[PATCH] ext3: more comments about block allocation/reservation code
Signed-off-by: Mingming Cao <cmm@us.ibm.com>
Acked-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
diff --git a/fs/ext3/balloc.c b/fs/ext3/balloc.c
index 2bd836f..f1897fe 100644
--- a/fs/ext3/balloc.c
+++ b/fs/ext3/balloc.c
@@ -38,6 +38,13 @@
#define in_range(b, first, len) ((b) >= (first) && (b) <= (first) + (len) - 1)
+/**
+ * ext3_get_group_desc() -- load group descriptor from disk
+ * @sb: super block
+ * @block_group: given block group
+ * @bh: pointer to the buffer head to store the block
+ * group descriptor
+ */
struct ext3_group_desc * ext3_get_group_desc(struct super_block * sb,
unsigned int block_group,
struct buffer_head ** bh)
@@ -73,7 +80,11 @@
return desc + offset;
}
-/*
+/**
+ * read_block_bitmap()
+ * @sb: super block
+ * @block_group: given block group
+ *
* Read the bitmap for a given block_group, reading into the specified
* slot in the superblock's bitmap cache.
*
@@ -103,13 +114,20 @@
* Operations include:
* dump, find, add, remove, is_empty, find_next_reservable_window, etc.
*
- * We use sorted double linked list for the per-filesystem reservation
- * window list. (like in vm_region).
+ * We use a red-black tree to represent per-filesystem reservation
+ * windows.
*
- * Initially, we keep those small operations in the abstract functions,
- * so later if we need a better searching tree than double linked-list,
- * we could easily switch to that without changing too much
- * code.
+ */
+
+/**
+ * __rsv_window_dump() -- Dump the filesystem block allocation reservation map
+ * @rb_root: root of per-filesystem reservation rb tree
+ * @verbose: verbose mode
+ * @fn: function which wishes to dump the reservation map
+ *
+ * If verbose is turned on, it will print the whole block reservation
+ * windows(start, end). Otherwise, it will only print out the "bad" windows,
+ * those windows that overlap with their immediate neighbors.
*/
#if 1
static void __rsv_window_dump(struct rb_root *root, int verbose,
@@ -161,6 +179,22 @@
#define rsv_window_dump(root, verbose) do {} while (0)
#endif
+/**
+ * goal_in_my_reservation()
+ * @rsv: inode's reservation window
+ * @grp_goal: given goal block relative to the allocation block group
+ * @group: the current allocation block group
+ * @sb: filesystem super block
+ *
+ * Test if the given goal block (group relative) is within the file's
+ * own block reservation window range.
+ *
+ * If the reservation window is outside the goal allocation group, return 0;
+ * grp_goal (given goal block) could be -1, which means no specific
+ * goal block. In this case, always return 1.
+ * If the goal block is within the reservation window, return 1;
+ * otherwise, return 0;
+ */
static int
goal_in_my_reservation(struct ext3_reserve_window *rsv, ext3_grpblk_t grp_goal,
unsigned int group, struct super_block * sb)
@@ -179,7 +213,11 @@
return 1;
}
-/*
+/**
+ * search_reserve_window()
+ * @rb_root: root of reservation tree
+ * @goal: target allocation block
+ *
* Find the reserved window which includes the goal, or the previous one
* if the goal is not in any window.
* Returns NULL if there are no windows or if all windows start after the goal.
@@ -216,6 +254,13 @@
return rsv;
}
+/**
+ * ext3_rsv_window_add() -- Insert a window to the block reservation rb tree.
+ * @sb: super block
+ * @rsv: reservation window to add
+ *
+ * Must be called with rsv_lock hold.
+ */
void ext3_rsv_window_add(struct super_block *sb,
struct ext3_reserve_window_node *rsv)
{
@@ -246,6 +291,15 @@
rb_insert_color(node, root);
}
+/**
+ * ext3_rsv_window_remove() -- unlink a window from the reservation rb tree
+ * @sb: super block
+ * @rsv: reservation window to remove
+ *
+ * Mark the block reservation window as not allocated, and unlink it
+ * from the filesystem reservation window rb tree. Must be called with
+ * rsv_lock hold.
+ */
static void rsv_window_remove(struct super_block *sb,
struct ext3_reserve_window_node *rsv)
{
@@ -255,11 +309,39 @@
rb_erase(&rsv->rsv_node, &EXT3_SB(sb)->s_rsv_window_root);
}
+/*
+ * rsv_is_empty() -- Check if the reservation window is allocated.
+ * @rsv: given reservation window to check
+ *
+ * returns 1 if the end block is EXT3_RESERVE_WINDOW_NOT_ALLOCATED.
+ */
static inline int rsv_is_empty(struct ext3_reserve_window *rsv)
{
/* a valid reservation end block could not be 0 */
- return (rsv->_rsv_end == EXT3_RESERVE_WINDOW_NOT_ALLOCATED);
+ return rsv->_rsv_end == EXT3_RESERVE_WINDOW_NOT_ALLOCATED;
}
+
+/**
+ * ext3_init_block_alloc_info()
+ * @inode: file inode structure
+ *
+ * Allocate and initialize the reservation window structure, and
+ * link the window to the ext3 inode structure at last
+ *
+ * The reservation window structure is only dynamically allocated
+ * and linked to ext3 inode the first time the open file
+ * needs a new block. So, before every ext3_new_block(s) call, for
+ * regular files, we should check whether the reservation window
+ * structure exists or not. In the latter case, this function is called.
+ * Fail to do so will result in block reservation being turned off for that
+ * open file.
+ *
+ * This function is called from ext3_get_blocks_handle(), also called
+ * when setting the reservation window size through ioctl before the file
+ * is open for write (needs block allocation).
+ *
+ * Needs truncate_mutex protection prior to call this function.
+ */
void ext3_init_block_alloc_info(struct inode *inode)
{
struct ext3_inode_info *ei = EXT3_I(inode);
@@ -289,6 +371,19 @@
ei->i_block_alloc_info = block_i;
}
+/**
+ * ext3_discard_reservation()
+ * @inode: inode
+ *
+ * Discard(free) block reservation window on last file close, or truncate
+ * or at last iput().
+ *
+ * It is being called in three cases:
+ * ext3_release_file(): last writer close the file
+ * ext3_clear_inode(): last iput(), when nobody link to this file.
+ * ext3_truncate(): when the block indirect map is about to change.
+ *
+ */
void ext3_discard_reservation(struct inode *inode)
{
struct ext3_inode_info *ei = EXT3_I(inode);
@@ -308,7 +403,14 @@
}
}
-/* Free given blocks, update quota and i_blocks field */
+/**
+ * ext3_free_blocks_sb() -- Free given blocks and update quota
+ * @handle: handle to this transaction
+ * @sb: super block
+ * @block: start physcial block to free
+ * @count: number of blocks to free
+ * @pdquot_freed_blocks: pointer to quota
+ */
void ext3_free_blocks_sb(handle_t *handle, struct super_block *sb,
ext3_fsblk_t block, unsigned long count,
unsigned long *pdquot_freed_blocks)
@@ -492,7 +594,13 @@
return;
}
-/* Free given blocks, update quota and i_blocks field */
+/**
+ * ext3_free_blocks() -- Free given blocks and update quota
+ * @handle: handle for this transaction
+ * @inode: inode
+ * @block: start physical block to free
+ * @count: number of blocks to count
+ */
void ext3_free_blocks(handle_t *handle, struct inode *inode,
ext3_fsblk_t block, unsigned long count)
{
@@ -510,7 +618,11 @@
return;
}
-/*
+/**
+ * ext3_test_allocatable()
+ * @nr: given allocation block group
+ * @bh: bufferhead contains the bitmap of the given block group
+ *
* For ext3 allocations, we must not reuse any blocks which are
* allocated in the bitmap buffer's "last committed data" copy. This
* prevents deletes from freeing up the page for reuse until we have
@@ -543,6 +655,16 @@
return ret;
}
+/**
+ * bitmap_search_next_usable_block()
+ * @start: the starting block (group relative) of the search
+ * @bh: bufferhead contains the block group bitmap
+ * @maxblocks: the ending block (group relative) of the reservation
+ *
+ * The bitmap search --- search forward alternately through the actual
+ * bitmap on disk and the last-committed copy in journal, until we find a
+ * bit free in both bitmaps.
+ */
static ext3_grpblk_t
bitmap_search_next_usable_block(ext3_grpblk_t start, struct buffer_head *bh,
ext3_grpblk_t maxblocks)
@@ -550,11 +672,6 @@
ext3_grpblk_t next;
struct journal_head *jh = bh2jh(bh);
- /*
- * The bitmap search --- search forward alternately through the actual
- * bitmap and the last-committed copy until we find a bit free in
- * both
- */
while (start < maxblocks) {
next = ext3_find_next_zero_bit(bh->b_data, maxblocks, start);
if (next >= maxblocks)
@@ -570,8 +687,14 @@
return -1;
}
-/*
- * Find an allocatable block in a bitmap. We honour both the bitmap and
+/**
+ * find_next_usable_block()
+ * @start: the starting block (group relative) to find next
+ * allocatable block in bitmap.
+ * @bh: bufferhead contains the block group bitmap
+ * @maxblocks: the ending block (group relative) for the search
+ *
+ * Find an allocatable block in a bitmap. We honor both the bitmap and
* its last-committed copy (if that exists), and perform the "most
* appropriate allocation" algorithm of looking for a free block near
* the initial goal; then for a free byte somewhere in the bitmap; then
@@ -622,7 +745,11 @@
return here;
}
-/*
+/**
+ * claim_block()
+ * @block: the free block (group relative) to allocate
+ * @bh: the bufferhead containts the block group bitmap
+ *
* We think we can allocate this block in this bitmap. Try to set the bit.
* If that succeeds then check that nobody has allocated and then freed the
* block since we saw that is was not marked in b_committed_data. If it _was_
@@ -648,7 +775,26 @@
return ret;
}
-/*
+/**
+ * ext3_try_to_allocate()
+ * @sb: superblock
+ * @handle: handle to this transaction
+ * @group: given allocation block group
+ * @bitmap_bh: bufferhead holds the block bitmap
+ * @grp_goal: given target block within the group
+ * @count: target number of blocks to allocate
+ * @my_rsv: reservation window
+ *
+ * Attempt to allocate blocks within a give range. Set the range of allocation
+ * first, then find the first free bit(s) from the bitmap (within the range),
+ * and at last, allocate the blocks by claiming the found free bit as allocated.
+ *
+ * To set the range of this allocation:
+ * if there is a reservation window, only try to allocate block(s) from the
+ * file's own reservation window;
+ * Otherwise, the allocation range starts from the give goal block, ends at
+ * the block group's last block.
+ *
* If we failed to allocate the desired block then we may end up crossing to a
* new bitmap. In that case we must release write access to the old one via
* ext3_journal_release_buffer(), else we'll run out of credits.
@@ -705,7 +851,8 @@
}
start = grp_goal;
- if (!claim_block(sb_bgl_lock(EXT3_SB(sb), group), grp_goal, bitmap_bh)) {
+ if (!claim_block(sb_bgl_lock(EXT3_SB(sb), group),
+ grp_goal, bitmap_bh)) {
/*
* The block was allocated by another thread, or it was
* allocated and then freed by another thread
@@ -720,7 +867,8 @@
grp_goal++;
while (num < *count && grp_goal < end
&& ext3_test_allocatable(grp_goal, bitmap_bh)
- && claim_block(sb_bgl_lock(EXT3_SB(sb), group), grp_goal, bitmap_bh)) {
+ && claim_block(sb_bgl_lock(EXT3_SB(sb), group),
+ grp_goal, bitmap_bh)) {
num++;
grp_goal++;
}
@@ -931,9 +1079,10 @@
if ((my_rsv->rsv_alloc_hit >
(my_rsv->rsv_end - my_rsv->rsv_start + 1) / 2)) {
/*
- * if we previously allocation hit ration is greater than half
- * we double the size of reservation window next time
- * otherwise keep the same
+ * if the previously allocation hit ratio is
+ * greater than 1/2, then we double the size of
+ * the reservation window the next time,
+ * otherwise we keep the same size window
*/
size = size * 2;
if (size > EXT3_MAX_RESERVE_BLOCKS)
@@ -1012,6 +1161,23 @@
goto retry;
}
+/**
+ * try_to_extend_reservation()
+ * @my_rsv: given reservation window
+ * @sb: super block
+ * @size: the delta to extend
+ *
+ * Attempt to expand the reservation window large enough to have
+ * required number of free blocks
+ *
+ * Since ext3_try_to_allocate() will always allocate blocks within
+ * the reservation window range, if the window size is too small,
+ * multiple blocks allocation has to stop at the end of the reservation
+ * window. To make this more efficient, given the total number of
+ * blocks needed and the current size of the window, we try to
+ * expand the reservation window size if necessary on a best-effort
+ * basis before ext3_new_blocks() tries to allocate blocks,
+ */
static void try_to_extend_reservation(struct ext3_reserve_window_node *my_rsv,
struct super_block *sb, int size)
{
@@ -1037,7 +1203,17 @@
spin_unlock(rsv_lock);
}
-/*
+/**
+ * ext3_try_to_allocate_with_rsv()
+ * @sb: superblock
+ * @handle: handle to this transaction
+ * @group: given allocation block group
+ * @bitmap_bh: bufferhead holds the block bitmap
+ * @grp_goal: given target block within the group
+ * @count: target number of blocks to allocate
+ * @my_rsv: reservation window
+ * @errp: pointer to store the error code
+ *
* This is the main function used to allocate a new block and its reservation
* window.
*
@@ -1053,9 +1229,7 @@
* reservation), and there are lots of free blocks, but they are all
* being reserved.
*
- * We use a sorted double linked list for the per-filesystem reservation list.
- * The insert, remove and find a free space(non-reserved) operations for the
- * sorted double linked list should be fast.
+ * We use a red-black tree for the per-filesystem reservation list.
*
*/
static ext3_grpblk_t
@@ -1120,7 +1294,8 @@
*/
while (1) {
if (rsv_is_empty(&my_rsv->rsv_window) || (ret < 0) ||
- !goal_in_my_reservation(&my_rsv->rsv_window, grp_goal, group, sb)) {
+ !goal_in_my_reservation(&my_rsv->rsv_window,
+ grp_goal, group, sb)) {
if (my_rsv->rsv_goal_size < *count)
my_rsv->rsv_goal_size = *count;
ret = alloc_new_reservation(my_rsv, grp_goal, sb,
@@ -1128,19 +1303,22 @@
if (ret < 0)
break; /* failed */
- if (!goal_in_my_reservation(&my_rsv->rsv_window, grp_goal, group, sb))
+ if (!goal_in_my_reservation(&my_rsv->rsv_window,
+ grp_goal, group, sb))
grp_goal = -1;
- } else if (grp_goal > 0 && (my_rsv->rsv_end-grp_goal+1) < *count)
+ } else if (grp_goal > 0 &&
+ (my_rsv->rsv_end-grp_goal+1) < *count)
try_to_extend_reservation(my_rsv, sb,
*count-my_rsv->rsv_end + grp_goal - 1);
- if ((my_rsv->rsv_start >= group_first_block + EXT3_BLOCKS_PER_GROUP(sb))
+ if ((my_rsv->rsv_start >= group_first_block +
+ EXT3_BLOCKS_PER_GROUP(sb))
|| (my_rsv->rsv_end < group_first_block)) {
rsv_window_dump(&EXT3_SB(sb)->s_rsv_window_root, 1);
BUG();
}
- ret = ext3_try_to_allocate(sb, handle, group, bitmap_bh, grp_goal,
- &num, &my_rsv->rsv_window);
+ ret = ext3_try_to_allocate(sb, handle, group, bitmap_bh,
+ grp_goal, &num, &my_rsv->rsv_window);
if (ret >= 0) {
my_rsv->rsv_alloc_hit += num;
*count = num;
@@ -1165,6 +1343,12 @@
return ret;
}
+/**
+ * ext3_has_free_blocks()
+ * @sbi: in-core super block structure.
+ *
+ * Check if filesystem has at least 1 free block available for allocation.
+ */
static int ext3_has_free_blocks(struct ext3_sb_info *sbi)
{
ext3_fsblk_t free_blocks, root_blocks;
@@ -1179,11 +1363,17 @@
return 1;
}
-/*
+/**
+ * ext3_should_retry_alloc()
+ * @sb: super block
+ * @retries number of attemps has been made
+ *
* ext3_should_retry_alloc() is called when ENOSPC is returned, and if
* it is profitable to retry the operation, this function will wait
* for the current or commiting transaction to complete, and then
* return TRUE.
+ *
+ * if the total number of retries exceed three times, return FALSE.
*/
int ext3_should_retry_alloc(struct super_block *sb, int *retries)
{
@@ -1195,13 +1385,19 @@
return journal_force_commit_nested(EXT3_SB(sb)->s_journal);
}
-/*
- * ext3_new_block uses a goal block to assist allocation. If the goal is
- * free, or there is a free block within 32 blocks of the goal, that block
- * is allocated. Otherwise a forward search is made for a free block; within
- * each block group the search first looks for an entire free byte in the block
- * bitmap, and then for any free bit if that fails.
- * This function also updates quota and i_blocks field.
+/**
+ * ext3_new_blocks() -- core block(s) allocation function
+ * @handle: handle to this transaction
+ * @inode: file inode
+ * @goal: given target block(filesystem wide)
+ * @count: target number of blocks to allocate
+ * @errp: error code
+ *
+ * ext3_new_blocks uses a goal block to assist allocation. It tries to
+ * allocate block(s) from the block group contains the goal block first. If that
+ * fails, it will try to allocate block(s) from other block groups without
+ * any specific goal block.
+ *
*/
ext3_fsblk_t ext3_new_blocks(handle_t *handle, struct inode *inode,
ext3_fsblk_t goal, unsigned long *count, int *errp)
@@ -1432,7 +1628,7 @@
spin_lock(sb_bgl_lock(sbi, group_no));
gdp->bg_free_blocks_count =
- cpu_to_le16(le16_to_cpu(gdp->bg_free_blocks_count) - num);
+ cpu_to_le16(le16_to_cpu(gdp->bg_free_blocks_count)-num);
spin_unlock(sb_bgl_lock(sbi, group_no));
percpu_counter_mod(&sbi->s_freeblocks_counter, -num);
@@ -1475,6 +1671,12 @@
return ext3_new_blocks(handle, inode, goal, &count, errp);
}
+/**
+ * ext3_count_free_blocks() -- count filesystem free blocks
+ * @sb: superblock
+ *
+ * Adds up the number of free blocks from each block group.
+ */
ext3_fsblk_t ext3_count_free_blocks(struct super_block *sb)
{
ext3_fsblk_t desc_count;