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Re: [dm-devel] [PATCH] md: dm-verity: Fix to avoid a deadlock in dm-bufio



Looks good to me.

On Tue, Mar 5, 2013 at 3:30 PM, Paul Taysom <taysom google com> wrote:
> seconds_kernel_to_login: 8.17s
> Passed all my other tests
>
> On Mon, Mar 4, 2013 at 2:15 PM, Mikulas Patocka <mpatocka redhat com> wrote:
>>
>>
>> On Mon, 4 Mar 2013, Paul Taysom wrote:
>>
>>> On Mon, Mar 4, 2013 at 9:42 AM, Alasdair G Kergon <agk redhat com> wrote:
>>> > On Mon, Mar 04, 2013 at 08:45:48AM -0800, Paul Taysom wrote:
>>> >> @@ -449,8 +468,14 @@ static void verity_prefetch_io(struct dm_verity *v, struct dm_verity_io *io)
>>> >>                               hash_block_end = v->hash_blocks - 1;
>>> >>               }
>>> >>  no_prefetch_cluster:
>>> >> -             dm_bufio_prefetch(v->bufio, hash_block_start,
>>> >> -                               hash_block_end - hash_block_start + 1);
>>> >> +             vw = kmalloc(sizeof(*vw), GFP_KERNEL);
>>> >
>>> > kmalloc?  mempool? ...
>>> >
>>> > Alasdair
>>> >
>>> The use of mempool would be a separate patch that would have to be
>>> measured for performance impact.
>>> -Paul
>>>
>>
>> You don't have to use mempool. Just avoid prefetching if there is not
>> enough memory for the prefetch structure.
>>
>> I reworked the patch, is uses an allocation that can fail and it generates
>> just one workqueue entry for one request (the original patch generated one
>> workqueue entry for hash tree level).
>>
>> Please test the patch and if it works and performs well, let's submit it.
>>
>> Mikulas
>>
>> ---
>>
>> Changed the dm-verity prefetching to use a worker thread to avoid
>> a deadlock in dm-bufio.
>>
>> If generic_make_request is called recursively, it queues the I/O
>> request on the current->bio_list without making the I/O request
>> and returns. The routine making the recursive call cannot wait
>> for the I/O to complete.
>>
>> The deadlock occurred when one thread grabbed the bufio_client
>> mutex and waited for an I/O to complete but the I/O was queued
>> on another thread's current->bio_list and it was waiting to get
>> the mutex held by the first thread.
>>
>> The fix allows only one I/O request from dm-verity to dm-bufio
>> per thread. To do this, the prefetch requests were queued on worker
>> threads.
>>
>> In addition to avoiding the deadlock, this fix made a slight
>> improvement in performance.
>>
>> seconds_kernel_to_login:
>>   with prefetch:    8.43s
>>   without prefetch: 9.2s
>>   worker prefetch:  8.28s
>>
>> Signed-off-by: Paul Taysom <taysom chromium org>
>> Signed-off-by: Mikulas Patocka <mpatocka redhat com>
>> Cc: stable kernel org
>> ---
>>  drivers/md/dm-bufio.c  |    2 ++
>>  drivers/md/dm-verity.c |   37 +++++++++++++++++++++++++++++++++----
>>  2 files changed, 35 insertions(+), 4 deletions(-)
>>
>> Index: linux-3.8-fast/drivers/md/dm-verity.c
>> ===================================================================
>> --- linux-3.8-fast.orig/drivers/md/dm-verity.c  2013-03-04 22:49:20.000000000 +0100
>> +++ linux-3.8-fast/drivers/md/dm-verity.c       2013-03-04 23:10:45.000000000 +0100
>> @@ -93,6 +93,13 @@ struct dm_verity_io {
>>          */
>>  };
>>
>> +struct dm_verity_prefetch_work {
>> +       struct work_struct work;
>> +       struct dm_verity *v;
>> +       sector_t block;
>> +       unsigned n_blocks;
>> +};
>> +
>>  static struct shash_desc *io_hash_desc(struct dm_verity *v, struct dm_verity_io *io)
>>  {
>>         return (struct shash_desc *)(io + 1);
>> @@ -424,15 +431,18 @@ static void verity_end_io(struct bio *bi
>>   * The root buffer is not prefetched, it is assumed that it will be cached
>>   * all the time.
>>   */
>> -static void verity_prefetch_io(struct dm_verity *v, struct dm_verity_io *io)
>> +static void verity_prefetch_io(struct work_struct *work)
>>  {
>> +       struct dm_verity_prefetch_work *pw =
>> +               container_of(work, struct dm_verity_prefetch_work, work);
>> +       struct dm_verity *v = pw->v;
>>         int i;
>>
>>         for (i = v->levels - 2; i >= 0; i--) {
>>                 sector_t hash_block_start;
>>                 sector_t hash_block_end;
>> -               verity_hash_at_level(v, io->block, i, &hash_block_start, NULL);
>> -               verity_hash_at_level(v, io->block + io->n_blocks - 1, i, &hash_block_end, NULL);
>> +               verity_hash_at_level(v, pw->block, i, &hash_block_start, NULL);
>> +               verity_hash_at_level(v, pw->block + pw->n_blocks - 1, i, &hash_block_end, NULL);
>>                 if (!i) {
>>                         unsigned cluster = ACCESS_ONCE(dm_verity_prefetch_cluster);
>>
>> @@ -452,6 +462,25 @@ no_prefetch_cluster:
>>                 dm_bufio_prefetch(v->bufio, hash_block_start,
>>                                   hash_block_end - hash_block_start + 1);
>>         }
>> +
>> +       kfree(pw);
>> +}
>> +
>> +static void verity_submit_prefetch(struct dm_verity *v, struct dm_verity_io *io)
>> +{
>> +       struct dm_verity_prefetch_work *pw;
>> +
>> +       pw = kmalloc(sizeof(struct dm_verity_prefetch_work),
>> +               GFP_NOIO | __GFP_NORETRY | __GFP_NOMEMALLOC | __GFP_NOWARN);
>> +
>> +       if (!pw)
>> +               return;
>> +
>> +       INIT_WORK(&pw->work, verity_prefetch_io);
>> +       pw->v = v;
>> +       pw->block = io->block;
>> +       pw->n_blocks = io->n_blocks;
>> +       queue_work(v->verify_wq, &pw->work);
>>  }
>>
>>  /*
>> @@ -498,7 +527,7 @@ static int verity_map(struct dm_target *
>>         memcpy(io->io_vec, bio_iovec(bio),
>>                io->io_vec_size * sizeof(struct bio_vec));
>>
>> -       verity_prefetch_io(v, io);
>> +       verity_submit_prefetch(v, io);
>>
>>         generic_make_request(bio);
>>
>> Index: linux-3.8-fast/drivers/md/dm-bufio.c
>> ===================================================================
>> --- linux-3.8-fast.orig/drivers/md/dm-bufio.c   2013-03-04 23:03:14.000000000 +0100
>> +++ linux-3.8-fast/drivers/md/dm-bufio.c        2013-03-04 23:04:19.000000000 +0100
>> @@ -1026,6 +1026,8 @@ void dm_bufio_prefetch(struct dm_bufio_c
>>  {
>>         struct blk_plug plug;
>>
>> +       BUG_ON(dm_bufio_in_request());
>> +
>>         blk_start_plug(&plug);
>>         dm_bufio_lock(c);
>>


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