Re: algif_hash: splice of data > 2**16

Stephan Mueller
2014-12-24T09:03:24Z

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