linux-kernel - Re: [PATCH] NFS: fix usage of mempools.

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Message-ID: <87r310vt19.fsf@notabene.neil.brown.name>
Date:   Tue, 11 Apr 2017 07:42:26 +1000
From:   NeilBrown <neilb@...e.com>
To:     Anna Schumaker <Anna.Schumaker@...app.com>,
        Trond Myklebust <trond.myklebust@...marydata.com>
Cc:     linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] NFS: fix usage of mempools.

On Mon, Apr 10 2017, Anna Schumaker wrote:

> Hi Neil,
>
> On 04/09/2017 10:22 PM, NeilBrown wrote:
>> 
>> +		/* It is OK to do some reclaim, not no safe to wait
>> +		 * for anything to be returned to the pool.
>> +		 * mempool_alloc() cannot handle that particular combination,
>> +		 * so we need two separate attempts.
>> +		 */
>> +		p = mempool_alloc(nfs_commit_mempool, GFP_NOWAIT);
>> +		if (!p)
>> +			p = kmem_cache_alloc(nfs_cdata_cachep, GFP_NOIO |
>> +					     __GFP_NOWARN | __GFP_NORETRY);
>
> Do we need to add something to the nfs_commit_data structure to
> properly free a kmem_cache_alloc()-ed object?  Right now it looks like
> nfs_commit_free() calls mempool_free() unconditionally. 

Good question.  I should have clarified that in the patch comment.
Can you add this to that comment please.

  The objects returned by kmem_cache_alloc() will still be freed
  by mempool_free().  This is safe as mempool_alloc() uses
  exactly the same function to allocate objects (since the mempool
  was created with mempool_create_slab_pool()).  The object returned
  by mempool_alloc() and kmem_cache_alloc() are indistinguishable
  so mempool_free() will handle both identically, either adding to the
  pool or calling kmem_cache_free().

Thanks,
NeilBrown

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