Re: [PATCH 8/8 v3] ext4: introduce lseek SEEK_DATA/SEEK_HOLE support

[Jeff Liu <jeff.liu@oracle.com>]

Hi Zheng,

On 10/26/2012 09:23 PM, Zheng Liu wrote:
> From: Zheng Liu <wenqing.lz@taobao.com>
>
> This patch makes ext4 really support SEEK_DATA/SEEK_HOLE flags.  Block-mapped
> and extent-mapped files are fully implemented together because ext4_map_blocks
> hides this differences.
>
> After applying this patch, it will cause a failure in xfstest #285 when the file
> is block-mapped due to block-mapped file isn't support fallocate(2).
>
> I had tried to use ext4_ext_walk_space() to retrieve the offset for a
> extent-mapped file.  But finally I decide to keep using ext4_map_blocks() to
> support SEEK_DATA/SEEK_HOLE because ext4_map_blocks() can hide the difference
> between block-mapped file and extent-mapped file.  Moreover, in next step,
> extent status tree will track all extent status, and we can get all mappings
> from this tree.  So I think that using ext4_map_blocks() is a better choice.
>
> CC: "Theodore Ts'o" <tytso@mit.edu>
> CC: Hugh Dickins <hughd@google.com>
> Signed-off-by: Jie Liu <jeff.liu@oracle.com>
> Signed-off-by: Zheng Liu <wenqing.lz@taobao.com>
> ---
> v3 <- v2:
>   - determine an unwritten extent as a data/hole according to the whether there
>     has some data in page cache or not.
>   - remove ext4_seek_data_hole(), and call ext4_seek_data/hole() directly.
>   - fix some bugs
>
> Please apply this patch before testing this patch using xfstest #285 due to
> there is a bug in xfstest (http://patchwork.xfs.org/patch/4276/).
>
> The function ext4_find_unwritten_pgoff() is inspired by Jeff's work for xfs
> (d126d43f631f996daeee5006714fed914be32368).  Thus, I add a Signed-off-by for
> his crediting.
>
> Jeff, I really appreicate if you could allow me to add Signed-off-by in this
> patch.  Please let me know if you think it is not well.  Thanks!
I'm fine. :)
Finally, maybe I'll try to tweak that routine with page cache lookup for 
unwritten extents to be a
VFS helper so that Btrfs can make use of it as well, thank you!

-Jeff
>
>   fs/ext4/file.c | 334 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
>   1 file changed, 332 insertions(+), 2 deletions(-)
>
> diff --git a/fs/ext4/file.c b/fs/ext4/file.c
> index bf3966b..2f5759e 100644
> --- a/fs/ext4/file.c
> +++ b/fs/ext4/file.c
> @@ -24,6 +24,7 @@
>   #include <linux/mount.h>
>   #include <linux/path.h>
>   #include <linux/quotaops.h>
> +#include <linux/pagevec.h>
>   #include "ext4.h"
>   #include "ext4_jbd2.h"
>   #include "xattr.h"
> @@ -286,6 +287,324 @@ static int ext4_file_open(struct inode * inode, struct file * filp)
>   }
>   
>   /*
> + * Here we use ext4_map_blocks() to get a block mapping for a extent-based
> + * file rather than ext4_ext_walk_space() because we can introduce
> + * SEEK_DATA/SEEK_HOLE for block-mapped and extent-mapped file at the same
> + * function.  When extent status tree has been fully implemented, it will
> + * track all extent status for a file and we can directly use it to
> + * retrieve the offset for SEEK_DATA/SEEK_HOLE.
> + */
> +
> +/*
> + * When we retrieve the offset for SEEK_DATA/SEEK_HOLE, we would need to
> + * lookup page cache to check whether or not there has some data between
> + * [startoff, endoff] because, if this range contains an unwritten extent,
> + * we determine this extent as a data or a hole according to whether the
> + * page cache has data or not.
> + */
> +static int ext4_find_unwritten_pgoff(struct inode *inode,
> +				     int origin,
> +				     struct ext4_map_blocks *map,
> +				     loff_t *offset)
> +{
> +	struct pagevec pvec;
> +	unsigned int blkbits;
> +	pgoff_t index;
> +	pgoff_t end;
> +	loff_t endoff;
> +	loff_t startoff;
> +	loff_t lastoff;
> +	int found = 0;
> +
> +	blkbits = inode->i_sb->s_blocksize_bits;
> +	startoff = *offset;
> +	lastoff = startoff;
> +	endoff = (map->m_lblk + map->m_len) << blkbits;
> +
> +	index = startoff >> PAGE_CACHE_SHIFT;
> +	end = endoff >> PAGE_CACHE_SHIFT;
> +
> +	pagevec_init(&pvec, 0);
> +	do {
> +		int i, num;
> +		unsigned long nr_pages;
> +
> +		num = min_t(pgoff_t, end - index, PAGEVEC_SIZE);
> +		nr_pages = pagevec_lookup(&pvec, inode->i_mapping, index,
> +					  (pgoff_t)num);
> +		if (nr_pages == 0) {
> +			if (origin == SEEK_DATA)
> +				break;
> +
> +			BUG_ON(origin != SEEK_HOLE);
> +			/*
> +			 * If this is the first time to go into the loop and
> +			 * offset is not beyond the end offset, it will be a
> +			 * hole at this offset
> +			 */
> +			if (lastoff == startoff || lastoff < endoff)
> +				found = 1;
> +			break;
> +		}
> +
> +		/*
> +		 * If this is the first time to go into the loop and
> +		 * offset is smaller than the first page offset, it will be a
> +		 * hole at this offset.
> +		 */
> +		if (lastoff == startoff && origin == SEEK_HOLE &&
> +		    lastoff < page_offset(pvec.pages[0])) {
> +			found = 1;
> +			break;
> +		}
> +
> +		for (i = 0; i < nr_pages; i++) {
> +			struct page *page = pvec.pages[i];
> +			struct buffer_head *bh, *head;
> +
> +			/*
> +			 * If the current offset is not beyond the end of given
> +			 * range, it will be a hole.
> +			 */
> +			if (lastoff < endoff && origin == SEEK_HOLE &&
> +			    page->index > end) {
> +				found = 1;
> +				*offset = lastoff;
> +				goto out;
> +			}
> +
> +			lock_page(page);
> +
> +			if (unlikely(page->mapping != inode->i_mapping)) {
> +				unlock_page(page);
> +				continue;
> +			}
> +
> +			if (!page_has_buffers(page)) {
> +				unlock_page(page);
> +				continue;
> +			}
> +
> +			if (page_has_buffers(page)) {
> +				lastoff = page_offset(page);
> +				bh = head = page_buffers(page);
> +				do {
> +					if (buffer_uptodate(bh) ||
> +					    buffer_unwritten(bh)) {
> +						if (origin == SEEK_DATA)
> +							found = 1;
> +					} else {
> +						if (origin == SEEK_HOLE)
> +							found = 1;
> +					}
> +					if (found) {
> +						*offset = max_t(loff_t,
> +							startoff, lastoff);
> +						unlock_page(page);
> +						goto out;
> +					}
> +					lastoff += bh->b_size;
> +					bh = bh->b_this_page;
> +				} while (bh != head);
> +			}
> +
> +			lastoff = page_offset(page) + PAGE_SIZE;
> +			unlock_page(page);
> +		}
> +
> +		/*
> +		 * The no. of pages is less than our desired, that would be a
> +		 * hole in there.
> +		 */
> +		if (nr_pages < num && origin == SEEK_HOLE) {
> +			found = 1;
> +			*offset = lastoff;
> +			break;
> +		}
> +
> +		index = pvec.pages[i - 1]->index + 1;
> +		pagevec_release(&pvec);
> +	} while (index <= end);
> +
> +out:
> +	pagevec_release(&pvec);
> +	return found;
> +}
> +
> +/*
> + * ext4_seek_data() retrieves the offset for SEEK_DATA.
> + */
> +static loff_t ext4_seek_data(struct file *file, loff_t offset, loff_t maxsize)
> +{
> +	struct inode *inode = file->f_mapping->host;
> +	struct ext4_map_blocks map;
> +	struct extent_status es;
> +	ext4_lblk_t start, last, end;
> +	loff_t dataoff, isize;
> +	int blkbits;
> +	int ret = 0;
> +
> +	mutex_lock(&inode->i_mutex);
> +
> +	isize = i_size_read(inode);
> +	if (offset >= isize) {
> +		mutex_unlock(&inode->i_mutex);
> +		return -ENXIO;
> +	}
> +
> +	blkbits = inode->i_sb->s_blocksize_bits;
> +	start = offset >> blkbits;
> +	last = start;
> +	end = isize >> blkbits;
> +	dataoff = offset;
> +
> +	do {
> +		map.m_lblk = last;
> +		map.m_len = end - last + 1;
> +		ret = ext4_map_blocks(NULL, inode, &map, 0);
> +		if (ret > 0 && !(map.m_flags & EXT4_MAP_UNWRITTEN)) {
> +			if (last != start)
> +				dataoff = last << blkbits;
> +			break;
> +		}
> +
> +		/*
> +		 * If there is a delay extent at this offset,
> +		 * it will be as a data.
> +		 */
> +		es.start = last;
> +		(void)ext4_es_find_extent(inode, &es);
> +		if (last >= es.start &&
> +		    last < es.start + es.len) {
> +			if (last != start)
> +				dataoff = last << blkbits;
> +			break;
> +		}
> +
> +		/*
> +		 * If there is a unwritten extent at this offset,
> +		 * it will be as a data or a hole according to page
> +		 * cache that has data or not.
> +		 */
> +		if (map.m_flags & EXT4_MAP_UNWRITTEN) {
> +			int unwritten;
> +			unwritten = ext4_find_unwritten_pgoff(inode, SEEK_DATA,
> +							      &map, &dataoff);
> +			if (unwritten)
> +				break;
> +		}
> +
> +		last++;
> +		dataoff = last << blkbits;
> +	} while (last <= end);
> +
> +	mutex_unlock(&inode->i_mutex);
> +
> +	if (dataoff > isize)
> +		return -ENXIO;
> +
> +	if (dataoff < 0 && !(file->f_mode & FMODE_UNSIGNED_OFFSET))
> +		return -EINVAL;
> +	if (dataoff > maxsize)
> +		return -EINVAL;
> +
> +	if (dataoff != file->f_pos) {
> +		file->f_pos = dataoff;
> +		file->f_version = 0;
> +	}
> +
> +	return dataoff;
> +}
> +
> +/*
> + * ext4_seek_hole() retrieves the offset for SEEK_HOLE.
> + */
> +static loff_t ext4_seek_hole(struct file *file, loff_t offset, loff_t maxsize)
> +{
> +	struct inode *inode = file->f_mapping->host;
> +	struct ext4_map_blocks map;
> +	struct extent_status es;
> +	ext4_lblk_t start, last, end;
> +	loff_t holeoff, isize;
> +	int blkbits;
> +	int ret = 0;
> +
> +	mutex_lock(&inode->i_mutex);
> +
> +	isize = i_size_read(inode);
> +	if (offset >= isize) {
> +		mutex_unlock(&inode->i_mutex);
> +		return -ENXIO;
> +	}
> +
> +	blkbits = inode->i_sb->s_blocksize_bits;
> +	start = offset >> blkbits;
> +	last = start;
> +	end = isize >> blkbits;
> +	holeoff = offset;
> +
> +	do {
> +		map.m_lblk = last;
> +		map.m_len = end - last + 1;
> +		ret = ext4_map_blocks(NULL, inode, &map, 0);
> +		if (ret > 0 && !(map.m_flags & EXT4_MAP_UNWRITTEN)) {
> +			last += ret;
> +			holeoff = last << blkbits;
> +			continue;
> +		}
> +
> +		/*
> +		 * If there is a delay extent at this offset,
> +		 * we will skip this extent.
> +		 */
> +		es.start = last;
> +		(void)ext4_es_find_extent(inode, &es);
> +		if (last >= es.start &&
> +		    last < es.start + es.len) {
> +			last = es.start + es.len;
> +			holeoff = last << blkbits;
> +			continue;
> +		}
> +
> +		/*
> +		 * If there is a unwritten extent at this offset,
> +		 * it will be as a data or a hole according to page
> +		 * cache that has data or not.
> +		 */
> +		if (map.m_flags & EXT4_MAP_UNWRITTEN) {
> +			int unwritten;
> +			unwritten = ext4_find_unwritten_pgoff(inode, SEEK_HOLE,
> +							      &map, &holeoff);
> +			if (!unwritten) {
> +				last += ret;
> +				holeoff = last << blkbits;
> +				continue;
> +			}
> +		}
> +
> +		/* find a hole */
> +		break;
> +	} while (last <= end);
> +
> +	mutex_unlock(&inode->i_mutex);
> +
> +	if (holeoff > isize)
> +		holeoff = isize;
> +
> +	if (holeoff < 0 && !(file->f_mode & FMODE_UNSIGNED_OFFSET))
> +		return -EINVAL;
> +	if (holeoff > maxsize)
> +		return -EINVAL;
> +
> +	if (holeoff != file->f_pos) {
> +		file->f_pos = holeoff;
> +		file->f_version = 0;
> +	}
> +
> +	return holeoff;
> +}
> +
> +/*
>    * ext4_llseek() handles both block-mapped and extent-mapped maxbytes values
>    * by calling generic_file_llseek_size() with the appropriate maxbytes
>    * value for each.
> @@ -300,8 +619,19 @@ loff_t ext4_llseek(struct file *file, loff_t offset, int origin)
>   	else
>   		maxbytes = inode->i_sb->s_maxbytes;
>   
> -	return generic_file_llseek_size(file, offset, origin,
> -					maxbytes, i_size_read(inode));
> +	switch (origin) {
> +	case SEEK_SET:
> +	case SEEK_CUR:
> +	case SEEK_END:
> +		return generic_file_llseek_size(file, offset, origin,
> +						maxbytes, i_size_read(inode));
> +	case SEEK_DATA:
> +		return ext4_seek_data(file, offset, maxbytes);
> +	case SEEK_HOLE:
> +		return ext4_seek_hole(file, offset, maxbytes);
> +	}
> +
> +	return -EINVAL;
>   }
>   
>   const struct file_operations ext4_file_operations = {