linux-kernel - Re: algif_hash: splice of data

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Message-ID: <11733044.fQnf94a6to@tachyon.chronox.de>
Date:	Wed, 24 Dec 2014 10:03:13 +0100
From:	Stephan Mueller <smueller@...onox.de>
To:	leroy christophe <christophe.leroy@....fr>
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

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.

-- 
Ciao
Stephan
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