[Qemu-devel] [PATCH v5 07/11] block: introduce backup-top filter driver

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

Backup-top filter does copy-before-write operation. It should be
inserted above active disk and has a target node for CBW, like the
following:

    +-------+
    | Guest |
    +---+---+
        |r,w
        v
    +---+-----------+  target   +---------------+
    | backup_top    |---------->| target(qcow2) |
    +---+-----------+   CBW     +---+-----------+
        |
backing |r,w
        v
    +---+---------+
    | Active disk |
    +-------------+

The driver will be used in backup instead of write-notifiers.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 block/backup-top.h  |  43 +++++++
 block/backup-top.c  | 306 ++++++++++++++++++++++++++++++++++++++++++++
 block/Makefile.objs |   2 +
 3 files changed, 351 insertions(+)
 create mode 100644 block/backup-top.h
 create mode 100644 block/backup-top.c

diff --git a/block/backup-top.h b/block/backup-top.h
new file mode 100644
index 0000000000..abe4dda574
--- /dev/null
+++ b/block/backup-top.h
@@ -0,0 +1,43 @@
+/*
+ * backup-top filter driver
+ *
+ * The driver performs Copy-Before-Write (CBW) operation: it is injected above
+ * some node, and before each write it copies _old_ data to the target node.
+ *
+ * Copyright (c) 2018 Virtuozzo International GmbH. All rights reserved.
+ *
+ * Author:
+ *  Sementsov-Ogievskiy Vladimir <vsementsov@virtuozzo.com>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "qemu/osdep.h"
+
+#include "block/block_int.h"
+
+typedef struct BDRVBackupTopState {
+    HBitmap *copy_bitmap; /* what should be copied to @target on guest write. */
+    BdrvChild *target;
+
+    uint64_t bytes_copied;
+} BDRVBackupTopState;
+
+void bdrv_backup_top_drop(BlockDriverState *bs);
+uint64_t bdrv_backup_top_progress(BlockDriverState *bs);
+
+BlockDriverState *bdrv_backup_top_append(BlockDriverState *source,
+                                         BlockDriverState *target,
+                                         HBitmap *copy_bitmap,
+                                         Error **errp);
diff --git a/block/backup-top.c b/block/backup-top.c
new file mode 100644
index 0000000000..0e7b3e3142
--- /dev/null
+++ b/block/backup-top.c
@@ -0,0 +1,306 @@
+/*
+ * backup-top filter driver
+ *
+ * The driver performs Copy-Before-Write (CBW) operation: it is injected above
+ * some node, and before each write it copies _old_ data to the target node.
+ *
+ * Copyright (c) 2018 Virtuozzo International GmbH. All rights reserved.
+ *
+ * Author:
+ *  Sementsov-Ogievskiy Vladimir <vsementsov@virtuozzo.com>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "qemu/osdep.h"
+
+#include "qemu/cutils.h"
+#include "qapi/error.h"
+#include "block/block_int.h"
+#include "block/qdict.h"
+
+#include "block/backup-top.h"
+
+static coroutine_fn int backup_top_co_preadv(
+        BlockDriverState *bs, uint64_t offset, uint64_t bytes,
+        QEMUIOVector *qiov, int flags)
+{
+    /*
+     * Features to be implemented:
+     * F1. COR. save read data to fleecing target for fast access
+     *     (to reduce reads). This possibly may be done with use of copy-on-read
+     *     filter, but we need an ability to make COR requests optional: for
+     *     example, if target is a ram-cache, and if it is full now, we should
+     *     skip doing COR request, as it is actually not necessary.
+     *
+     * F2. Feature for guest: read from fleecing target if data is in ram-cache
+     *     and is unchanged
+     */
+
+    return bdrv_co_preadv(bs->backing, offset, bytes, qiov, flags);
+}
+
+static coroutine_fn int backup_top_cbw(BlockDriverState *bs, uint64_t offset,
+                                          uint64_t bytes)
+{
+    int ret = 0;
+    BDRVBackupTopState *s = bs->opaque;
+    uint64_t gran = 1UL << hbitmap_granularity(s->copy_bitmap);
+    uint64_t end = QEMU_ALIGN_UP(offset + bytes, gran);
+    uint64_t off = QEMU_ALIGN_DOWN(offset, gran), len;
+    size_t align = MAX(bdrv_opt_mem_align(bs->backing->bs),
+                       bdrv_opt_mem_align(s->target->bs));
+    struct iovec iov = {
+        .iov_base = qemu_memalign(align, end - off),
+        .iov_len = end - off
+    };
+    QEMUIOVector qiov;
+
+    qemu_iovec_init_external(&qiov, &iov, 1);
+
+    /*
+     * Features to be implemented:
+     * F3. parallelize copying loop
+     * F4. detect zeros
+     * F5. use block_status ?
+     * F6. don't copy clusters which are already cached by COR [see F1]
+     * F7. if target is ram-cache and it is full, there should be a possibility
+     *     to drop not necessary data (cached by COR [see F1]) to handle CBW
+     *     fast.
+     */
+
+    len = end - off;
+    while (hbitmap_next_dirty_area(s->copy_bitmap, &off, &len)) {
+        iov.iov_len = qiov.size = len;
+
+        hbitmap_reset(s->copy_bitmap, off, len);
+
+        ret = bdrv_co_preadv(bs->backing, off, len, &qiov,
+                             BDRV_REQ_NO_SERIALISING);
+        if (ret < 0) {
+            hbitmap_set(s->copy_bitmap, off, len);
+            goto finish;
+        }
+
+        ret = bdrv_co_pwritev(s->target, off, len, &qiov, BDRV_REQ_SERIALISING);
+        if (ret < 0) {
+            hbitmap_set(s->copy_bitmap, off, len);
+            goto finish;
+        }
+
+        s->bytes_copied += len;
+        off += len;
+        if (off >= end) {
+            break;
+        }
+        len = end - off;
+    }
+
+finish:
+    qemu_vfree(iov.iov_base);
+
+    /*
+     * F8. we fail guest request in case of error. We can alter it by
+     * possibility to fail copying process instead, or retry several times, or
+     * may be guest pause, etc.
+     */
+    return ret;
+}
+
+static int coroutine_fn backup_top_co_pdiscard(BlockDriverState *bs,
+                                                  int64_t offset, int bytes)
+{
+    int ret = backup_top_cbw(bs, offset, bytes);
+    if (ret < 0) {
+        return ret;
+    }
+
+    /*
+     * Features to be implemented:
+     * F9. possibility of lazy discard: just defer the discard after fleecing
+     *     completion. If write (or new discard) occurs to the same area, just
+     *     drop deferred discard.
+     */
+
+    return bdrv_co_pdiscard(bs->backing, offset, bytes);
+}
+
+static int coroutine_fn backup_top_co_pwrite_zeroes(BlockDriverState *bs,
+        int64_t offset, int bytes, BdrvRequestFlags flags)
+{
+    int ret = backup_top_cbw(bs, offset, bytes);
+    if (ret < 0) {
+        return ret;
+    }
+
+    return bdrv_co_pwrite_zeroes(bs->backing, offset, bytes, flags);
+}
+
+static coroutine_fn int backup_top_co_pwritev(BlockDriverState *bs,
+                                                 uint64_t offset,
+                                                 uint64_t bytes,
+                                                 QEMUIOVector *qiov, int flags)
+{
+    int ret = backup_top_cbw(bs, offset, bytes);
+    if (ret < 0) {
+        return ret;
+    }
+
+    return bdrv_co_pwritev(bs->backing, offset, bytes, qiov, flags);
+}
+
+static int coroutine_fn backup_top_co_flush(BlockDriverState *bs)
+{
+    if (!bs->backing) {
+        return 0;
+    }
+
+    return bdrv_co_flush(bs->backing->bs);
+}
+
+static void backup_top_refresh_filename(BlockDriverState *bs, QDict *opts)
+{
+    if (bs->backing == NULL) {
+        /*
+         * we can be here after failed bdrv_attach_child in
+         * bdrv_set_backing_hd
+         */
+        return;
+    }
+    bdrv_refresh_filename(bs->backing->bs);
+    pstrcpy(bs->exact_filename, sizeof(bs->exact_filename),
+            bs->backing->bs->filename);
+}
+
+static void backup_top_child_perm(BlockDriverState *bs, BdrvChild *c,
+                                       const BdrvChildRole *role,
+                                       BlockReopenQueue *reopen_queue,
+                                       uint64_t perm, uint64_t shared,
+                                       uint64_t *nperm, uint64_t *nshared)
+{
+    bdrv_filter_default_perms(bs, c, role, reopen_queue, perm, shared, nperm,
+                              nshared);
+
+    if (role == &child_file) {
+        /*
+         * share write to target, to not interfere guest writes to it's disk
+         * which will be in target backing chain
+         */
+        *nshared = *nshared | BLK_PERM_WRITE;
+        *nperm = *nperm | BLK_PERM_WRITE;
+    } else {
+        *nperm = *nperm | BLK_PERM_CONSISTENT_READ;
+    }
+}
+
+BlockDriver bdrv_backup_top_filter = {
+    .format_name = "backup-top",
+    .instance_size = sizeof(BDRVBackupTopState),
+
+    .bdrv_co_preadv             = backup_top_co_preadv,
+    .bdrv_co_pwritev            = backup_top_co_pwritev,
+    .bdrv_co_pwrite_zeroes      = backup_top_co_pwrite_zeroes,
+    .bdrv_co_pdiscard           = backup_top_co_pdiscard,
+    .bdrv_co_flush              = backup_top_co_flush,
+
+    .bdrv_co_block_status       = bdrv_co_block_status_from_backing,
+
+    .bdrv_refresh_filename      = backup_top_refresh_filename,
+
+    .bdrv_child_perm            = backup_top_child_perm,
+
+    .is_filter = true,
+};
+
+BlockDriverState *bdrv_backup_top_append(BlockDriverState *source,
+                                         BlockDriverState *target,
+                                         HBitmap *copy_bitmap,
+                                         Error **errp)
+{
+    Error *local_err = NULL;
+    BDRVBackupTopState *state;
+    BlockDriverState *top = bdrv_new_open_driver(&bdrv_backup_top_filter,
+                                                 NULL, BDRV_O_RDWR, errp);
+
+    if (!top) {
+        return NULL;
+    }
+
+    top->implicit = true;
+    top->total_sectors = source->total_sectors;
+    top->opaque = state = g_new0(BDRVBackupTopState, 1);
+    state->copy_bitmap = copy_bitmap;
+
+    bdrv_ref(target);
+    state->target = bdrv_attach_child(top, target, "target", &child_file, errp);
+    if (!state->target) {
+        bdrv_unref(target);
+        bdrv_unref(top);
+        return NULL;
+    }
+
+    bdrv_set_aio_context(top, bdrv_get_aio_context(source));
+    bdrv_set_aio_context(target, bdrv_get_aio_context(source));
+
+    bdrv_drained_begin(source);
+
+    bdrv_ref(top);
+    bdrv_append(top, source, &local_err);
+
+    if (local_err) {
+        bdrv_unref(top);
+    }
+
+    bdrv_drained_end(source);
+
+    if (local_err) {
+        bdrv_unref_child(top, state->target);
+        bdrv_unref(top);
+        error_propagate(errp, local_err);
+        return NULL;
+    }
+
+    return top;
+}
+
+void bdrv_backup_top_drop(BlockDriverState *bs)
+{
+    BDRVBackupTopState *s = bs->opaque;
+
+    AioContext *aio_context = bdrv_get_aio_context(bs);
+
+    aio_context_acquire(aio_context);
+
+    bdrv_drained_begin(bs);
+
+    bdrv_child_try_set_perm(bs->backing, 0, BLK_PERM_ALL, &error_abort);
+    bdrv_replace_node(bs, backing_bs(bs), &error_abort);
+    bdrv_set_backing_hd(bs, NULL, &error_abort);
+
+    bdrv_drained_end(bs);
+
+    if (s->target) {
+        bdrv_unref_child(bs, s->target);
+    }
+    bdrv_unref(bs);
+
+    aio_context_release(aio_context);
+}
+
+uint64_t bdrv_backup_top_progress(BlockDriverState *bs)
+{
+    BDRVBackupTopState *s = bs->opaque;
+
+    return s->bytes_copied;
+}
diff --git a/block/Makefile.objs b/block/Makefile.objs
index 7a81892a52..9072115c09 100644
--- a/block/Makefile.objs
+++ b/block/Makefile.objs
@@ -40,6 +40,8 @@ block-obj-y += throttle.o copy-on-read.o
 
 block-obj-y += crypto.o
 
+block-obj-y += backup-top.o
+
 common-obj-y += stream.o
 
 nfs.o-libs         := $(LIBNFS_LIBS)
-- 
2.18.0