Re: [PATCH v6 07/10] SUNRPC: Add the ability to expand holes in data pages

[Trond Myklebust <trondmy@hammerspace.com>]

On Mon, 2020-09-28 at 13:09 -0400, schumaker.anna@gmail.com wrote:
> From: Anna Schumaker <Anna.Schumaker@Netapp.com>
> 
> This patch adds the ability to "read a hole" into a set of XDR data
> pages by taking the following steps:
> 
> 1) Shift all data after the current xdr->p to the right, possibly
> into
>    the tail,
> 2) Zero the specified range, and
> 3) Update xdr->p to point beyond the hole.
> 
> Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
> ---
>  include/linux/sunrpc/xdr.h |  1 +
>  net/sunrpc/xdr.c           | 73
> ++++++++++++++++++++++++++++++++++++++
>  2 files changed, 74 insertions(+)
> 
> diff --git a/include/linux/sunrpc/xdr.h b/include/linux/sunrpc/xdr.h
> index 026edbd041d5..36a81c29542e 100644
> --- a/include/linux/sunrpc/xdr.h
> +++ b/include/linux/sunrpc/xdr.h
> @@ -252,6 +252,7 @@ extern __be32 *xdr_inline_decode(struct
> xdr_stream *xdr, size_t nbytes);
>  extern unsigned int xdr_read_pages(struct xdr_stream *xdr, unsigned
> int len);
>  extern void xdr_enter_page(struct xdr_stream *xdr, unsigned int
> len);
>  extern int xdr_process_buf(struct xdr_buf *buf, unsigned int offset,
> unsigned int len, int (*actor)(struct scatterlist *, void *), void
> *data);
> +extern uint64_t xdr_expand_hole(struct xdr_stream *, uint64_t,
> uint64_t);
>  
>  /**
>   * xdr_stream_remaining - Return the number of bytes remaining in
> the stream
> diff --git a/net/sunrpc/xdr.c b/net/sunrpc/xdr.c
> index d8c9555c6f2b..24baf052e6e6 100644
> --- a/net/sunrpc/xdr.c
> +++ b/net/sunrpc/xdr.c
> @@ -390,6 +390,42 @@ _copy_from_pages(char *p, struct page **pages,
> size_t pgbase, size_t len)
>  }
>  EXPORT_SYMBOL_GPL(_copy_from_pages);
>  
> +/**
> + * _zero_pages
> + * @pages: array of pages
> + * @pgbase: beginning page vector address
> + * @len: length
> + */
> +static void
> +_zero_pages(struct page **pages, size_t pgbase, size_t len)
> +{
> +	struct page **page;
> +	char *vpage;
> +	size_t zero;
> +
> +	page = pages + (pgbase >> PAGE_SHIFT);
> +	pgbase &= ~PAGE_MASK;
> +
> +	do {
> +		zero = PAGE_SIZE - pgbase;
> +		if (zero > len)
> +			zero = len;
> +
> +		vpage = kmap_atomic(*page);
> +		memset(vpage + pgbase, 0, zero);
> +		kunmap_atomic(vpage);
> +
> +		pgbase += zero;
> +		if (pgbase == PAGE_SIZE) {

Hmm.... Do we need to make this conditional? After all, if pgbase !=
PAGE_SIZE in this case, then zero == len, in which case we still want
to flush_dcache_page(*page) and then exit.

> +			flush_dcache_page(*page);
> +			pgbase = 0;
> +			page++;
> +		}
> +
> +	} while ((len -= zero) != 0);
> +	flush_dcache_page(*page);
> +}
> +
>  /**
>   * xdr_shrink_bufhead
>   * @buf: xdr_buf
> @@ -1141,6 +1177,43 @@ unsigned int xdr_read_pages(struct xdr_stream
> *xdr, unsigned int len)
>  }
>  EXPORT_SYMBOL_GPL(xdr_read_pages);
>  
> +uint64_t xdr_expand_hole(struct xdr_stream *xdr, uint64_t offset,
> uint64_t length)
> +{
> +	struct xdr_buf *buf = xdr->buf;
> +	unsigned int bytes;
> +	unsigned int from;
> +	unsigned int truncated = 0;
> +
> +	if ((offset + length) < offset ||
> +	    (offset + length) > buf->page_len)
> +		length = buf->page_len - offset;
> +
> +	xdr_realign_pages(xdr);
> +	from = xdr_page_pos(xdr);
> +	bytes = xdr->nwords << 2;

Hmm... Won't this mean that we also include the padding at the end of
the read buffer in 'bytes'?

> +
> +	if (offset + length + bytes > buf->page_len) {
> +		unsigned int shift = (offset + length + bytes) - buf-
> >page_len;
> +		unsigned int res = _shift_data_right_tail(buf, from +
> bytes - shift, shift);
> +		truncated = shift - res;
> +		xdr->nwords -= XDR_QUADLEN(truncated);
> +		bytes -= shift;
> +	}
> +
> +	/* Now move the page data over and zero pages */
> +	if (bytes > 0)
> +		_shift_data_right_pages(buf->pages,
> +					buf->page_base + offset +
> length,
> +					buf->page_base + from,
> +					bytes);
> +	_zero_pages(buf->pages, buf->page_base + offset, length);
> +
> +	buf->len += length - (from - offset) - truncated;
> +	xdr_set_page(xdr, offset + length, PAGE_SIZE);
> +	return length;
> +}
> +EXPORT_SYMBOL_GPL(xdr_expand_hole);
> +
>  /**
>   * xdr_enter_page - decode data from the XDR page
>   * @xdr: pointer to xdr_stream struct
-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@hammerspace.com