULL, src->start, src->len, GFP_ATOMIC);
+        if (new == NULL)
+                return NULL;
+
+        for (i = 0; i < num_pages; i++) {
+                p = alloc_page(GFP_ATOMIC);
+                BUG_ON(!p);
+                attach_extent_buffer_page(new, p);
+                WARN_ON(PageDirty(p));
+                SetPageUptodate(p);
+                new->pages[i] = p;
+        }
+
+        copy_extent_buffer(new, src, 0, 0, src->len);
+        set_bit(EXTENT_BUFFER_UPTODATE, &new->bflags);
+        set_bit(EXTENT_BUFFER_DUMMY, &new->bflags);
+
+        return new;
+}
+
+struct extent_buffer *alloc_dummy_extent_buffer(u64 start, unsigned long len)
+{
+        struct extent_buffer *eb;
+        unsigned long num_pages = num_extent_pages(0, len);
+        unsigned long i;
+
+        eb = __alloc_extent_buffer(NULL, start, len, GFP_ATOMIC);
+        if (!eb)
+                return NULL;
+
+        for (i = 0; i < num_pages; i++) {
+                eb->pages[i] = alloc_page(GFP_ATOMIC);
+                if (!eb->pages[i])
+                        goto err;
+        }
+        set_extent_buffer_uptodate(eb);
+        btrfs_set_header_nritems(eb, 0);
+        set_bit(EXTENT_BUFFER_DUMMY, &eb->bflags);
+
+        return eb;
+err:
+        for (i--; i > 0; i--)
+                __free_page(eb->pages[i]);
+        __free_extent_buffer(eb);
+        return NULL;
+}
+
 static int extent_buffer_under_io(struct extent_buffer *eb)
 {
         return (atomic_read(&eb->io_pages) ||
@@ -3982,6 +4037,7 @@ static void btrfs_release_extent_buffer_page(struct extent_buffer *eb,
 {
         unsigned long index;
         struct page *page;
+        int mapped = !test_bit(EXTENT_BUFFER_DUMMY, &eb->bflags);
 
         BUG_ON(extent_buffer_under_io(eb));
 
@@ -3992,7 +4048,7 @@ static void btrfs_release_extent_buffer_page(struct extent_buffer *eb,
         do {
                 index--;
                 page = extent_buffer_page(eb, index);
-                if (page) {
+                if (page && mapped) {
                         spin_lock(&page->mapping->private_lock);
                         /*
                          * We do this since we'll remove the pages after we've
@@ -4017,6 +4073,8 @@ static void btrfs_release_extent_buffer_page(struct extent_buffer *eb,
                         }
                         spin_unlock(&page->mapping->private_lock);
 
+                }
+                if (page) {
                         /* One for when we alloced the page */
                         page_cache_release(page);
                 }
@@ -4235,14 +4293,18 @@ static void release_extent_buffer(struct extent_buffer *eb, gfp_t mask)
 {
         WARN_ON(atomic_read(&eb->refs) == 0);
         if (atomic_dec_and_test(&eb->refs)) {
-                struct extent_io_tree *tree = eb->tree;
+                if (test_bit(EXTENT_BUFFER_DUMMY, &eb->bflags)) {
+                        spin_unlock(&eb->refs_lock);
+                } else {
+                        struct extent_io_tree *tree = eb->tree;
 
-                spin_unlock(&eb->refs_lock);
+                        spin_unlock(&eb->refs_lock);
 
-                spin_lock(&tree->buffer_lock);
-                radix_tree_delete(&tree->buffer,
-                                  eb->start >> PAGE_CACHE_SHIFT);
-                spin_unlock(&tree->buffer_lock);
+                        spin_lock(&tree->buffer_lock);
+                        radix_tree_delete(&tree->buffer,
+                                          eb->start >> PAGE_CACHE_SHIFT);
+                        spin_unlock(&tree->buffer_lock);
+                }
 
                 /* Should be safe to release our pages at this point */
                 btrfs_release_extent_buffer_page(eb, 0);
@@ -4260,6 +4322,10 @@ void free_extent_buffer(struct extent_buffer *eb)
 
         spin_lock(&eb->refs_lock);
         if (atomic_read(&eb->refs) == 2 &&
+            test_bit(EXTENT_BUFFER_DUMMY, &eb->bflags))
+                atomic_dec(&eb->refs);
+
+        if (atomic_read(&eb->refs) == 2 &&
             test_bit(EXTENT_BUFFER_STALE, &eb->bflags) &&
             !extent_buffer_under_io(eb) &&
             test_and_clear_bit(EXTENT_BUFFER_TREE_REF, &eb->bflags))