linux-kernel - Re: [PATCH] lightnvm: add full block direct to the gc list

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Date:	Tue, 5 Jan 2016 17:58:06 +0800
From:	Wenwei Tao <ww.tao0320@...il.com>
To:	Matias Bjørling <mb@...htnvm.io>
Cc:	linux-kernel@...r.kernel.org, linux-block@...r.kernel.org
Subject: Re: [PATCH] lightnvm: add full block direct to the gc list

You are right, a deadlock might occur if interrupt is not disabled.

We might add the block to prio_list when we find the block is full in
rrpc_alloc_addr and check whether all the writes are complete in
rrpc_lun_gc, in this way we may avoid gcb allocation fail and irq
disable issues.

But this still has a problem. We allocate page from block before
write, but the bio submission may fail, the bio never get execute and
rrpc_end_io never get called on this bio, this may lead to a
situation: a block's pages are all allocated, but not all of them are
used. So this block is not fully used now, and will not get reclaimed
for further use.

I think we may need to put the page back when the page is not actually
used/programmed.

2016-01-04 19:24 GMT+08:00 Matias Bjørling <mb@...htnvm.io>:
> On 01/04/2016 10:54 AM, Wenwei Tao wrote:
>>
>> We allocate gcb to queue full block to the gc list,
>> but gcb allocation may fail, if that happens, the
>> block will not get reclaimed. So add the full block
>> direct to the gc list, omit the queuing step.
>>
>> Signed-off-by: Wenwei Tao <ww.tao0320@...il.com>
>> ---
>>   drivers/lightnvm/rrpc.c | 47
>> ++++++++++-------------------------------------
>>   1 file changed, 10 insertions(+), 37 deletions(-)
>>
>> diff --git a/drivers/lightnvm/rrpc.c b/drivers/lightnvm/rrpc.c
>> index 40b0309..27fb98d 100644
>> --- a/drivers/lightnvm/rrpc.c
>> +++ b/drivers/lightnvm/rrpc.c
>> @@ -475,24 +475,6 @@ static void rrpc_lun_gc(struct work_struct *work)
>>         /* TODO: Hint that request queue can be started again */
>>   }
>>
>> -static void rrpc_gc_queue(struct work_struct *work)
>> -{
>> -       struct rrpc_block_gc *gcb = container_of(work, struct
>> rrpc_block_gc,
>> -
>> ws_gc);
>> -       struct rrpc *rrpc = gcb->rrpc;
>> -       struct rrpc_block *rblk = gcb->rblk;
>> -       struct nvm_lun *lun = rblk->parent->lun;
>> -       struct rrpc_lun *rlun = &rrpc->luns[lun->id - rrpc->lun_offset];
>> -
>> -       spin_lock(&rlun->lock);
>> -       list_add_tail(&rblk->prio, &rlun->prio_list);
>> -       spin_unlock(&rlun->lock);
>> -
>> -       mempool_free(gcb, rrpc->gcb_pool);
>> -       pr_debug("nvm: block '%lu' is full, allow GC (sched)\n",
>> -                                                       rblk->parent->id);
>> -}
>> -
>>   static const struct block_device_operations rrpc_fops = {
>>         .owner          = THIS_MODULE,
>>   };
>> @@ -620,39 +602,30 @@ err:
>>         return NULL;
>>   }
>>
>> -static void rrpc_run_gc(struct rrpc *rrpc, struct rrpc_block *rblk)
>> -{
>> -       struct rrpc_block_gc *gcb;
>> -
>> -       gcb = mempool_alloc(rrpc->gcb_pool, GFP_ATOMIC);
>> -       if (!gcb) {
>> -               pr_err("rrpc: unable to queue block for gc.");
>> -               return;
>> -       }
>> -
>> -       gcb->rrpc = rrpc;
>> -       gcb->rblk = rblk;
>> -
>> -       INIT_WORK(&gcb->ws_gc, rrpc_gc_queue);
>> -       queue_work(rrpc->kgc_wq, &gcb->ws_gc);
>> -}
>> -
>>   static void rrpc_end_io_write(struct rrpc *rrpc, struct rrpc_rq *rrqd,
>>                                                 sector_t laddr, uint8_t
>> npages)
>>   {
>>         struct rrpc_addr *p;
>>         struct rrpc_block *rblk;
>>         struct nvm_lun *lun;
>> +       struct rrpc_lun *rlun;
>>         int cmnt_size, i;
>>
>>         for (i = 0; i < npages; i++) {
>>                 p = &rrpc->trans_map[laddr + i];
>>                 rblk = p->rblk;
>>                 lun = rblk->parent->lun;
>> +               rlun = &rrpc->luns[lun->id - rrpc->lun_offset];
>>
>>                 cmnt_size = atomic_inc_return(&rblk->data_cmnt_size);
>> -               if (unlikely(cmnt_size == rrpc->dev->pgs_per_blk))
>> -                       rrpc_run_gc(rrpc, rblk);
>> +               if (unlikely(cmnt_size == rrpc->dev->pgs_per_blk)) {
>> +                       pr_debug("nvm: block '%lu' is full, allow GC
>> (sched)\n",
>> +                                                       rblk->parent->id);
>> +                       spin_lock(&rlun->lock);
>
>
> A deadlock might occur, as the lock can be called from interrupt context.
> The other ->rlun usages will have to be converted to
> spinlock_irqsave/spinlock_irqrestore to be valid.
>
> The reason for the queueing is that the ->rlun lock is held for a while in
> rrpc_lun_gc. Therefore, it rather takes the queueing overhead, than disable
> interrupts on the CPU for the duration of the ->prio sorting and selection
> of victim block. My assumptions about this optimization might be premature.
> So I like to be proved wrong.
>
>
>> +                       list_add_tail(&rblk->prio, &rlun->prio_list);
>> +                       spin_unlock(&rlun->lock);
>> +
>> +               }
>>         }
>>   }
>>
>>
>
>
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