Re: Broken nfsd in recent kernels

[Nick Piggin <nickpiggin@yahoo.com.au>]

Neil Brown wrote:
> On Tuesday February 13, nickpiggin@yahoo.com.au wrote:

>>It would be interesting to know whether Norman's test case actually is
>>using writev...
> 
> 
> He is just use NFS.  NFSD does use writev.

OK that makes a lot of sense.

> A typical 32K write arrives as a bunch of IP packets most of which
> hold 1448 bytes.  These are all presented to the filesystem in a
> writev (vfs_writev actually).
> 
> I presume the problem is that we cannot fault_in_pages_readable two
> different buffers as the first might disappear while the second is
> being paged in....

Yeah that, and also I don't think we ever actually did the
fault_in_pages_readable for subsequent segments past the first one,
so it could be quite trivial to trigger the problem delibrately.

> Would it be possible to count how much of the iovec is in
> kernel-space, or maybe how much is *not* part of the file being
> written too, and allow that much to be processed all at once?
> Or is there something more subtle that I am missing?
> 
> The following patch is rather gross, but seems to work and should be
> safe... what do you think?

I think it seems like a very good idea. There is no reason to worry
about faults if we're dealing with kernel constructed buffers.

> 
> NeilBrown
> 
> Signed-off-by: Neil Brown <neilb@suse.de>
> 
> diff .prev/mm/filemap.c ./mm/filemap.c
> --- .prev/mm/filemap.c	2007-02-13 15:10:32.000000000 +1100
> +++ ./mm/filemap.c	2007-02-13 15:19:20.000000000 +1100
> @@ -2163,9 +2163,11 @@ generic_file_buffered_write(struct kiocb
>  		/*
>  		 * Limit the size of the copy to that of the current segment,
>  		 * because fault_in_pages_readable() doesn't know how to walk
> -		 * segments.
> +		 * segments, but don't worry about such technicalities if nfsd
> +		 * is writing, as prefault isn't needed then.
>  		 */
> -		bytes = min(bytes, cur_iov->iov_len - iov_base);
> +		if (!segment_eq(get_fs(), KERNEL_DS))
> +			bytes = min(bytes, cur_iov->iov_len - iov_base);
>  
>  		/*
>  		 * Bring in the user page that we will copy from _first_.
> @@ -2173,7 +2175,8 @@ generic_file_buffered_write(struct kiocb
>  		 * same page as we're writing to, without it being marked
>  		 * up-to-date.
>  		 */
> -		fault_in_pages_readable(buf, bytes);
> +		if (!segment_eq(get_fs(), KERNEL_DS))
> +			fault_in_pages_readable(buf, bytes);
>  
>  		page = __grab_cache_page(mapping,index,&cached_page,&lru_pvec);
>  		if (!page) {
> 


-- 
SUSE Labs, Novell Inc.
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