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Commit 36faadc1 authored by Mingming Cao's avatar Mingming Cao Committed by Linus Torvalds
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[PATCH] ext3: more comments about block allocation/reservation code 

Signed-off-by: default avatarMingming Cao <cmm@us.ibm.com>
Acked-by: default avatarRandy Dunlap <rdunlap@xenotime.net>
Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
parent 321fb9e8
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Showing with 247 additions and 45 deletions
fs/ext3/balloc.c
View file @ 36faadc1
......@@ -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 @@ struct ext3_group_desc * ext3_get_group_desc(struct super_block * sb,
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 @@ error_out:
* 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 @@ restart:
#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 @@ goal_in_my_reservation(struct ext3_reserve_window *rsv, ext3_grpblk_t grp_goal,
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 @@ search_reserve_window(struct rb_root *root, ext3_fsblk_t goal)
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 @@ void ext3_rsv_window_add(struct super_block *sb,
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 @@ static void rsv_window_remove(struct super_block *sb,
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 @@ void ext3_init_block_alloc_info(struct inode *inode)
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 @@ void ext3_discard_reservation(struct inode *inode)
}
}
/* 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 @@ error_return:
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 @@ void ext3_free_blocks(handle_t *handle, struct inode *inode,
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 @@ static int ext3_test_allocatable(ext3_grpblk_t nr, struct buffer_head *bh)
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 @@ bitmap_search_next_usable_block(ext3_grpblk_t start, struct buffer_head *bh,
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 @@ bitmap_search_next_usable_block(ext3_grpblk_t start, struct buffer_head *bh,
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 @@ find_next_usable_block(ext3_grpblk_t start, struct buffer_head *bh,
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 @@ claim_block(spinlock_t *lock, ext3_grpblk_t block, struct buffer_head *bh)
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 @@ repeat:
}
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 @@ repeat:
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 @@ static int alloc_new_reservation(struct ext3_reserve_window_node *my_rsv,
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 @@ retry:
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 @@ static void try_to_extend_reservation(struct ext3_reserve_window_node *my_rsv,
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 @@ static void try_to_extend_reservation(struct ext3_reserve_window_node *my_rsv,
* 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 @@ ext3_try_to_allocate_with_rsv(struct super_block *sb, handle_t *handle,
*/
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 @@ ext3_try_to_allocate_with_rsv(struct super_block *sb, handle_t *handle,
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 @@ out:
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 @@ static int ext3_has_free_blocks(struct ext3_sb_info *sbi)
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 @@ int ext3_should_retry_alloc(struct super_block *sb, int *retries)
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 @@ allocated:
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 @@ ext3_fsblk_t ext3_new_block(handle_t *handle, struct inode *inode,
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;
......
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