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

Stefan Reiter s.reiter at proxmox.com
Tue Jan 12 12:29:05 CET 2021


On 12/01/2021 11:54, Wolfgang Bumiller wrote:
> 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?
> 

Not intentional, but the only place where bs->backing is assumed is in 
track_co_block_status. Fix for this is simple enough, since according to 
the comment in 'include/block/block_int.h' when you return 0, the block 
layer automatically checks if a backing image is attached and assumes 
zero otherwise.

So basically just to avoid a potential NULL deref:

-----------------
diff --git a/block/alloc-track.c b/block/alloc-track.c
index f2a4080579..b1a41be05a 100644
--- a/block/alloc-track.c
+++ b/block/alloc-track.c
@@ -222,10 +222,10 @@ static int coroutine_fn 
track_co_block_status(BlockDriverState *bs,
      if (alloc) {
          *file = bs->file->bs;
          return BDRV_BLOCK_RAW | BDRV_BLOCK_OFFSET_VALID;
-    } else {
+    } else if (bs->backing) {
          *file = bs->backing->bs;
-        return 0;
      }
+    return 0;
  }

  static void track_child_perm(BlockDriverState *bs, BdrvChild *c,
-----------------

Yeah?

> 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 ;-)
> 

But zeroinit operates on writes, not reads? Not sure how this driver 
would help...

>> +            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




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