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