ext4: introduce lseek SEEK_DATA/SEEK_HOLE support

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: Hugh Dickins <hughd@google.com>
Signed-off-by: Jie Liu <jeff.liu@oracle.com>
Signed-off-by: Zheng Liu <wenqing.lz@taobao.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

1 files changed, 332 insertions, 2 deletions

diff --git a/fs/ext4/file.c b/fs/ext4/file.c
index bf3966bccd34..2f5759eb9f89 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 = {