Re: [PATCH 5/5] ext4: write support for preallocated blocks/extents

Andrew Morton
2007-05-04T04:32:51Z

On Thu, 26 Apr 2007 23:46:23 +0530 "Amit K. Arora" <aarora@linux.vnet.ibm.com> wrote:

> This patch adds write support for preallocated (using fallocate system
> call) blocks/extents. The preallocated extents in ext4 are marked
> "uninitialized", hence they need special handling especially while
> writing to them. This patch takes care of that.
>
> ...
>
> /*
> + * ext4_ext_try_to_merge:
> + * tries to merge the "ex" extent to the next extent in the tree.
> + * It always tries to merge towards right. If you want to merge towards
> + * left, pass "ex - 1" as argument instead of "ex".
> + * Returns 0 if the extents (ex and ex+1) were _not_ merged and returns
> + * 1 if they got merged.

OK.

> + */
> +int ext4_ext_try_to_merge(struct inode *inode,
> + struct ext4_ext_path *path,
> + struct ext4_extent *ex)
> +{
> + struct ext4_extent_header *eh;
> + unsigned int depth, len;
> + int merge_done=0, uninitialized = 0;

space around "=", please.

Many people prefer not to do the multiple-definitions-per-line, btw:

int merge_done = 0;
int uninitialized = 0;

reasons:

- If gives you some space for a nice comment

- It makes patches much more readable, and it makes rejects easier to fix

- standardisation.

> + depth = ext_depth(inode);
> + BUG_ON(path[depth].p_hdr == NULL);
> + eh = path[depth].p_hdr;
> +
> + while (ex < EXT_LAST_EXTENT(eh)) {
> + if (!ext4_can_extents_be_merged(inode, ex, ex + 1))
> + break;
> + /* merge with next extent! */
> + if (ext4_ext_is_uninitialized(ex))
> + uninitialized = 1;
> + ex->ee_len = cpu_to_le16(ext4_ext_get_actual_len(ex)
> + + ext4_ext_get_actual_len(ex + 1));
> + if (uninitialized)
> + ext4_ext_mark_uninitialized(ex);
> +
> + if (ex + 1 < EXT_LAST_EXTENT(eh)) {
> + len = (EXT_LAST_EXTENT(eh) - ex - 1)
> + * sizeof(struct ext4_extent);
> + memmove(ex + 1, ex + 2, len);
> + }
> + eh->eh_entries = cpu_to_le16(le16_to_cpu(eh->eh_entries)-1);

Kenrel convention is to put spaces around "-"

> + merge_done = 1;
> + BUG_ON(eh->eh_entries == 0);

eek, scary BUG_ON. Do we really need to be that severe? Would it be
better to warn and run ext4_error() here?

> + }
> +
> + return merge_done;
> +}
> +
> +
>
> ...
>
> +/*
> + * ext4_ext_convert_to_initialized:
> + * this function is called by ext4_ext_get_blocks() if someone tries to write
> + * to an uninitialized extent. It may result in splitting the uninitialized
> + * extent into multiple extents (upto three). Atleast one initialized extent
> + * and atmost two uninitialized extents can result.

There are some typos here

> + * There are three possibilities:
> + * a> No split required: Entire extent should be initialized.
> + * b> Split into two extents: Only one end of the extent is being written to.
> + * c> Split into three extents: Somone is writing in middle of the extent.

and here

> + */
> +int ext4_ext_convert_to_initialized(handle_t *handle, struct inode *inode,
> + struct ext4_ext_path *path,
> + ext4_fsblk_t iblock,
> + unsigned long max_blocks)
> +{
> + struct ext4_extent *ex, *ex1 = NULL, *ex2 = NULL, *ex3 = NULL, newex;
> + struct ext4_extent_header *eh;
> + unsigned int allocated, ee_block, ee_len, depth;
> + ext4_fsblk_t newblock;
> + int err = 0, ret = 0;
> +
> + depth = ext_depth(inode);
> + eh = path[depth].p_hdr;
> + ex = path[depth].p_ext;
> + ee_block = le32_to_cpu(ex->ee_block);
> + ee_len = ext4_ext_get_actual_len(ex);
> + allocated = ee_len - (iblock - ee_block);
> + newblock = iblock - ee_block + ext_pblock(ex);
> + ex2 = ex;
> +
> + /* ex1: ee_block to iblock - 1 : uninitialized */
> + if (iblock > ee_block) {
> + ex1 = ex;
> + ex1->ee_len = cpu_to_le16(iblock - ee_block);
> + ext4_ext_mark_uninitialized(ex1);
> + ex2 = &newex;
> + }
> + /* for sanity, update the length of the ex2 extent before
> + * we insert ex3, if ex1 is NULL. This is to avoid temporary
> + * overlap of blocks.
> + */
> + if (!ex1 && allocated > max_blocks)
> + ex2->ee_len = cpu_to_le16(max_blocks);
> + /* ex3: to ee_block + ee_len : uninitialised */
> + if (allocated > max_blocks) {
> + unsigned int newdepth;
> + ex3 = &newex;
> + ex3->ee_block = cpu_to_le32(iblock + max_blocks);
> + ext4_ext_store_pblock(ex3, newblock + max_blocks);
> + ex3->ee_len = cpu_to_le16(allocated - max_blocks);
> + ext4_ext_mark_uninitialized(ex3);
> + err = ext4_ext_insert_extent(handle, inode, path, ex3);
> + if (err)
> + goto out;
> + /* The depth, and hence eh & ex might change
> + * as part of the insert above.
> + */
> + newdepth = ext_depth(inode);
> + if (newdepth != depth)
> + {

Use

if (newdepth != depth) {

> + depth=newdepth;

spaces

> + path = ext4_ext_find_extent(inode, iblock, NULL);
> + if (IS_ERR(path)) {
> + err = PTR_ERR(path);
> + path = NULL;
> + goto out;
> + }
> + eh = path[depth].p_hdr;
> + ex = path[depth].p_ext;
> + if (ex2 != &newex)
> + ex2 = ex;
> + }
> + allocated = max_blocks;
> + }
> + /* If there was a change of depth as part of the
> + * insertion of ex3 above, we need to update the length
> + * of the ex1 extent again here
> + */
> + if (ex1 && ex1 != ex) {
> + ex1 = ex;
> + ex1->ee_len = cpu_to_le16(iblock - ee_block);
> + ext4_ext_mark_uninitialized(ex1);
> + ex2 = &newex;
> + }
> + /* ex2: iblock to iblock + maxblocks-1 : initialised */
> + ex2->ee_block = cpu_to_le32(iblock);
> + ex2->ee_start = cpu_to_le32(newblock);
> + ext4_ext_store_pblock(ex2, newblock);
> + ex2->ee_len = cpu_to_le16(allocated);
> + if (ex2 != ex)
> + goto insert;
> + if ((err = ext4_ext_get_access(handle, inode, path + depth)))
> + goto out;

The preferred style is

err = ext4_ext_get_access(handle, inode, path + depth);
if (err)
goto out;

> + /* New (initialized) extent starts from the first block
> + * in the current extent. i.e., ex2 == ex
> + * We have to see if it can be merged with the extent
> + * on the left.
> + */
> + if (ex2 > EXT_FIRST_EXTENT(eh)) {
> + /* To merge left, pass "ex2 - 1" to try_to_merge(),
> + * since it merges towards right _only_.
> + */
> + ret = ext4_ext_try_to_merge(inode, path, ex2 - 1);
> + if (ret) {
> + err = ext4_ext_correct_indexes(handle, inode, path);
> + if (err)
> + goto out;
> + depth = ext_depth(inode);
> + ex2--;
> + }
> + }
> + /* Try to Merge towards right. This might be required
> + * only when the whole extent is being written to.
> + * i.e. ex2==ex and ex3==NULL.
> + */
> + if (!ex3) {
> + ret = ext4_ext_try_to_merge(inode, path, ex2);
> + if (ret) {
> + err = ext4_ext_correct_indexes(handle, inode, path);
> + if (err)
> + goto out;
> + }
> + }
> + /* Mark modified extent as dirty */
> + err = ext4_ext_dirty(handle, inode, path + depth);
> + goto out;
> +insert:
> + err = ext4_ext_insert_extent(handle, inode, path, &newex);
> +out:
> + return err ? err : allocated;
> +}

Sigh. I hope you guys know how all this works, because the extent code is
a mystery to me. Is the on-disk layout and the allocation strategy
described anywhere?

> +extern int ext4_ext_try_to_merge(struct inode *, struct ext4_ext_path *, struct ext4_extent *);

Again, I do think that sticking the identifiers in there helps
readability. Although it is not as important in a boring old declaration
as it is in, say, inode_operations, etc.

Please try to keep the code looking nice in an 80-column display.