Re: [PATCH 3/3] raid5: allow r5l_io_unit allocations to fail

[NeilBrown <neilb@suse.com>]

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On Wed, Dec 23 2015, Christoph Hellwig wrote:

>> I wonder if we should have a mempool for these io units too.
>> We would allocate with GFP_ATOMIC (or similar) so the allocation woult
>> fail instead of blocking, but we would then know that an allocation
>> could only fail if there was another request in flight.  So the place
>> where we free an io_unit would be the obviously correct place to trigger
>> a retry of the delayed-due-to-mem-allocation-failure stripes.
>> 
>> So I think I would prefer two lists, another mempool, and very well
>> defined places to retry the two lists.  Is that over-engineering?
>
> How about the variant below (relative to md/for-next)?  This implements
> the above and passes testing fine:
>
> diff --git a/drivers/md/raid5-cache.c b/drivers/md/raid5-cache.c
> index 18de1fc..4fa9457 100644
> --- a/drivers/md/raid5-cache.c
> +++ b/drivers/md/raid5-cache.c
> @@ -75,7 +75,10 @@ struct r5l_log {
>  	struct list_head finished_ios;	/* io_units which settle down in log disk */
>  	struct bio flush_bio;
>  
> +	struct list_head no_mem_stripes;   /* pending stripes, -ENOMEM */
> +
>  	struct kmem_cache *io_kc;
> +	mempool_t *io_pool;
>  	struct bio_set *bs;
>  	mempool_t *meta_pool;
>  
> @@ -287,9 +290,10 @@ static struct r5l_io_unit *r5l_new_meta(struct r5l_log *log)
>  	struct r5l_io_unit *io;
>  	struct r5l_meta_block *block;
>  
> -	io = kmem_cache_zalloc(log->io_kc, GFP_ATOMIC);
> +	io = mempool_alloc(log->io_pool, GFP_ATOMIC);
>  	if (!io)
>  		return NULL;
> +	memset(io, 0, sizeof(*io));
>  
>  	io->log = log;
>  	INIT_LIST_HEAD(&io->log_sibling);
> @@ -490,24 +494,25 @@ int r5l_write_stripe(struct r5l_log *log, struct stripe_head *sh)
>  	mutex_lock(&log->io_mutex);
>  	/* meta + data */
>  	reserve = (1 + write_disks) << (PAGE_SHIFT - 9);
> -	if (!r5l_has_free_space(log, reserve))
> -		goto err_retry;
> +	if (!r5l_has_free_space(log, reserve)) {
> +		spin_lock(&log->no_space_stripes_lock);
> +		list_add_tail(&sh->log_list, &log->no_space_stripes);
> +		spin_unlock(&log->no_space_stripes_lock);
> +
> +		r5l_wake_reclaim(log, reserve);
> +		goto out_unlock;
> +	}
>  
>  	ret = r5l_log_stripe(log, sh, data_pages, parity_pages);
> -	if (ret)
> -		goto err_retry;
> +	if (ret) {
> +		spin_lock_irq(&log->io_list_lock);
> +		list_add_tail(&sh->log_list, &log->no_mem_stripes);
> +		spin_unlock_irq(&log->io_list_lock);
> +	}
>  
>  out_unlock:
>  	mutex_unlock(&log->io_mutex);
>  	return 0;
> -
> -err_retry:
> -	spin_lock(&log->no_space_stripes_lock);
> -	list_add_tail(&sh->log_list, &log->no_space_stripes);
> -	spin_unlock(&log->no_space_stripes_lock);
> -
> -	r5l_wake_reclaim(log, reserve);
> -	goto out_unlock;
>  }
>  
>  void r5l_write_stripe_run(struct r5l_log *log)
> @@ -559,6 +564,21 @@ static sector_t r5l_reclaimable_space(struct r5l_log *log)
>  				 log->next_checkpoint);
>  }
>  
> +static void r5l_run_no_mem_stripe(struct r5l_log *log)
> +{
> +	struct stripe_head *sh;
> +
> +	assert_spin_locked(&log->io_list_lock);
> +
> +	if (!list_empty(&log->no_mem_stripes)) {
> +		sh = list_first_entry(&log->no_mem_stripes,
> +				      struct stripe_head, log_list);
> +		list_del_init(&sh->log_list);
> +		set_bit(STRIPE_HANDLE, &sh->state);
> +		raid5_release_stripe(sh);
> +	}
> +}
> +
>  static bool r5l_complete_finished_ios(struct r5l_log *log)
>  {
>  	struct r5l_io_unit *io, *next;
> @@ -575,7 +595,8 @@ static bool r5l_complete_finished_ios(struct r5l_log *log)
>  		log->next_cp_seq = io->seq;
>  
>  		list_del(&io->log_sibling);
> -		kmem_cache_free(log->io_kc, io);
> +		mempool_free(io, log->io_pool);
> +		r5l_run_no_mem_stripe(log);
>  
>  		found = true;
>  	}
> @@ -1189,6 +1210,10 @@ int r5l_init_log(struct r5conf *conf, struct md_rdev *rdev)
>  	if (!log->io_kc)
>  		goto io_kc;
>  
> +	log->io_pool = mempool_create_slab_pool(R5L_POOL_SIZE, log->io_kc);
> +	if (!log->io_pool)
> +		goto io_pool;
> +
>  	log->bs = bioset_create(R5L_POOL_SIZE, 0);
>  	if (!log->bs)
>  		goto io_bs;
> @@ -1203,6 +1228,8 @@ int r5l_init_log(struct r5conf *conf, struct md_rdev *rdev)
>  		goto reclaim_thread;
>  	init_waitqueue_head(&log->iounit_wait);
>  
> +	INIT_LIST_HEAD(&log->no_mem_stripes);
> +
>  	INIT_LIST_HEAD(&log->no_space_stripes);
>  	spin_lock_init(&log->no_space_stripes_lock);
>  
> @@ -1219,6 +1246,8 @@ reclaim_thread:
>  out_mempool:
>  	bioset_free(log->bs);
>  io_bs:
> +	mempool_destroy(log->io_pool);
> +io_pool:
>  	kmem_cache_destroy(log->io_kc);
>  io_kc:
>  	kfree(log);

Yes, that looks just right - thanks.
I feel a lot more confident that this code won't deadlock.

Thanks,
NeilBrown

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