[pve-devel] [PATCH qemu 03/11] block: add alloc-track driver

Wolfgang Bumiller w.bumiller at proxmox.com
Tue Jan 12 11:54:11 CET 2021


one inline comment, otherwise lgtm

On Mon, Jan 11, 2021 at 12:14:01PM +0100, Stefan Reiter wrote:
> Add a new filter node 'alloc-track', which seperates reads and writes to
> different children, thus allowing to put a backing image behind any
> blockdev (regardless of driver support). Since we can't detect any
> pre-allocated blocks, we can only track new writes, hence the write
> target ('file') for this node must always be empty.
> 
> Intended use case is for live restoring, i.e. add a backup image as a
> block device into a VM, then put an alloc-track on the restore target
> and set the backup as backing. With this, one can use a regular
> 'block-stream' to restore the image, while the VM can already run in the
> background. Copy-on-read will help make progress as the VM reads as
> well.
> 
> This only worked if the target supports backing images, so up until now
> only for qcow2, with alloc-track any driver for the target can be used.
> 
> If 'auto-remove' is set, alloc-track will automatically detach itself
> once the backing image is removed. It will be replaced by 'file'.
> 
> Signed-off-by: Stefan Reiter <s.reiter at proxmox.com>
> ---
> 
> Potentially for upstream as well, but let's maybe test it a bit here first ;)
> 
>  block/Makefile.objs |   1 +
>  block/alloc-track.c | 319 ++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 320 insertions(+)
>  create mode 100644 block/alloc-track.c
> 
> diff --git a/block/Makefile.objs b/block/Makefile.objs
> index 28fb3b7f7c..04f91b8cc9 100644
> --- a/block/Makefile.objs
> +++ b/block/Makefile.objs
> @@ -13,6 +13,7 @@ block-obj-$(CONFIG_QED) += qed-check.o
>  block-obj-y += vhdx.o vhdx-endian.o vhdx-log.o
>  block-obj-y += quorum.o
>  block-obj-y += zeroinit.o
> +block-obj-y += alloc-track.o
>  block-obj-y += blkdebug.o blkverify.o blkreplay.o
>  block-obj-$(CONFIG_PARALLELS) += parallels.o
>  block-obj-y += blklogwrites.o
> diff --git a/block/alloc-track.c b/block/alloc-track.c
> new file mode 100644
> index 0000000000..f2a4080579
> --- /dev/null
> +++ b/block/alloc-track.c
> @@ -0,0 +1,319 @@
> +/*
> + * Filter to allow backing images to be applied to any node. Assumes a blank
> + * image to begin with, only new writes are tracked as allocated, thus this
> + * must never be put on a node that already contains data.
> + *
> + * Copyright (c) 2020 Proxmox Server Solutions GmbH
> + * Copyright (c) 2020 Stefan Reiter <s.reiter at proxmox.com>
> + *
> + * This work is licensed under the terms of the GNU GPL, version 2 or later.
> + * See the COPYING file in the top-level directory.
> + */
> +
> +#include "qemu/osdep.h"
> +#include "qapi/error.h"
> +#include "block/block_int.h"
> +#include "qapi/qmp/qdict.h"
> +#include "qapi/qmp/qstring.h"
> +#include "qemu/cutils.h"
> +#include "qemu/option.h"
> +#include "qemu/module.h"
> +
> +#define TRACK_OPT_AUTO_REMOVE "auto-remove"
> +
> +typedef struct {
> +    BdrvDirtyBitmap *bitmap;
> +    bool dropping;
> +    bool auto_remove;
> +} BDRVAllocTrackState;
> +
> +static QemuOptsList runtime_opts = {
> +    .name = "alloc-track",
> +    .head = QTAILQ_HEAD_INITIALIZER(runtime_opts.head),
> +    .desc = {
> +        {
> +            .name = TRACK_OPT_AUTO_REMOVE,
> +            .type = QEMU_OPT_BOOL,
> +            .help = "automatically replace this node with 'file' when 'backing'"
> +                    "is detached",
> +        },
> +        { /* end of list */ }
> +    },
> +};
> +
> +static void track_refresh_limits(BlockDriverState *bs, Error **errp)
> +{
> +    BlockDriverInfo bdi;
> +
> +    if (!bs->file) {
> +        return;
> +    }
> +
> +    /* always use alignment from underlying write device so RMW cycle for
> +     * bdrv_pwritev reads data from our backing via track_co_preadv (no partial
> +     * cluster allocation in 'file') */
> +    bdrv_get_info(bs->file->bs, &bdi);
> +    bs->bl.request_alignment = MAX(bs->file->bs->bl.request_alignment,
> +                                   MAX(bdi.cluster_size, BDRV_SECTOR_SIZE));
> +}
> +
> +static int track_open(BlockDriverState *bs, QDict *options, int flags,
> +                      Error **errp)
> +{
> +    BDRVAllocTrackState *s = bs->opaque;
> +    QemuOpts *opts;
> +    Error *local_err = NULL;
> +    int ret = 0;
> +
> +    opts = qemu_opts_create(&runtime_opts, NULL, 0, &error_abort);
> +    qemu_opts_absorb_qdict(opts, options, &local_err);
> +    if (local_err) {
> +        error_propagate(errp, local_err);
> +        ret = -EINVAL;
> +        goto fail;
> +    }
> +
> +    s->auto_remove = qemu_opt_get_bool(opts, TRACK_OPT_AUTO_REMOVE, false);
> +
> +    /* open the target (write) node, backing will be attached by block layer */
> +    bs->file = bdrv_open_child(NULL, options, "file", bs, &child_of_bds,
> +                               BDRV_CHILD_DATA | BDRV_CHILD_METADATA, false,
> +                               &local_err);
> +    if (local_err) {
> +        ret = -EINVAL;
> +        error_propagate(errp, local_err);
> +        goto fail;
> +    }
> +
> +    track_refresh_limits(bs, errp);
> +    uint64_t gran = bs->bl.request_alignment;
> +    s->bitmap = bdrv_create_dirty_bitmap(bs->file->bs, gran, NULL, &local_err);
> +    if (local_err) {
> +        ret = -EIO;
> +        error_propagate(errp, local_err);
> +        goto fail;
> +    }
> +
> +    s->dropping = false;
> +
> +fail:
> +    if (ret < 0) {
> +        bdrv_unref_child(bs, bs->file);
> +        if (s->bitmap) {
> +            bdrv_release_dirty_bitmap(s->bitmap);
> +        }
> +    }
> +    qemu_opts_del(opts);
> +    return ret;
> +}
> +
> +static void track_close(BlockDriverState *bs)
> +{
> +    BDRVAllocTrackState *s = bs->opaque;
> +    if (s->bitmap) {
> +        bdrv_release_dirty_bitmap(s->bitmap);
> +    }
> +}
> +
> +static int64_t track_getlength(BlockDriverState *bs)
> +{
> +    return bdrv_getlength(bs->file->bs);
> +}
> +
> +static int coroutine_fn track_co_preadv(BlockDriverState *bs,
> +    uint64_t offset, uint64_t bytes, QEMUIOVector *qiov, int flags)
> +{
> +    BDRVAllocTrackState *s = bs->opaque;
> +    QEMUIOVector local_qiov;
> +    int ret;
> +
> +    /* 'cur_offset' is relative to 'offset', 'local_offset' to image start */
> +    uint64_t cur_offset, local_offset;
> +    int64_t local_bytes;
> +    bool alloc;
> +
> +    /* a read request can span multiple granularity-sized chunks, and can thus
> +     * contain blocks with different allocation status - we could just iterate
> +     * granularity-wise, but for better performance use bdrv_dirty_bitmap_next_X
> +     * to find the next flip and consider everything up to that in one go */
> +    for (cur_offset = 0; cur_offset < bytes; cur_offset += local_bytes) {
> +        local_offset = offset + cur_offset;
> +        alloc = bdrv_dirty_bitmap_get(s->bitmap, local_offset);
> +        if (alloc) {
> +            local_bytes = bdrv_dirty_bitmap_next_zero(s->bitmap, local_offset,
> +                                                      bytes - cur_offset);
> +        } else {
> +            local_bytes = bdrv_dirty_bitmap_next_dirty(s->bitmap, local_offset,
> +                                                       bytes - cur_offset);
> +        }
> +
> +        /* _bitmap_next_X return is -1 if no end found within limit, otherwise
> +         * offset of next flip (to start of image) */
> +        local_bytes = local_bytes < 0 ?
> +            bytes - cur_offset :
> +            local_bytes - local_offset;
> +
> +        qemu_iovec_init_slice(&local_qiov, qiov, cur_offset, local_bytes);
> +
> +        if (alloc) {
> +            ret = bdrv_co_preadv(bs->file, local_offset, local_bytes,
> +                                 &local_qiov, flags);
> +        } else if (bs->backing) {
> +            ret = bdrv_co_preadv(bs->backing, local_offset, local_bytes,
> +                                 &local_qiov, flags);
> +        } else {

Here you handle the case where there's no backing file, but other
functions (eg. track_co_block_status) simply assume that `bs->backing`
is always valid.

Is this intentional?

It makes me also wonder - since you just zero out the iov below - if
this driver could maybe also replace the current zeroinit filter?
Otherwise it's at least a good reminder that the zeroinit code could
start making use of bitmaps as well ;-)

> +            ret = qemu_iovec_memset(&local_qiov, cur_offset, 0, local_bytes);
> +        }
> +
> +        if (ret != 0) {
> +            break;
> +        }
> +    }
> +
> +    return ret;
> +}
> +
> +static int coroutine_fn track_co_pwritev(BlockDriverState *bs,
> +    uint64_t offset, uint64_t bytes, QEMUIOVector *qiov, int flags)
> +{
> +    return bdrv_co_pwritev(bs->file, offset, bytes, qiov, flags);
> +}
> +
> +static int coroutine_fn track_co_pwrite_zeroes(BlockDriverState *bs,
> +    int64_t offset, int count, BdrvRequestFlags flags)
> +{
> +    return bdrv_pwrite_zeroes(bs->file, offset, count, flags);
> +}
> +
> +static int coroutine_fn track_co_pdiscard(BlockDriverState *bs,
> +    int64_t offset, int count)
> +{
> +    return bdrv_co_pdiscard(bs->file, offset, count);
> +}
> +
> +static coroutine_fn int track_co_flush(BlockDriverState *bs)
> +{
> +    return bdrv_co_flush(bs->file->bs);
> +}
> +
> +static int coroutine_fn track_co_block_status(BlockDriverState *bs,
> +                                              bool want_zero,
> +                                              int64_t offset,
> +                                              int64_t bytes,
> +                                              int64_t *pnum,
> +                                              int64_t *map,
> +                                              BlockDriverState **file)
> +{
> +    BDRVAllocTrackState *s = bs->opaque;
> +
> +    bool alloc = bdrv_dirty_bitmap_get(s->bitmap, offset);
> +    int64_t next_flipped;
> +    if (alloc) {
> +        next_flipped = bdrv_dirty_bitmap_next_zero(s->bitmap, offset, bytes);
> +    } else {
> +        next_flipped = bdrv_dirty_bitmap_next_dirty(s->bitmap, offset, bytes);
> +    }
> +
> +    /* in case not the entire region has the same state, we need to set pnum to
> +     * indicate for how many bytes our result is valid */
> +    *pnum = next_flipped == -1 ? bytes : next_flipped - offset;
> +    *map = offset;
> +
> +    if (alloc) {
> +        *file = bs->file->bs;
> +        return BDRV_BLOCK_RAW | BDRV_BLOCK_OFFSET_VALID;
> +    } else {
> +        *file = bs->backing->bs;
> +        return 0;
> +    }
> +}
> +
> +static void track_child_perm(BlockDriverState *bs, BdrvChild *c,
> +                             BdrvChildRole role, BlockReopenQueue *reopen_queue,
> +                             uint64_t perm, uint64_t shared,
> +                             uint64_t *nperm, uint64_t *nshared)
> +{
> +    BDRVAllocTrackState *s = bs->opaque;
> +
> +    *nshared = BLK_PERM_ALL;
> +
> +    /* in case we're currently dropping ourselves, claim to not use any
> +     * permissions at all - which is fine, since from this point on we will
> +     * never issue a read or write anymore */
> +    if (s->dropping) {
> +        *nperm = 0;
> +        return;
> +    }
> +
> +    if (role & BDRV_CHILD_DATA) {
> +        *nperm = perm & DEFAULT_PERM_PASSTHROUGH;
> +    } else {
> +        /* 'backing' is also a child of our BDS, but we don't expect it to be
> +         * writeable, so we only forward 'consistent read' */
> +        *nperm = perm & BLK_PERM_CONSISTENT_READ;
> +    }
> +}
> +
> +static void track_drop(void *opaque)
> +{
> +    BlockDriverState *bs = (BlockDriverState*)opaque;
> +    BDRVAllocTrackState *s = bs->opaque;
> +
> +    bdrv_drained_begin(bs);
> +
> +    s->dropping = true;
> +
> +    bdrv_child_refresh_perms(bs, bs->file, &error_abort);
> +    bdrv_replace_node(bs, bs->file->bs, &error_abort);
> +    bdrv_set_backing_hd(bs, NULL, &error_abort);
> +
> +    bdrv_drained_end(bs);
> +}
> +
> +static int track_change_backing_file(BlockDriverState *bs,
> +                                     const char *backing_file,
> +                                     const char *backing_fmt)
> +{
> +    if (backing_file == NULL && backing_fmt == NULL) {
> +        /* backing file has been disconnected, there's no longer any use for
> +         * this node, so let's remove ourselves from the block graph - we need
> +         * to schedule this for later however, since when this function is
> +         * called, the blockjob modifying us is probably not done yet and has a
> +         * blocker on 'bs' */
> +        aio_bh_schedule_oneshot(qemu_get_aio_context(), track_drop, (void*)bs);
> +    }
> +
> +    return 0;
> +}
> +
> +static BlockDriver bdrv_alloc_track = {
> +    .format_name                      = "alloc-track",
> +    .instance_size                    = sizeof(BDRVAllocTrackState),
> +
> +    .bdrv_file_open                   = track_open,
> +    .bdrv_close                       = track_close,
> +    .bdrv_getlength                   = track_getlength,
> +    .bdrv_child_perm                  = track_child_perm,
> +    .bdrv_refresh_limits              = track_refresh_limits,
> +
> +    .bdrv_co_pwrite_zeroes            = track_co_pwrite_zeroes,
> +    .bdrv_co_pwritev                  = track_co_pwritev,
> +    .bdrv_co_preadv                   = track_co_preadv,
> +    .bdrv_co_pdiscard                 = track_co_pdiscard,
> +
> +    .bdrv_co_flush                    = track_co_flush,
> +    .bdrv_co_flush_to_disk            = track_co_flush,
> +
> +    .is_filter                        = true,
> +    .supports_backing                 = true,
> +
> +    .bdrv_co_block_status             = track_co_block_status,
> +    .bdrv_change_backing_file         = track_change_backing_file,
> +};
> +
> +static void bdrv_alloc_track_init(void)
> +{
> +    bdrv_register(&bdrv_alloc_track);
> +}
> +
> +block_init(bdrv_alloc_track_init);
> -- 
> 2.20.1




More information about the pve-devel mailing list