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    Subject[PATCH 3.8 033/124] dm thin: fix discard support to a previously shared block
    Date
    3.8.13.18 -stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Joe Thornber <ejt@redhat.com>

    commit 19fa1a6756ed9e92daa9537c03b47d6b55cc2316 upstream.

    If a snapshot is created and later deleted the origin dm_thin_device's
    snapshotted_time will have been updated to reflect the snapshot's
    creation time. The 'shared' flag in the dm_thin_lookup_result struct
    returned from dm_thin_find_block() is an approximation based on
    snapshotted_time -- this is done to avoid 0(n), or worse, time
    complexity. In this case, the shared flag would be true.

    But because the 'shared' flag reflects an approximation a block can be
    incorrectly assumed to be shared (e.g. false positive for 'shared'
    because the snapshot no longer exists). This could result in discards
    issued to a thin device not being passed down to the pool's underlying
    data device.

    To fix this we double check that a thin block is really still in-use
    after a mapping is removed using dm_pool_block_is_used(). If the
    reference count for a block is now zero the discard is allowed to be
    passed down.

    Also add a 'definitely_not_shared' member to the dm_thin_new_mapping
    structure -- reflects that the 'shared' flag in the response from
    dm_thin_find_block() can only be held as definitive if false is
    returned.

    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1043527

    Signed-off-by: Joe Thornber <ejt@redhat.com>
    Signed-off-by: Mike Snitzer <snitzer@redhat.com>
    Signed-off-by: Kamal Mostafa <kamal@canonical.com>
    ---
    drivers/md/dm-thin-metadata.c | 20 ++++++++++++++++++++
    drivers/md/dm-thin-metadata.h | 2 ++
    drivers/md/dm-thin.c | 14 ++++++++++++--
    3 files changed, 34 insertions(+), 2 deletions(-)

    diff --git a/drivers/md/dm-thin-metadata.c b/drivers/md/dm-thin-metadata.c
    index 0368a69..6727958 100644
    --- a/drivers/md/dm-thin-metadata.c
    +++ b/drivers/md/dm-thin-metadata.c
    @@ -1349,6 +1349,12 @@ dm_thin_id dm_thin_dev_id(struct dm_thin_device *td)
    return td->id;
    }

    +/*
    + * Check whether @time (of block creation) is older than @td's last snapshot.
    + * If so then the associated block is shared with the last snapshot device.
    + * Any block on a device created *after* the device last got snapshotted is
    + * necessarily not shared.
    + */
    static bool __snapshotted_since(struct dm_thin_device *td, uint32_t time)
    {
    return td->snapshotted_time > time;
    @@ -1458,6 +1464,20 @@ int dm_thin_remove_block(struct dm_thin_device *td, dm_block_t block)
    return r;
    }

    +int dm_pool_block_is_used(struct dm_pool_metadata *pmd, dm_block_t b, bool *result)
    +{
    + int r;
    + uint32_t ref_count;
    +
    + down_read(&pmd->root_lock);
    + r = dm_sm_get_count(pmd->data_sm, b, &ref_count);
    + if (!r)
    + *result = (ref_count != 0);
    + up_read(&pmd->root_lock);
    +
    + return r;
    +}
    +
    bool dm_thin_changed_this_transaction(struct dm_thin_device *td)
    {
    int r;
    diff --git a/drivers/md/dm-thin-metadata.h b/drivers/md/dm-thin-metadata.h
    index 050a9ad..149d326 100644
    --- a/drivers/md/dm-thin-metadata.h
    +++ b/drivers/md/dm-thin-metadata.h
    @@ -180,6 +180,8 @@ int dm_pool_get_data_block_size(struct dm_pool_metadata *pmd, sector_t *result);

    int dm_pool_get_data_dev_size(struct dm_pool_metadata *pmd, dm_block_t *result);

    +int dm_pool_block_is_used(struct dm_pool_metadata *pmd, dm_block_t b, bool *result);
    +
    /*
    * Returns -ENOSPC if the new size is too small and already allocated
    * blocks would be lost.
    diff --git a/drivers/md/dm-thin.c b/drivers/md/dm-thin.c
    index b6fdaf7..5e62738 100644
    --- a/drivers/md/dm-thin.c
    +++ b/drivers/md/dm-thin.c
    @@ -440,6 +440,7 @@ struct dm_thin_new_mapping {
    unsigned quiesced:1;
    unsigned prepared:1;
    unsigned pass_discard:1;
    + unsigned definitely_not_shared:1;

    struct thin_c *tc;
    dm_block_t virt_block;
    @@ -610,7 +611,15 @@ static void process_prepared_discard_passdown(struct dm_thin_new_mapping *m)
    cell_defer_no_holder(tc, m->cell2);

    if (m->pass_discard)
    - remap_and_issue(tc, m->bio, m->data_block);
    + if (m->definitely_not_shared)
    + remap_and_issue(tc, m->bio, m->data_block);
    + else {
    + bool used = false;
    + if (dm_pool_block_is_used(tc->pool->pmd, m->data_block, &used) || used)
    + bio_endio(m->bio, 0);
    + else
    + remap_and_issue(tc, m->bio, m->data_block);
    + }
    else
    bio_endio(m->bio, 0);

    @@ -957,7 +966,8 @@ static void process_discard(struct thin_c *tc, struct bio *bio)
    */
    m = get_next_mapping(pool);
    m->tc = tc;
    - m->pass_discard = (!lookup_result.shared) && pool->pf.discard_passdown;
    + m->pass_discard = pool->pf.discard_passdown;
    + m->definitely_not_shared = !lookup_result.shared;
    m->virt_block = block;
    m->data_block = lookup_result.block;
    m->cell = cell;
    --
    1.8.3.2


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