Re: [PATCH] NFS: Use GFP_NOFS in nfs_direct_req_alloc

[Chuck Lever <chuck.lever@oracle.com>]

On Sep 8, 2009, at 6:32 PM, Trond Myklebust wrote:
> On Tue, 2009-09-08 at 18:05 -0400, Chuck Lever wrote:
>> Don't dive into memory reclaim in the NFS direct I/O paths, otherwise
>> we can deadlock.
>>
>> Reported by: Wengang Wang <wen.gang.wang@oracle.com>
>> Fix-suggested-by: Zach Brown <zach.brown@oracle.com>
>> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
>
> Wait... What??? How does an O_DIRECT read or write allocation deadlock
> with memory reclaim? Both the read and the write path call
> nfs_direct_req_alloc() before they pin any user pages in memory.

This may be an issue only for loopback mounts where the backing device  
is an NFS O_DIRECT file.  This type of deadlock may not be able to  
happen in upstream kernels at this point.

Even so, it makes sense for this allocation to be consistent with  
similar allocations in the other NFS I/O paths.

> Trond
>
>> ---
>>
>> Trond-
>>
>> Thoughts?
>>
>> fs/nfs/direct.c |    2 +-
>> 1 files changed, 1 insertions(+), 1 deletions(-)
>>
>> diff --git a/fs/nfs/direct.c b/fs/nfs/direct.c
>> index e4e089a..00f06b8 100644
>> --- a/fs/nfs/direct.c
>> +++ b/fs/nfs/direct.c
>> @@ -149,7 +149,7 @@ static inline struct nfs_direct_req  
>> *nfs_direct_req_alloc(void)
>> {
>> 	struct nfs_direct_req *dreq;
>>
>> -	dreq = kmem_cache_alloc(nfs_direct_cachep, GFP_KERNEL);
>> +	dreq = kmem_cache_alloc(nfs_direct_cachep, GFP_NOFS);
>> 	if (!dreq)
>> 		return NULL;
>>
>>
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>
> -- 
> Trond Myklebust
> Linux NFS client maintainer
>
> NetApp
> Trond.Myklebust@netapp.com
> www.netapp.com
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--
Chuck Lever
chuck[dot]lever[at]oracle[dot]com