linux-kernel - Re: algif_hash: splice of data

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Date:	Wed, 24 Dec 2014 15:10:14 +0100
From:	leroy christophe <christophe.leroy@....fr>
To:	Stephan Mueller <smueller@...onox.de>
CC:	Herbert Xu <herbert@...dor.apana.org.au>,
	linux-crypto <linux-crypto@...r.kernel.org>,
	'LKML' <linux-kernel@...r.kernel.org>
Subject: Re: algif_hash: splice of data > 2**16


Le 24/12/2014 10:03, Stephan Mueller a écrit :
> Am Dienstag, 23. Dezember 2014, 18:16:01 schrieb leroy christophe:
>
> Hi leroy,
>
>> Le 20/12/2014 07:37, Stephan Mueller a écrit :
>>> Am Donnerstag, 18. Dezember 2014, 13:22:20 schrieb leroy christophe:
>>>
>>> Hi Christophe,
>>>
>>>> Le 18/12/2014 13:15, Stephan Mueller a écrit :
>>>>> Hi Herbert,
>>>>>
>>>>> While testing the vmsplice/splice interface of algif_hash I was made
>>>>> aware of the problem that data blobs larger than 16 pages do not seem to
>>>>> be hashed properly.
>>>>>
>>>>> For testing, a file is mmap()ed and handed to vmsplice / splice. If the
>>>>> file is smaller than 2**16, the interface returns the proper hash.
>>>>> However, when the file is larger, only the first 2**16 bytes seem to be
>>>>> used.
>>>>>
>>>>> When adding printk's to hash_sendpage, I see that this function is
>>>>> invoked exactly 16 times where the first 15 invocations have the
>>>>> MSG_MORE flag set and the last invocation does not have MSG_MORE.
>>>> Hi Stephan,
>>>>
>>>> I have already noticed the same issue and proposed a patch, but I never
>>>> got any feedback and it has never been merged, allthought I pinged it a
>>>> few times.
>>>>
>>>> See https://lkml.org/lkml/2014/4/18/276
>>> After testing, this patch does not work for me. The operation still stops
>>> after 16 pages.
>> Yes, it looks like the function I fixed is exclusively used by
>> sendfile() system call.
>> So there is probably the same kind of fix to be done in another function.
> I do not believe that is the case. IMHO the blocking issue is found in the
> following code:
>
> splice_from_pipe_feed walks the pipe->nrbufs. And vmsplice_to_pipe defines the
> maximum number of nrbufs as PIPE_DEF_BUFFERS -- which is 16. As subsequent
> functions allocate memory based on PIPE_DEF_BUFFERS, there is no trivial way
> to increase the number of pages to be processed.
>
> Thus I see that the vmsplice/splice combo can at most operate on a chunk of 16
> pages. Thus, you have to segment your input buffer into chunks of that size
> and invoke the vmsplice/splice syscalls for each segment.
>
Yes your are probably right. There splice needs to be called with 
SPLICE_F_MORE flag, hope that works.


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