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Date: Wed, 29 Aug 2012 17:24:34 +0900
From: Akira Fujita <a-fujita@...jp.nec.com>
To: Dmitry Monakhov <dmonakhov@...nvz.org>
CC: linux-ext4@...r.kernel.org, tytso@....edu
Subject: Re: [PATCH 3/3] ext4: reimplement uninit extent optimization for
move_extent_per_page
Hi Dmitry,
(2012/08/29 1:21), Dmitry Monakhov wrote:
> Uninitialized extent may became initialized(parallel writeback task)
> at any moment after we drop i_data_sem, so we have to recheck extent's
> state after we hold page's lock and i_data_sem.
>
> If we about to change page's mapping we must hold page's lock in order to
> serialize other users.
> ---
> fs/ext4/move_extent.c | 96 ++++++++++++++++++++++++++++++++++++++++++++-----
> 1 files changed, 87 insertions(+), 9 deletions(-)
>
> diff --git a/fs/ext4/move_extent.c b/fs/ext4/move_extent.c
> index c5ca317..b62defa 100644
> --- a/fs/ext4/move_extent.c
> +++ b/fs/ext4/move_extent.c
> @@ -629,6 +629,42 @@ mext_calc_swap_extents(struct ext4_extent *tmp_dext,
> }
>
> /**
> + * mext_check_range_coverage - Check that all extents in range has the same type
> + *
> + * @inode: inode in question
> + * @from: block offset of inode
> + * @count: block count to be checked
> + * @uninit: extents expected to be uninitialized
> + * @err: pointer to save error value
> + *
> + * Return 1 if all extents in range has expected type, and zero otherwise.
> + */
> +int mext_check_range_coverage(struct inode *inode, ext4_lblk_t from, ext4_lblk_t count,
> + int uninit, int *err)
> +{
> + struct ext4_ext_path *path = NULL;
> + struct ext4_extent *ext;
> + ext4_lblk_t last = from;
ext4_lblk_t last = from + count;
> + while (from < last) {
> + *err = get_ext_path(inode, from, &path);
> + if (*err)
> + return 0;
> + ext = path[ext_depth(inode)].p_ext;
> + if (!ext) {
> + ext4_ext_drop_refs(path);
> + return 0;
> + }
> + if (uninit != ext4_ext_is_uninitialized(ext)) {
> + ext4_ext_drop_refs(path);
> + return 0;
> + }
> + from += ext4_ext_get_actual_len(ext);
> + ext4_ext_drop_refs(path);
> + }
> + return 1;
> +}
> +
> +/**
> * mext_replace_branches - Replace original extents with new extents
> *
> * @handle: journal handle
> @@ -663,9 +699,6 @@ mext_replace_branches(handle_t *handle, struct inode *orig_inode,
> int replaced_count = 0;
> int dext_alen;
>
> - /* Protect extent trees against block allocations via delalloc */
> - double_down_write_data_sem(orig_inode, donor_inode);
> -
> /* Get the original extent for the block "orig_off" */
> *err = get_ext_path(orig_inode, orig_off, &orig_path);
> if (*err)
> @@ -764,8 +797,6 @@ out:
> ext4_ext_invalidate_cache(orig_inode);
> ext4_ext_invalidate_cache(donor_inode);
>
> - double_up_write_data_sem(orig_inode, donor_inode);
> -
> return replaced_count;
> }
>
> @@ -807,10 +838,6 @@ move_extent_per_page(struct file *o_filp, struct inode *donor_inode,
> int replaced_count = 0;
> int blocks_per_page = PAGE_CACHE_SIZE >> orig_inode->i_blkbits;
>
> - /*
> - * It needs twice the amount of ordinary journal buffers because
> - * inode and donor_inode may change each different metadata blocks.
> - */
> jblocks = ext4_writepage_trans_blocks(orig_inode) * 2;
>
> if (segment_eq(get_fs(), KERNEL_DS))
> @@ -819,6 +846,55 @@ move_extent_per_page(struct file *o_filp, struct inode *donor_inode,
> orig_blk_offset = orig_page_offset * blocks_per_page +
> data_offset_in_page;
>
> + /*
> + * If orig extent was uninitialized, but it can become initialized at any
> + * time after i_data_sem was dropped, in order to serialize with delalloc
> + * we have recheck extent while we hold page's lock, if it is still the
> + * case data copy is not necessery, just swap data blocks between orig
> + * and donor.
> + */
> + if (uninit) {
> + handle = ext4_journal_start(orig_inode, jblocks);
> + if (!handle) {
> + *err = -ENOMEM;
> + return 0;
> + }
It would be better we use IS_ERR(handle)
because ext4_journal_start() can return various error values.
> + page = find_or_create_page(mapping,orig_page_offset, GFP_NOFS);
You might want to put a white space after "mapping,".
> + if (!page) {
> + ext4_journal_stop(handle);
> + *err = -ENOMEM;
> + return 0;
> + }
> + double_down_write_data_sem(orig_inode, donor_inode);
> + /* If any of extents in range became initialized we have to
> + * fallback to data copying */
> + if (!mext_check_range_coverage(orig_inode, orig_blk_offset,
> + block_len_in_page, 1, err) ||
> + !mext_check_range_coverage(donor_inode, orig_blk_offset,
> + block_len_in_page, 1, &err2)) {
> + double_up_write_data_sem(orig_inode, donor_inode);
> + unlock_page(page);
> + page_cache_release(page);
> + ext4_journal_stop(handle);
> + if (*err || err2)
> + goto out;
> + goto data_copy;
> + }
> + if (!try_to_release_page(page, 0))
> + goto drop_data_sem;
> +
> + if (!PageUptodate(page)) {
> + zero_user(page, 0, PAGE_SIZE);
> + SetPageUptodate(page);
> + }
> + replaced_count = mext_replace_branches(handle, orig_inode,
> + donor_inode, orig_blk_offset,
> + block_len_in_page, err);
> + drop_data_sem:
> + double_up_write_data_sem(orig_inode, donor_inode);
> + goto drop_page;
> + }
> +data_copy:
> offs = (long long)orig_blk_offset << orig_inode->i_blkbits;
>
> /* Calculate data_size */
> @@ -884,9 +960,11 @@ move_extent_per_page(struct file *o_filp, struct inode *donor_inode,
> if (ext4_journal_extend(handle, jblocks) < 0)
> goto write_end;
>
> + double_down_write_data_sem(orig_inode, donor_inode);
> replaced_count = mext_replace_branches(handle, orig_inode, donor_inode,
> orig_blk_offset, block_len_in_page,
> &err2);
> + double_up_write_data_sem(orig_inode, donor_inode);
> if (err2) {
> if (replaced_count) {
> block_len_in_page = replaced_count;
>
Regards,
Akira Fujita
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