From: Amir Goldstein <amir73il@xxxxxxxxxxxx>
On active snapshot page read, the function ext4_snapshot_get_block()
is called to map the page to a disk block. If the page is not mapped
in the snapshot file a direct mapping to the block device is returned.
Signed-off-by: Amir Goldstein <amir73il@xxxxxxxxxxxx>
Signed-off-by: Yongqiang Yang <xiaoqiangnk@xxxxxxxxx>
---
fs/ext4/inode.c | 3 +-
fs/ext4/snapshot_inode.c | 244 ++++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 245 insertions(+), 2 deletions(-)
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 0468ef2..f44f7d3 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -4158,8 +4158,7 @@ static int ext4_no_writepage(struct page *page,
* the snapshot COW bitmaps and a few initial blocks copied on snapshot_take().
*/
static const struct address_space_operations ext4_snapfile_aops = {
- .readpage = ext4_readpage,
- .readpages = ext4_readpages,
+ .readpage = ext4_snapshot_readpage,
.writepage = ext4_no_writepage,
.bmap = ext4_bmap,
.invalidatepage = ext4_invalidatepage,
diff --git a/fs/ext4/snapshot_inode.c b/fs/ext4/snapshot_inode.c
index 2de017a..74b455d 100644
--- a/fs/ext4/snapshot_inode.c
+++ b/fs/ext4/snapshot_inode.c
@@ -38,5 +38,249 @@
#include <trace/events/ext4.h>
#include "snapshot.h"
+/*
+ * ext4_snapshot_get_block_access() - called from ext4_snapshot_read_through()
+ * on snapshot file access.
+ * return value <0 indicates access not granted
+ * return value 0 indicates snapshot inode read through access
+ * in which case 'prev_snapshot' is pointed to the previous snapshot
+ * on the list or set to NULL to indicate read through to block device.
+ */
+static int ext4_snapshot_get_block_access(struct inode *inode,
+ struct inode **prev_snapshot)
+{
+ struct ext4_inode_info *ei = EXT4_I(inode);
+ unsigned long flags = ext4_get_snapstate_flags(inode);
+
+
+ *prev_snapshot = NULL;
+ if (ext4_snapshot_is_active(inode) ||
+ (flags & 1UL<<EXT4_SNAPSTATE_ACTIVE))
+ /* read through from active snapshot to block device */
+ return 0;
+
+ return -EPERM;
+}
+
+#ifdef CONFIG_EXT4_DEBUG
+/*
+ * ext4_snapshot_get_blockdev_access - get read through access to block device.
+ * Sanity test to verify that the read block is allocated and not excluded.
+ * This test has performance penalty and is only called if SNAPTEST_READ
+ * is enabled. An attempt to read through to block device of a non allocated
+ * or excluded block may indicate a corrupted filesystem, corrupted snapshot
+ * or corrupted exclude bitmap. However, it may also be a read-ahead, which
+ * was not implicitly requested by the user, so be sure to disable read-ahead
+ * on block device (blockdev --setra 0 <bdev>) before enabling SNAPTEST_READ.
+ *
+ * Return values:
+ * = 0 - block is allocated and not excluded
+ * < 0 - error (or block is not allocated or excluded)
+ */
+static int ext4_snapshot_get_blockdev_access(struct super_block *sb,
+ struct buffer_head *bh)
+{
+ unsigned long block_group = SNAPSHOT_BLOCK_GROUP(bh->b_blocknr);
+ ext4_grpblk_t bit = SNAPSHOT_BLOCK_GROUP_OFFSET(bh->b_blocknr);
+ struct buffer_head *bitmap_bh;
+ int err = 0;
+
+ if (PageReadahead(bh->b_page))
+ return 0;
+
+ bitmap_bh = ext4_read_block_bitmap(sb, block_group);
+ if (!bitmap_bh)
+ return -EIO;
+
+ if (!ext4_test_bit(bit, bitmap_bh->b_data)) {
+ snapshot_debug(2, "warning: attempt to read through to "
+ "non-allocated block [%d/%lu] - read ahead?\n",
+ bit, block_group);
+ brelse(bitmap_bh);
+ return -EIO;
+ }
+
+ brelse(bitmap_bh);
+ return err;
+}
+#endif
+
+/*
+ * ext4_snapshot_read_through - get snapshot image block.
+ * On read of snapshot file, an unmapped block is a peephole to prev snapshot.
+ * On read of active snapshot, an unmapped block is a peephole to the block
+ * device. On first block write, the peephole is filled forever.
+ */
+static int ext4_snapshot_read_through(struct inode *inode, sector_t iblock,
+ struct buffer_head *bh_result)
+{
+ int err;
+ struct ext4_map_blocks map;
+ struct inode *prev_snapshot;
+
+ map.m_lblk = iblock;
+ map.m_pblk = 0;
+ map.m_len = bh_result->b_size >> inode->i_blkbits;
+
+ prev_snapshot = NULL;
+ /* request snapshot file read access */
+ err = ext4_snapshot_get_block_access(inode, &prev_snapshot);
+ if (err < 0)
+ return err;
+ err = ext4_map_blocks(NULL, inode, &map, 0);
+ snapshot_debug(4, "ext4_snapshot_read_through(%lld): block = "
+ "(%lld), err = %d\n prev_snapshot = %u",
+ (long long)iblock, map.m_pblk, err,
+ prev_snapshot ? prev_snapshot->i_generation : 0);
+ if (err < 0)
+ return err;
+ if (!err)
+ /* hole in active snapshot - read though to block device */
+ return 0;
+
+ map_bh(bh_result, inode->i_sb, map.m_pblk);
+ bh_result->b_state = (bh_result->b_state & ~EXT4_MAP_FLAGS) |
+ map.m_flags;
+
+ return 0;
+}
+
+/*
+ * Check if @block is a bitmap block of @group.
+ * if @block is found to be a block/inode/exclude bitmap block, the return
+ * value is one of the non-zero values below:
+ */
+#define BLOCK_BITMAP 1
+#define INODE_BITMAP 2
+#define EXCLUDE_BITMAP 3
+
+static int ext4_snapshot_is_group_bitmap(struct super_block *sb,
+ ext4_fsblk_t block, ext4_group_t group)
+{
+ struct ext4_group_desc *gdp;
+ struct ext4_group_info *grp;
+ ext4_fsblk_t bitmap_blk;
+
+ gdp = ext4_get_group_desc(sb, group, NULL);
+ grp = ext4_get_group_info(sb, group);
+ if (!gdp || !grp)
+ return 0;
+
+ bitmap_blk = ext4_block_bitmap(sb, gdp);
+ if (bitmap_blk == block)
+ return BLOCK_BITMAP;
+ bitmap_blk = ext4_inode_bitmap(sb, gdp);
+ if (bitmap_blk == block)
+ return INODE_BITMAP;
+ bitmap_blk = ext4_exclude_bitmap(sb, gdp);
+ if (bitmap_blk == block)
+ return EXCLUDE_BITMAP;
+ return 0;
+}
+
+/*
+ * Check if @block is a bitmap block and of any block group.
+ * if @block is found to be a bitmap block, @bitmap_group is set to the
+ * block group described by the bitmap block.
+ */
+static int ext4_snapshot_is_bitmap(struct super_block *sb,
+ ext4_fsblk_t block, ext4_group_t *bitmap_group)
+{
+ ext4_group_t group = SNAPSHOT_BLOCK_GROUP(block);
+ ext4_group_t ngroups = ext4_get_groups_count(sb);
+ int flex_groups = ext4_flex_bg_size(EXT4_SB(sb));
+ int i, is_bitmap = 0;
+
+ /*
+ * When block is in the first block group of a flex group, we need to
+ * check all group desc of the flex group.
+ * The exclude bitmap can potentially be allocated from any group, if
+ * exclude inode was added not on mkfs. The worst case if we fail to
+ * identify a block as an exclude bitmap is that fsck sanity check of
+ * snapshots will fail, becasue exclude bitmap inside snapshot will
+ * not be all zeros.
+ */
+ if (!EXT4_HAS_INCOMPAT_FEATURE(sb, EXT4_FEATURE_INCOMPAT_FLEX_BG) ||
+ (group % flex_groups) != 0)
+ flex_groups = 1;
+
+ for (i = 0; i < flex_groups && group < ngroups; i++, group++) {
+ is_bitmap = ext4_snapshot_is_group_bitmap(sb, block, group);
+ if (is_bitmap)
+ break;
+ }
+ *bitmap_group = group;
+ return is_bitmap;
+}
+
+static int ext4_snapshot_get_block(struct inode *inode, sector_t iblock,
+ struct buffer_head *bh_result, int create)
+{
+ ext4_group_t block_group;
+ int err, is_bitmap;
+
+ BUG_ON(create != 0);
+ BUG_ON(buffer_tracked_read(bh_result));
+
+ err = ext4_snapshot_read_through(inode, SNAPSHOT_IBLOCK(iblock),
+ bh_result);
+ snapshot_debug(4, "ext4_snapshot_get_block(%lld): block = (%lld), "
+ "err = %d\n",
+ (long long)iblock, buffer_mapped(bh_result) ?
+ (long long)bh_result->b_blocknr : 0, err);
+ if (err < 0)
+ return err;
+
+ if (!buffer_tracked_read(bh_result))
+ return 0;
+
+ /* Check for read through to block or exclude bitmap block */
+ is_bitmap = ext4_snapshot_is_bitmap(inode->i_sb, bh_result->b_blocknr,
+ &block_group);
+ if (is_bitmap == BLOCK_BITMAP) {
+ /* copy fixed block bitmap directly to page buffer */
+ cancel_buffer_tracked_read(bh_result);
+ /* cancel_buffer_tracked_read() clears mapped flag */
+ set_buffer_mapped(bh_result);
+ snapshot_debug(2, "fixing snapshot block bitmap #%u\n",
+ block_group);
+ /*
+ * XXX: if we return unmapped buffer, the page will be zeroed
+ * but if we return mapped to block device and uptodate buffer
+ * next readpage may read directly from block device without
+ * fixing block bitmap. This only affects fsck of snapshots.
+ */
+ return ext4_snapshot_read_block_bitmap(inode->i_sb,
+ block_group, bh_result);
+ } else if (is_bitmap == EXCLUDE_BITMAP) {
+ /* return unmapped buffer to zero out page */
+ cancel_buffer_tracked_read(bh_result);
+ /* cancel_buffer_tracked_read() clears mapped flag */
+ snapshot_debug(2, "zeroing snapshot exclude bitmap #%u\n",
+ block_group);
+ return 0;
+ }
+
#ifdef CONFIG_EXT4_DEBUG
+ snapshot_debug(3, "started tracked read: block = [%llu/%llu]\n",
+ SNAPSHOT_BLOCK_TUPLE(bh_result->b_blocknr));
+ if (snapshot_enable_test[SNAPTEST_READ]) {
+ err = ext4_snapshot_get_blockdev_access(inode->i_sb,
+ bh_result);
+ if (err) {
+ /* read through access denied */
+ cancel_buffer_tracked_read(bh_result);
+ return err;
+ }
+ /* sleep 1 tunable delay unit */
+ snapshot_test_delay(SNAPTEST_READ);
+ }
#endif
+ return 0;
+}
+
+int ext4_snapshot_readpage(struct file *file, struct page *page)
+{
+ /* do read I/O with buffer heads to enable tracked reads */
+ return ext4_read_full_page(page, ext4_snapshot_get_block);
+}
--
1.7.4.1
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