linux-kernel - Re: [PATCH] NFS: Fix O_DIRECT read problem when another write is going on

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Message-ID: <c869daa9-cb66-6221-0a3b-c73fa9c39066@cn.fujitsu.com>
Date:   Mon, 7 Oct 2019 10:17:16 +0800
From:   Su Yanjun <suyj.fnst@...fujitsu.com>
To:     Trond Myklebust <trondmy@...merspace.com>
CC:     "linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] NFS: Fix O_DIRECT read problem when another write is
 going on


在 2019/10/1 2:06, Trond Myklebust 写道:
> Hi Su,
>
> On Mon, 2019-09-30 at 17:11 +0800, Su Yanjun wrote:
>> In xfstests generic/465 tests failed. Because O_DIRECT r/w use
>> async rpc calls, when r/w rpc calls are running concurrently we
>> may read partial data which is wrong.
>>
>> For example as follows.
>>   user buffer
>> /--------\
>>>     |XXXX|
>>   rpc0 rpc1
>>
>> When rpc0 runs it encounters eof so return 0, then another writes
>> something. When rpc1 runs it returns some data. The total data
>> buffer contains wrong data.
>>
>> In this patch we check eof mark for each direct request. If
>> encounters
>> eof then set eof mark in the request, when we meet it again report
>> -EAGAIN error. In nfs_direct_complete we convert -EAGAIN as if read
>> nothing. When the reader issue another read it will read ok.
>>
>> Signed-off-by: Su Yanjun <suyj.fnst@...fujitsu.com>
>> ---
>>   fs/nfs/direct.c | 14 +++++++++++++-
>>   1 file changed, 13 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/nfs/direct.c b/fs/nfs/direct.c
>> index 222d711..7f737a3 100644
>> --- a/fs/nfs/direct.c
>> +++ b/fs/nfs/direct.c
>> @@ -93,6 +93,7 @@ struct nfs_direct_req {
>>   				bytes_left,	/* bytes left to be
>> sent */
>>   				error;		/* any reported error
>> */
>>   	struct completion	completion;	/* wait for i/o completion */
>> +	int			eof;		/* eof mark in the
>> req */
>>   
>>   	/* commit state */
>>   	struct nfs_mds_commit_info mds_cinfo;	/* Storage for cinfo
>> */
>> @@ -380,6 +381,12 @@ static void nfs_direct_complete(struct
>> nfs_direct_req *dreq)
>>   {
>>   	struct inode *inode = dreq->inode;
>>   
>> +	/* read partial data just as read nothing */
>> +	if (dreq->error == -EAGAIN) {
>> +		dreq->count = 0;
>> +		dreq->error = 0;
>> +	}
>> +
>>   	inode_dio_end(inode);
>>   
>>   	if (dreq->iocb) {
>> @@ -413,8 +420,13 @@ static void nfs_direct_read_completion(struct
>> nfs_pgio_header *hdr)
>>   	if (hdr->good_bytes != 0)
>>   		nfs_direct_good_bytes(dreq, hdr);
>>   
>> -	if (test_bit(NFS_IOHDR_EOF, &hdr->flags))
>> +	if (dreq->eof)
>> +		dreq->error = -EAGAIN;
>> +
>> +	if (test_bit(NFS_IOHDR_EOF, &hdr->flags)) {
>>   		dreq->error = 0;
>> +		dreq->eof = 1;
>> +	}
>>   
>>   	spin_unlock(&dreq->lock);
>>   
> Thanks for looking into this issue. I agree with your analysis of what
> is going wrong in generic/465.
>
> However, I think the problem is greater than just EOF. I think we also
> need to look at the generic error handling, and ensure that it handles
> a truncated RPC call in the middle of a series of calls correctly.
>
> Please see the two patches I sent you just now and check if they fix
> the problem for you.

The patchset you sent works for generic/465.

Thanks a lot