linux-kernel - Re: algif_hash: splice of data

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Date:	Wed, 24 Dec 2014 16:12:53 +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 Mittwoch, 24. Dezember 2014, 15:10:14 schrieb leroy christophe:

Hi leroy,

> 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.

That is not the only one. Vmsplice works on a pipe. A pipe is backed by 16 
pages max. Thus, you have to call vmsplice once per 16 pages. The current code 
I am testing which seems to work is the following:

	while (inlen) {
		size_t datalen = (inlen > MAXLEN) ? MAXLEN : inlen;

		iov.iov_len = datalen;
		ret = _kcapi_common_vmsplice_data(handle, &iov, 1, datalen, 
1);
		if (0 > ret)
			return ret;
		processed += ret;
		iov.iov_base = (void*)(uintptr_t)(in + processed);
		inlen -= ret;
	}
	return _kcapi_common_read_data(handle, out, outlen);


-- 
Ciao
Stephan
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