[dm-devel] [PATCH 1/3] dm clone metadata: Track exact changes per transaction
Nikos Tsironis
ntsironis at arrikto.com
Wed Dec 4 14:06:52 UTC 2019
Extend struct dirty_map with a second bitmap which tracks the exact
regions that were hydrated during the current metadata transaction.
Moreover, fix __flush_dmap() to only commit the metadata of the regions
that were hydrated during the current transaction.
This is required by the following commits to fix a data corruption bug.
Fixes: 7431b7835f55 ("dm: add clone target")
Cc: stable at vger.kernel.org # v5.4+
Signed-off-by: Nikos Tsironis <ntsironis at arrikto.com>
---
drivers/md/dm-clone-metadata.c | 90 +++++++++++++++++++++++++++++-------------
1 file changed, 62 insertions(+), 28 deletions(-)
diff --git a/drivers/md/dm-clone-metadata.c b/drivers/md/dm-clone-metadata.c
index 08c552e5e41b..ee870a425ab8 100644
--- a/drivers/md/dm-clone-metadata.c
+++ b/drivers/md/dm-clone-metadata.c
@@ -67,23 +67,34 @@ struct superblock_disk {
* To save constantly doing look ups on disk we keep an in core copy of the
* on-disk bitmap, the region_map.
*
- * To further reduce metadata I/O overhead we use a second bitmap, the dmap
- * (dirty bitmap), which tracks the dirty words, i.e. longs, of the region_map.
+ * In order to track which regions are hydrated during a metadata transaction,
+ * we use a second set of bitmaps, the dmap (dirty bitmap), which includes two
+ * bitmaps, namely dirty_regions and dirty_words. The dirty_regions bitmap
+ * tracks the regions that got hydrated during the current metadata
+ * transaction. The dirty_words bitmap tracks the dirty words, i.e. longs, of
+ * the dirty_regions bitmap.
+ *
+ * This allows us to precisely track the regions that were hydrated during the
+ * current metadata transaction and update the metadata accordingly, when we
+ * commit the current transaction. This is important because dm-clone should
+ * only commit the metadata of regions that were properly flushed to the
+ * destination device beforehand. Otherwise, in case of a crash, we could end
+ * up with a corrupted dm-clone device.
*
* When a region finishes hydrating dm-clone calls
* dm_clone_set_region_hydrated(), or for discard requests
* dm_clone_cond_set_range(), which sets the corresponding bits in region_map
* and dmap.
*
- * During a metadata commit we scan the dmap for dirty region_map words (longs)
- * and update accordingly the on-disk metadata. Thus, we don't have to flush to
- * disk the whole region_map. We can just flush the dirty region_map words.
+ * During a metadata commit we scan dmap->dirty_words and dmap->dirty_regions
+ * and update the on-disk metadata accordingly. Thus, we don't have to flush to
+ * disk the whole region_map. We can just flush the dirty region_map bits.
*
- * We use a dirty bitmap, which is smaller than the original region_map, to
- * reduce the amount of memory accesses during a metadata commit. As dm-bitset
- * accesses the on-disk bitmap in 64-bit word granularity, there is no
- * significant benefit in tracking the dirty region_map bits with a smaller
- * granularity.
+ * We use the helper dmap->dirty_words bitmap, which is smaller than the
+ * original region_map, to reduce the amount of memory accesses during a
+ * metadata commit. Moreover, as dm-bitset also accesses the on-disk bitmap in
+ * 64-bit word granularity, the dirty_words bitmap helps us avoid useless disk
+ * accesses.
*
* We could update directly the on-disk bitmap, when dm-clone calls either
* dm_clone_set_region_hydrated() or dm_clone_cond_set_range(), buts this
@@ -92,12 +103,13 @@ struct superblock_disk {
* e.g., in a hooked overwrite bio's completion routine, and further reduce the
* I/O completion latency.
*
- * We maintain two dirty bitmaps. During a metadata commit we atomically swap
- * the currently used dmap with the unused one. This allows the metadata update
- * functions to run concurrently with an ongoing commit.
+ * We maintain two dirty bitmap sets. During a metadata commit we atomically
+ * swap the currently used dmap with the unused one. This allows the metadata
+ * update functions to run concurrently with an ongoing commit.
*/
struct dirty_map {
unsigned long *dirty_words;
+ unsigned long *dirty_regions;
unsigned int changed;
};
@@ -461,22 +473,40 @@ static size_t bitmap_size(unsigned long nr_bits)
return BITS_TO_LONGS(nr_bits) * sizeof(long);
}
-static int dirty_map_init(struct dm_clone_metadata *cmd)
+static int __dirty_map_init(struct dirty_map *dmap, unsigned long nr_words,
+ unsigned long nr_regions)
{
- cmd->dmap[0].changed = 0;
- cmd->dmap[0].dirty_words = kvzalloc(bitmap_size(cmd->nr_words), GFP_KERNEL);
+ dmap->changed = 0;
- if (!cmd->dmap[0].dirty_words) {
- DMERR("Failed to allocate dirty bitmap");
+ dmap->dirty_words = kvzalloc(bitmap_size(nr_words), GFP_KERNEL);
+ if (!dmap->dirty_words)
+ return -ENOMEM;
+
+ dmap->dirty_regions = kvzalloc(bitmap_size(nr_regions), GFP_KERNEL);
+ if (!dmap->dirty_regions) {
+ kvfree(dmap->dirty_words);
return -ENOMEM;
}
- cmd->dmap[1].changed = 0;
- cmd->dmap[1].dirty_words = kvzalloc(bitmap_size(cmd->nr_words), GFP_KERNEL);
+ return 0;
+}
- if (!cmd->dmap[1].dirty_words) {
+static void __dirty_map_exit(struct dirty_map *dmap)
+{
+ kvfree(dmap->dirty_words);
+ kvfree(dmap->dirty_regions);
+}
+
+static int dirty_map_init(struct dm_clone_metadata *cmd)
+{
+ if (__dirty_map_init(&cmd->dmap[0], cmd->nr_words, cmd->nr_regions)) {
DMERR("Failed to allocate dirty bitmap");
- kvfree(cmd->dmap[0].dirty_words);
+ return -ENOMEM;
+ }
+
+ if (__dirty_map_init(&cmd->dmap[1], cmd->nr_words, cmd->nr_regions)) {
+ DMERR("Failed to allocate dirty bitmap");
+ __dirty_map_exit(&cmd->dmap[0]);
return -ENOMEM;
}
@@ -487,8 +517,8 @@ static int dirty_map_init(struct dm_clone_metadata *cmd)
static void dirty_map_exit(struct dm_clone_metadata *cmd)
{
- kvfree(cmd->dmap[0].dirty_words);
- kvfree(cmd->dmap[1].dirty_words);
+ __dirty_map_exit(&cmd->dmap[0]);
+ __dirty_map_exit(&cmd->dmap[1]);
}
static int __load_bitset_in_core(struct dm_clone_metadata *cmd)
@@ -633,21 +663,23 @@ unsigned long dm_clone_find_next_unhydrated_region(struct dm_clone_metadata *cmd
return find_next_zero_bit(cmd->region_map, cmd->nr_regions, start);
}
-static int __update_metadata_word(struct dm_clone_metadata *cmd, unsigned long word)
+static int __update_metadata_word(struct dm_clone_metadata *cmd,
+ unsigned long *dirty_regions,
+ unsigned long word)
{
int r;
unsigned long index = word * BITS_PER_LONG;
unsigned long max_index = min(cmd->nr_regions, (word + 1) * BITS_PER_LONG);
while (index < max_index) {
- if (test_bit(index, cmd->region_map)) {
+ if (test_bit(index, dirty_regions)) {
r = dm_bitset_set_bit(&cmd->bitset_info, cmd->bitset_root,
index, &cmd->bitset_root);
-
if (r) {
DMERR("dm_bitset_set_bit failed");
return r;
}
+ __clear_bit(index, dirty_regions);
}
index++;
}
@@ -721,7 +753,7 @@ static int __flush_dmap(struct dm_clone_metadata *cmd, struct dirty_map *dmap)
if (word == cmd->nr_words)
break;
- r = __update_metadata_word(cmd, word);
+ r = __update_metadata_word(cmd, dmap->dirty_regions, word);
if (r)
return r;
@@ -802,6 +834,7 @@ int dm_clone_set_region_hydrated(struct dm_clone_metadata *cmd, unsigned long re
dmap = cmd->current_dmap;
__set_bit(word, dmap->dirty_words);
+ __set_bit(region_nr, dmap->dirty_regions);
__set_bit(region_nr, cmd->region_map);
dmap->changed = 1;
@@ -830,6 +863,7 @@ int dm_clone_cond_set_range(struct dm_clone_metadata *cmd, unsigned long start,
if (!test_bit(region_nr, cmd->region_map)) {
word = region_nr / BITS_PER_LONG;
__set_bit(word, dmap->dirty_words);
+ __set_bit(region_nr, dmap->dirty_regions);
__set_bit(region_nr, cmd->region_map);
dmap->changed = 1;
}
--
2.11.0
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