[dm-devel] [PATCH v2] dm thin: Fix bug wrt FUA request completion
Nikos Tsironis
ntsironis at arrikto.com
Fri Feb 15 14:33:29 UTC 2019
On 2/15/19 3:54 PM, Joe Thornber wrote:
> Ack.
>
> Thanks for this I was under the mistaken impression that FUA requests got split
> by core dm into separate payload and PREFLUSH requests.
>
> I've audited dm-cache and that looks ok.
>
> How did you test this patch? That missing bio_list_init() in V1 must
> have caused memory corruption?
>
> - Joe
Hi Joe,
bio_list_init() initializes the bio list's head and tail pointers to
NULL and pool_create() allocates the struct pool structure using
kzalloc() so the bio list was implicitly correctly initialized and no
memory corruption occurred.
Nikos
>
>
> On Fri, Feb 15, 2019 at 01:21:38AM +0200, Nikos Tsironis wrote:
>> When provisioning a new data block for a virtual block, either because
>> the block was previously unallocated or because we are breaking sharing,
>> if the whole block of data is being overwritten the bio that triggered
>> the provisioning is issued immediately, skipping copying or zeroing of
>> the data block.
>>
>> When this bio completes the new mapping is inserted in to the pool's
>> metadata by process_prepared_mapping(), where the bio completion is
>> signaled to the upper layers.
>>
>> This completion is signaled without first committing the metadata. If
>> the bio in question has the REQ_FUA flag set and the system crashes
>> right after its completion and before the next metadata commit, then the
>> write is lost despite the REQ_FUA flag requiring that I/O completion for
>> this request is only signaled after the data has been committed to
>> non-volatile storage.
>>
>> Fix this by deferring the completion of overwrite bios, with the REQ_FUA
>> flag set, after the metadata has been committed.
>>
>> Signed-off-by: Nikos Tsironis <ntsironis at arrikto.com>
>> ---
>> drivers/md/dm-thin.c | 55 +++++++++++++++++++++++++++++++++++++++++++++++-----
>> 1 file changed, 50 insertions(+), 5 deletions(-)
>>
>> Changes in v2:
>> - Add missing bio_list_init() in pool_create()
>>
>> v1: https://www.redhat.com/archives/dm-devel/2019-February/msg00064.html
>>
>> diff --git a/drivers/md/dm-thin.c b/drivers/md/dm-thin.c
>> index ca8af21bf644..e83b63608262 100644
>> --- a/drivers/md/dm-thin.c
>> +++ b/drivers/md/dm-thin.c
>> @@ -257,6 +257,7 @@ struct pool {
>>
>> spinlock_t lock;
>> struct bio_list deferred_flush_bios;
>> + struct bio_list deferred_flush_completions;
>> struct list_head prepared_mappings;
>> struct list_head prepared_discards;
>> struct list_head prepared_discards_pt2;
>> @@ -956,6 +957,39 @@ static void process_prepared_mapping_fail(struct dm_thin_new_mapping *m)
>> mempool_free(m, &m->tc->pool->mapping_pool);
>> }
>>
>> +static void complete_overwrite_bio(struct thin_c *tc, struct bio *bio)
>> +{
>> + struct pool *pool = tc->pool;
>> + unsigned long flags;
>> +
>> + /*
>> + * If the bio has the REQ_FUA flag set we must commit the metadata
>> + * before signaling its completion.
>> + */
>> + if (!bio_triggers_commit(tc, bio)) {
>> + bio_endio(bio);
>> + return;
>> + }
>> +
>> + /*
>> + * Complete bio with an error if earlier I/O caused changes to the
>> + * metadata that can't be committed, e.g, due to I/O errors on the
>> + * metadata device.
>> + */
>> + if (dm_thin_aborted_changes(tc->td)) {
>> + bio_io_error(bio);
>> + return;
>> + }
>> +
>> + /*
>> + * Batch together any bios that trigger commits and then issue a
>> + * single commit for them in process_deferred_bios().
>> + */
>> + spin_lock_irqsave(&pool->lock, flags);
>> + bio_list_add(&pool->deferred_flush_completions, bio);
>> + spin_unlock_irqrestore(&pool->lock, flags);
>> +}
>> +
>> static void process_prepared_mapping(struct dm_thin_new_mapping *m)
>> {
>> struct thin_c *tc = m->tc;
>> @@ -988,7 +1022,7 @@ static void process_prepared_mapping(struct dm_thin_new_mapping *m)
>> */
>> if (bio) {
>> inc_remap_and_issue_cell(tc, m->cell, m->data_block);
>> - bio_endio(bio);
>> + complete_overwrite_bio(tc, bio);
>> } else {
>> inc_all_io_entry(tc->pool, m->cell->holder);
>> remap_and_issue(tc, m->cell->holder, m->data_block);
>> @@ -2317,7 +2351,7 @@ static void process_deferred_bios(struct pool *pool)
>> {
>> unsigned long flags;
>> struct bio *bio;
>> - struct bio_list bios;
>> + struct bio_list bios, bio_completions;
>> struct thin_c *tc;
>>
>> tc = get_first_thin(pool);
>> @@ -2328,26 +2362,36 @@ static void process_deferred_bios(struct pool *pool)
>> }
>>
>> /*
>> - * If there are any deferred flush bios, we must commit
>> - * the metadata before issuing them.
>> + * If there are any deferred flush bios, we must commit the metadata
>> + * before issuing them or signaling their completion.
>> */
>> bio_list_init(&bios);
>> + bio_list_init(&bio_completions);
>> +
>> spin_lock_irqsave(&pool->lock, flags);
>> bio_list_merge(&bios, &pool->deferred_flush_bios);
>> bio_list_init(&pool->deferred_flush_bios);
>> +
>> + bio_list_merge(&bio_completions, &pool->deferred_flush_completions);
>> + bio_list_init(&pool->deferred_flush_completions);
>> spin_unlock_irqrestore(&pool->lock, flags);
>>
>> - if (bio_list_empty(&bios) &&
>> + if (bio_list_empty(&bios) && bio_list_empty(&bio_completions) &&
>> !(dm_pool_changed_this_transaction(pool->pmd) && need_commit_due_to_time(pool)))
>> return;
>>
>> if (commit(pool)) {
>> + bio_list_merge(&bios, &bio_completions);
>> +
>> while ((bio = bio_list_pop(&bios)))
>> bio_io_error(bio);
>> return;
>> }
>> pool->last_commit_jiffies = jiffies;
>>
>> + while ((bio = bio_list_pop(&bio_completions)))
>> + bio_endio(bio);
>> +
>> while ((bio = bio_list_pop(&bios)))
>> generic_make_request(bio);
>> }
>> @@ -2954,6 +2998,7 @@ static struct pool *pool_create(struct mapped_device *pool_md,
>> INIT_DELAYED_WORK(&pool->no_space_timeout, do_no_space_timeout);
>> spin_lock_init(&pool->lock);
>> bio_list_init(&pool->deferred_flush_bios);
>> + bio_list_init(&pool->deferred_flush_completions);
>> INIT_LIST_HEAD(&pool->prepared_mappings);
>> INIT_LIST_HEAD(&pool->prepared_discards);
>> INIT_LIST_HEAD(&pool->prepared_discards_pt2);
>> --
>> 2.11.0
>>
>
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