Re: [Qemu-devel] [PATCH v2 4/4] block/dirty-bitmaps: implement inconsistent bit

[Vladimir Sementsov-Ogievskiy]

23.02.2019 3:22, John Snow wrote:
> Set the inconsistent bit on load instead of rejecting such bitmaps.
> There is no way to un-set it; the only option is to delete it.
> 
> Obvervations:
> - bitmap loading does not need to update the header for in_use bitmaps.
> - inconsistent bitmaps don't need to have their data loaded; they're
>    glorified corruption sentinels.
> - bitmap saving does not need to save inconsistent bitmaps back to disk.
> - bitmap reopening DOES need to drop the readonly flag from inconsistent
>    bitmaps to allow reopening of qcow2 files with non-qemu-owned bitmaps
>    being eventually flushed back to disk.
> 
> Signed-off-by: John Snow <address@hidden>
> ---
>   block/qcow2-bitmap.c | 77 +++++++++++++++++++++++---------------------
>   1 file changed, 40 insertions(+), 37 deletions(-)
> 
> diff --git a/block/qcow2-bitmap.c b/block/qcow2-bitmap.c
> index 3ee524da4b..d1cc11da88 100644
> --- a/block/qcow2-bitmap.c
> +++ b/block/qcow2-bitmap.c
> @@ -343,9 +343,15 @@ static BdrvDirtyBitmap *load_bitmap(BlockDriverState *bs,
>       uint32_t granularity;
>       BdrvDirtyBitmap *bitmap = NULL;
>   
> +    granularity = 1U << bm->granularity_bits;
> +    bitmap = bdrv_create_dirty_bitmap(bs, granularity, bm->name, errp);
> +    if (bitmap == NULL) {
> +        goto fail;
> +    }
> +
>       if (bm->flags & BME_FLAG_IN_USE) {
> -        error_setg(errp, "Bitmap '%s' is in use", bm->name);
> -        goto fail;
> +        /* Data is unusable, skip loading it */
> +        return bitmap;
>       }
>   
>       ret = bitmap_table_load(bs, &bm->table, &bitmap_table);
> @@ -356,12 +362,6 @@ static BdrvDirtyBitmap *load_bitmap(BlockDriverState *bs,
>           goto fail;
>       }
>   
> -    granularity = 1U << bm->granularity_bits;
> -    bitmap = bdrv_create_dirty_bitmap(bs, granularity, bm->name, errp);
> -    if (bitmap == NULL) {
> -        goto fail;
> -    }
> -
>       ret = load_bitmap_data(bs, bitmap_table, bm->table.size, bitmap);
>       if (ret < 0) {
>           error_setg_errno(errp, -ret, "Could not read bitmap '%s' from 
> image",
> @@ -949,6 +949,7 @@ bool qcow2_load_dirty_bitmaps(BlockDriverState *bs, Error 
> **errp)
>       Qcow2Bitmap *bm;
>       GSList *created_dirty_bitmaps = NULL;
>       bool header_updated = false;
> +    bool needs_update = false;
>   
>       if (s->nb_bitmaps == 0) {
>           /* No bitmaps - nothing to do */
> @@ -962,23 +963,27 @@ bool qcow2_load_dirty_bitmaps(BlockDriverState *bs, 
> Error **errp)
>       }
>   
>       QSIMPLEQ_FOREACH(bm, bm_list, entry) {
> -        if (!(bm->flags & BME_FLAG_IN_USE)) {
> -            BdrvDirtyBitmap *bitmap = load_bitmap(bs, bm, errp);
> -            if (bitmap == NULL) {
> -                goto fail;
> -            }
> +        BdrvDirtyBitmap *bitmap = load_bitmap(bs, bm, errp);
> +        if (bitmap == NULL) {
> +            goto fail;
> +        }
>   
> -            if (!(bm->flags & BME_FLAG_AUTO)) {
> -                bdrv_disable_dirty_bitmap(bitmap);
> -            }
> -            bdrv_dirty_bitmap_set_persistance(bitmap, true);
> +        if (bm->flags & BME_FLAG_IN_USE) {
> +            bdrv_dirty_bitmap_set_inconsistent(bitmap);
> +        } else {
> +            /* NB: updated flags only get written if can_write(bs) is true. 
> */
>               bm->flags |= BME_FLAG_IN_USE;
> -            created_dirty_bitmaps =
> -                    g_slist_append(created_dirty_bitmaps, bitmap);
> +            needs_update = true;
>           }
> +        if (!(bm->flags & BME_FLAG_AUTO)) {
> +            bdrv_disable_dirty_bitmap(bitmap);
> +        }
> +        bdrv_dirty_bitmap_set_persistance(bitmap, true);

We define persistent bitmaps as which we are going to store.. But we are
not going to store inconsistent bitmaps. So inconsistent bitmaps are not 
persistent.

> +        created_dirty_bitmaps =
> +            g_slist_append(created_dirty_bitmaps, bitmap);
>       }
>   
> -    if (created_dirty_bitmaps != NULL) {
> +    if (needs_update) {

created_dirty_bitmaps list needed only for setting them readonly, if failed 
write that
they are IN_USE. So, instead of creating additional variable, better is just 
not include
IN_USE bitmaps to this list. And it may be renamed like set_in_use_list.

>           if (can_write(bs)) {
>               /* in_use flags must be updated */
>               int ret = update_ext_header_and_dir_in_place(bs, bm_list);
> @@ -1112,23 +1117,21 @@ int qcow2_reopen_bitmaps_rw_hint(BlockDriverState 
> *bs, bool *header_updated,
>       }
>   
>       QSIMPLEQ_FOREACH(bm, bm_list, entry) {
> -        if (!(bm->flags & BME_FLAG_IN_USE)) {
> -            BdrvDirtyBitmap *bitmap = bdrv_find_dirty_bitmap(bs, bm->name);
> -            if (bitmap == NULL) {
> -                continue;
> -            }
> -
> -            if (!bdrv_dirty_bitmap_readonly(bitmap)) {
> -                error_setg(errp, "Bitmap %s is not readonly but not marked"
> -                                 "'IN_USE' in the image. Something went 
> wrong,"
> -                                 "all the bitmaps may be corrupted", 
> bm->name);
> -                ret = -EINVAL;
> -                goto out;
> -            }
> +        BdrvDirtyBitmap *bitmap = bdrv_find_dirty_bitmap(bs, bm->name);
> +        if (bitmap == NULL) {
> +            continue;
> +        }

>   
> -            bm->flags |= BME_FLAG_IN_USE;
> -            ro_dirty_bitmaps = g_slist_append(ro_dirty_bitmaps, bitmap);
> +        if (!bdrv_dirty_bitmap_readonly(bitmap)) {

so, we'll fail, as we didn't marked inconsistetn bitmaps as readonly above. 
Should we?
Or we may check here inconsistance directly.

> +            error_setg(errp, "Bitmap %s was loaded prior to rw-reopen, but 
> was "
> +                       "not marked as readonly. This is a bug, something 
> went "
> +                       "wrong. All of the bitmaps may be corrupted", 
> bm->name);

Hmm, your wording covers invalid case when inconsistent bitmap is not IN_USE, 
and sounds
better for user, though may be less precise (as it may be not one reopen in a 
sequence..).
Ok for me.

> +            ret = -EINVAL;
> +            goto out;
>           }
> +
> +        bm->flags |= BME_FLAG_IN_USE;
> +        ro_dirty_bitmaps = g_slist_append(ro_dirty_bitmaps, bitmap);
>       }
>   
>       if (ro_dirty_bitmaps != NULL) {
> @@ -1424,8 +1427,8 @@ void 
> qcow2_store_persistent_dirty_bitmaps(BlockDriverState *bs, Error **errp)
>           Qcow2Bitmap *bm;
>   
>           if (!bdrv_dirty_bitmap_get_persistance(bitmap) ||
> -            bdrv_dirty_bitmap_readonly(bitmap))
> -        {
> +            bdrv_dirty_bitmap_readonly(bitmap) ||
> +            bdrv_dirty_bitmap_inconsistent(bitmap)) {
>               continue;
>           }
>   
> 


-- 
Best regards,
Vladimir