[Devel] [PATCH hci-8.1] drivers/md/dm-qcow2: truncate unused tail of image on suspend

[Alexander Ivanov]

On postsuspend we truncate the image file only down to
file_preallocated_area_start, i.e. we drop preallocations only.
Clusters at the end of the file freed by discard (refcount dropped
to zero) are never given back, so the file size never shrinks.

Add qcow2_calc_used_end(), which scans refcount blocks backward from
the end of the file and returns the end of the last cluster with
non-zero refcount. Refcount ranges with no allocated refblock are
skipped at once. Metadata pages are taken from md cache or read
synchronously. This is called on suspended device after md
writeback, so R1 and R2 can't change under us.

Truncate the file to min(file_preallocated_area_start, used end).
Skip the scan if md writeback failed, as refcounts on disk are stale
then, and fall back to the old behaviour on scan error.

https://virtuozzo.atlassian.net/browse/VSTOR-148171
Feature: dm-qcow2: block device over QCOW2 files driver

Signed-off-by: Alexander Ivanov <alexander.ivanov at virtuozzo.com>
---
 drivers/md/dm-qcow2-map.c    | 77 ++++++++++++++++++++++++++++++++++++
 drivers/md/dm-qcow2-target.c | 12 +++++-
 drivers/md/dm-qcow2.h        |  1 +
 3 files changed, 89 insertions(+), 1 deletion(-)

diff --git a/drivers/md/dm-qcow2-map.c b/drivers/md/dm-qcow2-map.c
index 78ee7cf5e881..39d465343d8b 100644
--- a/drivers/md/dm-qcow2-map.c
+++ b/drivers/md/dm-qcow2-map.c
@@ -2208,6 +2208,83 @@ int qcow2_truncate_safe(struct file *file, loff_t new_len)
 	return do_ftruncate(file, new_len, 0);
 }
 
+static int get_md_page_sync(struct qcow2 *qcow2, u64 id, struct md_page **md)
+{
+	int ret;
+
+	spin_lock_irq(&qcow2->md_pages_lock);
+	*md = qcow2_md_page_find_or_postpone(qcow2, id, NULL);
+	spin_unlock_irq(&qcow2->md_pages_lock);
+	if (*md)
+		return 0;
+
+	ret = qcow2_alloc_and_insert_md_page(qcow2, id, md);
+	if (ret)
+		return ret;
+
+	ret = qcow2_rw_page_sync(READ, qcow2, id, (*md)->page);
+	spin_lock_irq(&qcow2->md_pages_lock);
+	if (ret)
+		qcow2_md_page_erase(qcow2, *md);
+	else
+		(*md)->status |= MD_UPTODATE;
+	spin_unlock_irq(&qcow2->md_pages_lock);
+	if (ret)
+		qcow2_free_md_page(*md);
+	return ret;
+}
+
+/*
+ * Returns the end of the last cluster, which is used according
+ * to refcount table. Clusters freed by discard may leave the tail
+ * of file unused. This must be called on suspended device after
+ * md writeback, so nobody changes R1 and R2.
+ */
+loff_t qcow2_calc_used_end(struct qcow2 *qcow2)
+{
+	u32 clu_size = qcow2->clu_size;
+	struct qcow2_map_item r1, r2;
+	struct md_page *r1_md = NULL;
+	loff_t pos;
+	s32 i;
+	int ret;
+
+	pos = round_down(qcow2->file_size - 1, clu_size);
+	while (pos >= 0) {
+		if (calc_refcounters_map(qcow2, pos, &r1, &r2) < 0)
+			return -EIO;
+
+		if (!r1_md || r1_md->id != r1.page_id) {
+			ret = get_md_page_sync(qcow2, r1.page_id, &r1_md);
+			if (ret)
+				return ret;
+		}
+		r1.md = r1_md;
+
+		ret = calc_r2_page_id(qcow2, &r1, &r2);
+		if (ret == -ENOENT) {
+			/* No R2 cluster, so all clusters it covers are unused */
+			pos = (loff_t)r1.index * qcow2->refblock_entries * clu_size;
+			pos -= clu_size;
+			continue;
+		}
+
+		if (ret < 0)
+			return ret;
+
+		ret = get_md_page_sync(qcow2, r2.page_id, &r2.md);
+		if (ret)
+			return ret;
+
+		for (i = r2.index_in_page; i >= 0; i--, pos -= clu_size) {
+			if (get_r2_entry(qcow2, r2.md, i))
+				return pos + clu_size;
+		}
+	}
+
+	return -EIO;
+}
+
 static int truncate_prealloc_safe(struct qcow2 *qcow2, loff_t len, const char *func)
 {
 	loff_t prealloc_len, max_prealloc_len = qcow2->reftable_max_file_size;
diff --git a/drivers/md/dm-qcow2-target.c b/drivers/md/dm-qcow2-target.c
index be09615eb8e2..95c8b114d92f 100644
--- a/drivers/md/dm-qcow2-target.c
+++ b/drivers/md/dm-qcow2-target.c
@@ -1016,11 +1016,21 @@ static void qcow2_truncate_preallocations(struct dm_target *ti)
 {
 	struct qcow2_target *tgt = to_qcow2_target(ti);
 	struct qcow2 *qcow2 = top_qcow2_protected(ti);
-	loff_t end = qcow2->file_preallocated_area_start;
+	loff_t used_end, end = qcow2->file_preallocated_area_start;
 	int ret;
 
 	if (!(dm_table_get_mode(ti->table) & FMODE_WRITE))
 		return;
+
+	/* Refcounts on disk are stale after failed md writeback */
+	if (!tgt->md_writeback_error) {
+		used_end = qcow2_calc_used_end(qcow2);
+		if (used_end < 0)
+			QC_ERR(ti, "dm-qcow2: Can't find used end: %lld", used_end);
+		else if (used_end < end)
+			end = used_end;
+	}
+
 	if (end == qcow2->file_size)
 		return;
 
diff --git a/drivers/md/dm-qcow2.h b/drivers/md/dm-qcow2.h
index d9b8c38e093f..2c5eb7578316 100644
--- a/drivers/md/dm-qcow2.h
+++ b/drivers/md/dm-qcow2.h
@@ -409,6 +409,7 @@ void qcow2_ref_dec(struct qcow2_target *tgt, u8 ref_index);
 int qcow2_inflight_ref_switch(struct qcow2_target *tgt);
 void qcow2_flush_deferred_activity(struct qcow2_target *tgt, struct qcow2 *qcow2);
 int qcow2_truncate_safe(struct file *file, loff_t new_len);
+loff_t qcow2_calc_used_end(struct qcow2 *qcow2);
 
 void qcow2_merge_backward_work(struct work_struct *work);
 void qcow2_merge_backward_cancel(struct qcow2_target *tgt);
-- 
2.53.0