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Date: Tue, 27 Sep 2011 09:32:37 +0800
From: Yongqiang Yang <xiaoqiangnk@...il.com>
To: jeff.liu@...cle.com
Cc: linux-ext4@...r.kernel.org
Subject: Re: [PATCH] Add SEEK_DATA/SEEK_HOLE support
On Mon, Sep 26, 2011 at 9:50 PM, Jeff Liu <jeff.liu@...cle.com> wrote:
> Dear developer,
>
> Has anyone started working on SEEK_DATA/SEEK_HOLE yet?
> If not, I'd like to submit a patch for it, or please just ignore my noise. :-P.
>
> The mainly idea of how to retrieve the delayed extent info are copied from ext4_ext_fiemap_cb().
> And also, IMHO, for block based file, return -ENOTSUPP is ok?
>
> I have done the following tests, all works for me.
> 1. file start with hole.
> 2. file end with hole.
> 3. file with about 10001 extents on my test env which was created through:
> python -c "f=open('./sptest', 'w'); [(f.seek(x) or f.write(str(x))) for x in range(1, 1000000000, 99999)]; f.close()
>
> Any constructive criticism/comments are welcome, thank you!
>
> Signed-off-by: Jie Liu <jeff.liu@...cle.com>
>
> ---
> fs/ext4/ext4.h | 4 +
> fs/ext4/ext4_extents.h | 8 ++
> fs/ext4/extents.c | 234 ++++++++++++++++++++++++++++++++++++++++++++++++
> fs/ext4/file.c | 14 ++--
> 4 files changed, 253 insertions(+), 7 deletions(-)
>
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index e717dfd..05b9a3a 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -2222,6 +2222,10 @@ extern int ext4_move_extents(struct file *o_filp, struct file *d_filp,
> __u64 start_orig, __u64 start_donor,
> __u64 len, __u64 *moved_len);
>
> +/* extent.c */
> +extern int ext4_find_desired_extent(struct inode *inode, loff_t *offset,
> + int origin);
> +
> /* page-io.c */
> extern int __init ext4_init_pageio(void);
> extern void ext4_exit_pageio(void);
> diff --git a/fs/ext4/ext4_extents.h b/fs/ext4/ext4_extents.h
> index 095c36f..ca7a339 100644
> --- a/fs/ext4/ext4_extents.h
> +++ b/fs/ext4/ext4_extents.h
> @@ -116,6 +116,14 @@ struct ext4_ext_path {
> };
>
> /*
> + * Structure for retrieving extent offset for SEEK_DATA/SEEK_HOLE.
> + */
> +struct ext4_extent_seek_info {
> + unsigned int origin; /* SEEK_DATA or SEEK_HOLE */
> + loff_t offset; /* input/output offset */
> +};
> +
> +/*
> * structure for external API
> */
>
> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> index 57cf568..a339e00 100644
> --- a/fs/ext4/extents.c
> +++ b/fs/ext4/extents.c
> @@ -3901,6 +3901,240 @@ int ext4_convert_unwritten_extents(struct inode *inode, loff_t offset,
> }
>
> /*
> + * Callback function called for each extent to gather SEEK_DATA/SEEK_HOLE
> + * information.
> + */
> +static int ext4_ext_seek_cb(struct inode *inode, ext4_lblk_t next,
> + struct ext4_ext_cache *newex,
> + struct ext4_extent *ex,
> + void *data)
Hi,
We'd better implement SEEK_DATA and SEEK_HOLE based on FIEMAP, thus,
code will be much maintainable. You can have a look at btrfs's
implementation.
There is another important problem here. This function operates on
pages heavily to collect delayed extents, as Jan Kara pointed out we
should lock a page before we use the page. Now the problem comes,
the function is called with i_mutex_lock held, so we can not lock
page, otherwise, deadlock is easy to happen. To address the problem,
we have two solutions: 1. doing as filemap, releasing i_mutex_block
before calling this function, 2. collecting delayed extent info in an
in-memory extent tree, I am working on it. The question regarding the
2nd solution is if it is worth adding an in-memory extent tree.
bigalloc has similar problem with delayed extent.
You can have a look at fiemap using delayed extent tree via
http://patchwork.ozlabs.org/patch/112233/. It is very simple.
Thx!
Yongqiang.
> +{
> + __u64 logical;
> + __u32 flags = 0;
> + int ret = 0;
> + unsigned char blksize_bits;
> + struct ext4_extent_seek_info *ext_seek_info = data;
> +
> + blksize_bits = inode->i_sb->s_blocksize_bits;
> + logical = (__u64)newex->ec_block << blksize_bits;
> +
> + /*
> + * No extent in extent-tree contains block @newex->ec_start,
> + * then the block may stay in 1)a hole or 2)delayed-extent.
> + *
> + * Holes or delayed-extents are processed as follows.
> + * 1. lookup dirty pages with specified range in pagecache.
> + * If no page is got, then there is no delayed-extent and
> + * return with EXT_CONTINUE if SEEK_DATA; return EXT_BREAK
> + * with extent offset if SEEK_HOLE.
> + * 2. find the 1st mapped buffer,
> + * 3. check if the mapped buffer is both in the request range
> + * and a delayed buffer. If not, there is no delayed-extent,
> + * then return with EXT_CONTINUE if SEEK_DATA, or return
> + * EXT_BREAK if SEEK_HOLE and extent offset.
> + * 4. a delayed-extent is found, return the extent offset if
> + * SEEK_DATA and EXT_BREAK.
> + */
> + if (newex->ec_start == 0) {
> + ext4_lblk_t end = 0;
> + pgoff_t last_offset;
> + pgoff_t offset;
> + pgoff_t index;
> + pgoff_t start_index = 0;
> + struct page **pages = NULL;
> + struct buffer_head *bh = NULL;
> + struct buffer_head *head = NULL;
> + unsigned int nr_pages = PAGE_SIZE / sizeof(struct page *);
> +
> + pages = kmalloc(PAGE_SIZE, GFP_KERNEL);
> + if (pages == NULL)
> + return -ENOMEM;
> +
> + offset = logical >> PAGE_SHIFT;
> +repeat:
> + last_offset = offset;
> + head = NULL;
> +
> + ret = find_get_pages_tag(inode->i_mapping, &offset,
> + PAGECACHE_TAG_DIRTY, nr_pages, pages);
> +
> + /* Not a delay allocated extent. */
> + if (!(flags & FIEMAP_EXTENT_DELALLOC)) {
> + /* First time, try to find a mapped buffer. */
> + if (ret == 0) {
> +out:
> + for (index = 0; index < ret; index++)
> + page_cache_release(pages[index]);
> + /* just a hole. */
> + kfree(pages);
> +
> + /* If SEEK_HOLE, return the logical offset. */
> + if (ext_seek_info->origin == SEEK_HOLE) {
> + ext_seek_info->offset = logical;
> + return EXT_BREAK;
> + }
> +
> + return EXT_CONTINUE;
> + }
> +
> + index = 0;
> +next_page:
> + /* Try to find the 1st mapped buffer. */
> + end = ((__u64)pages[index]->index << PAGE_SHIFT) >>
> + blksize_bits;
> + if (!page_has_buffers(pages[index]))
> + goto out;
> + head = page_buffers(pages[index]);
> + if (!head)
> + goto out;
> +
> + index++;
> + bh = head;
> + do {
> + /* The buffer is out of the request range. */
> + if (end >= newex->ec_block + newex->ec_len)
> + goto out;
> +
> + if (buffer_mapped(bh) &&
> + end >= newex->ec_block) {
> + start_index = index - 1;
> + /* get the 1st mapped buffer. */
> + goto found_mapped_buffer;
> + }
> +
> + bh = bh->b_this_page;
> + end++;
> + } while (bh != head);
> +
> + /* No mapped buffer in the range found in this page,
> + * We need to look up next page.
> + */
> + if (index >= ret) {
> + /* There is no page left, but we need to limit
> + * newex->ec_len.
> + */
> + newex->ec_len = end - newex->ec_block;
> + goto out;
> + }
> + goto next_page;
> + } else {
> + /*Find contiguous delayed buffers. */
> + if (ret > 0 && pages[0]->index == last_offset)
> + head = page_buffers(pages[0]);
> + bh = head;
> + index = 1;
> + start_index = 0;
> + }
> +
> +found_mapped_buffer:
> + if (bh != NULL && buffer_delay(bh)) {
> + /* 1st or contiguous delayed buffer found. */
> + if (!(flags & FIEMAP_EXTENT_DELALLOC)) {
> + /*
> + * 1st delayed buffer found, record
> + * the start of extent.
> + */
> + flags |= FIEMAP_EXTENT_DELALLOC;
> + newex->ec_block = end;
> + logical = (__u64)end << blksize_bits;
> + }
> + /* Find contiguous delayed buffers. */
> + do {
> + if (!buffer_delay(bh))
> + goto found_delayed_extent;
> + bh = bh->b_this_page;
> + end++;
> + } while (bh != head);
> +
> + for (; index < ret; index++) {
> + if (!page_has_buffers(pages[index])) {
> + bh = NULL;
> + break;
> + }
> + head = page_buffers(pages[index]);
> + if (!head) {
> + bh = NULL;
> + break;
> + }
> +
> + if (pages[index]->index !=
> + pages[start_index]->index + index
> + - start_index) {
> + /* Blocks are not contiguous. */
> + bh = NULL;
> + break;
> + }
> + bh = head;
> + do {
> + if (!buffer_delay(bh))
> + /* Delayed-extent ends. */
> + goto found_delayed_extent;
> + bh = bh->b_this_page;
> + end++;
> + } while (bh != head);
> + }
> + } else if (!(flags & FIEMAP_EXTENT_DELALLOC))
> + /* a hole found. */
> + goto out;
> +
> +found_delayed_extent:
> + newex->ec_len = min(end - newex->ec_block,
> + (ext4_lblk_t)EXT_INIT_MAX_LEN);
> + if (ret == nr_pages && bh != NULL &&
> + newex->ec_len < EXT_INIT_MAX_LEN &&
> + buffer_delay(bh)) {
> + /* Have not collected an extent and continue. */
> + for (index = 0; index < ret; index++)
> + page_cache_release(pages[index]);
> + goto repeat;
> + }
> +
> + for (index = 0; index < ret; index++)
> + page_cache_release(pages[index]);
> + kfree(pages);
> + }
> +
> + if (ret < 0)
> + return ret;
> +
> + if (ext_seek_info->origin == SEEK_DATA) {
> + ext_seek_info->offset = logical;
> + return EXT_BREAK;
> + }
> +
> + return EXT_CONTINUE;
> +}
> +
> +int ext4_find_desired_extent(struct inode *inode, loff_t *offset, int origin)
> +{
> + ext4_lblk_t start_blk;
> + ext4_lblk_t end_blk;
> + ext4_lblk_t len_blks;
> + struct ext4_extent_seek_info ext_seek_info;
> + unsigned int blkbits = inode->i_sb->s_blocksize_bits;
> + int error = 0;
> +
> + /* Only support seeking extents based file. */
> + if (!(ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS)))
> + return -ENOTSUPP;
> +
> + start_blk = *offset >> blkbits;
> + if (start_blk >= EXT_MAX_BLOCKS)
> + return -ENXIO;
> +
> + end_blk = i_size_read(inode) >> blkbits;
> + len_blks = ((ext4_lblk_t)end_blk) - start_blk + 1;
> +
> + ext_seek_info.origin = origin;
> + error = ext4_ext_walk_space(inode, start_blk, len_blks,
> + ext4_ext_seek_cb, &ext_seek_info);
> + if (!error)
> + *offset = ext_seek_info.offset;
> +
> + return error;
> +}
> +
> +/*
> * Callback function called for each extent to gather FIEMAP information.
> */
> static int ext4_ext_fiemap_cb(struct inode *inode, ext4_lblk_t next,
> diff --git a/fs/ext4/file.c b/fs/ext4/file.c
> index e4095e9..48579d5 100644
> --- a/fs/ext4/file.c
> +++ b/fs/ext4/file.c
> @@ -219,6 +219,7 @@ loff_t ext4_llseek(struct file *file, loff_t offset, int origin)
> {
> struct inode *inode = file->f_mapping->host;
> loff_t maxbytes;
> + int ret;
>
> if (!(ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS)))
> maxbytes = EXT4_SB(inode->i_sb)->s_bitmap_maxbytes;
> @@ -241,21 +242,20 @@ loff_t ext4_llseek(struct file *file, loff_t offset, int origin)
> * In the generic case the entire file is data, so as long as
> * offset isn't at the end of the file then the offset is data.
> */
> - if (offset >= inode->i_size) {
> - mutex_unlock(&inode->i_mutex);
> - return -ENXIO;
> - }
> - break;
> case SEEK_HOLE:
> /*
> * There is a virtual hole at the end of the file, so as long as
> * offset isn't i_size or larger, return i_size.
> */
> - if (offset >= inode->i_size) {
> + if (offset >= i_size_read(inode)) {
> mutex_unlock(&inode->i_mutex);
> return -ENXIO;
> }
> - offset = inode->i_size;
> + ret = ext4_find_desired_extent(inode, &offset, origin);
> + if (ret) {
> + mutex_unlock(&inode->i_mutex);
> + return ret;
> + }
> break;
> }
>
> --
> 1.7.4.1
> --
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--
Best Wishes
Yongqiang Yang
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