Re: [PATCH v1] xprtrdma: Simplify rpcrdma_convert_kvec() and frwr_map()

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> On Feb 2, 2021, at 2:18 PM, Tom Talpey <tom@xxxxxxxxxx> wrote:
> 
> What's not to like about a log that uses the words "with aplomb"? :)
> 
> Minor related comment/question below.
> 
> On 2/2/2021 9:42 AM, Chuck Lever wrote:
>> Clean up.
>> Support for FMR was removed by commit ba69cd122ece ("xprtrdma:
>> Remove support for FMR memory registration") [Dec 2018]. That means
>> the buffer-splitting behavior of rpcrdma_convert_kvec(), added by
>> commit 821c791a0bde ("xprtrdma: Segment head and tail XDR buffers
>> on page boundaries") [Mar 2016], is no longer necessary. FRWR
>> memory registration handles this case with aplomb.
>> A related simplification removes an extra conditional branch from
>> the SGL set-up loop in frwr_map(): Instead of using either
>> sg_set_page() or sg_set_buf(), initialize the mr_page field properly
>> when rpcrdma_convert_kvec() converts the kvec to an SGL entry.
>> frwr_map() can then invoke sg_set_page() unconditionally.
>> Signed-off-by: Chuck Lever <chuck.lever@xxxxxxxxxx>
>> ---
>>  net/sunrpc/xprtrdma/frwr_ops.c  |   10 ++--------
>>  net/sunrpc/xprtrdma/rpc_rdma.c  |   21 +++++----------------
>>  net/sunrpc/xprtrdma/xprt_rdma.h |    2 +-
>>  3 files changed, 8 insertions(+), 25 deletions(-)
>> diff --git a/net/sunrpc/xprtrdma/frwr_ops.c b/net/sunrpc/xprtrdma/frwr_ops.c
>> index baca49fe83af..5eb044a5f0be 100644
>> --- a/net/sunrpc/xprtrdma/frwr_ops.c
>> +++ b/net/sunrpc/xprtrdma/frwr_ops.c
>> @@ -306,14 +306,8 @@ struct rpcrdma_mr_seg *frwr_map(struct rpcrdma_xprt *r_xprt,
>>  	if (nsegs > ep->re_max_fr_depth)
>>  		nsegs = ep->re_max_fr_depth;
>>  	for (i = 0; i < nsegs;) {
>> -		if (seg->mr_page)
>> -			sg_set_page(&mr->mr_sg[i],
>> -				    seg->mr_page,
>> -				    seg->mr_len,
>> -				    offset_in_page(seg->mr_offset));
>> -		else
>> -			sg_set_buf(&mr->mr_sg[i], seg->mr_offset,
>> -				   seg->mr_len);
>> +		sg_set_page(&mr->mr_sg[i], seg->mr_page,
>> +			    seg->mr_len, offset_in_page(seg->mr_offset));
>>    		++seg;
>>  		++i;
>> diff --git a/net/sunrpc/xprtrdma/rpc_rdma.c b/net/sunrpc/xprtrdma/rpc_rdma.c
>> index 8f5d0cb68360..529adb6ad4db 100644
>> --- a/net/sunrpc/xprtrdma/rpc_rdma.c
>> +++ b/net/sunrpc/xprtrdma/rpc_rdma.c
>> @@ -204,9 +204,7 @@ rpcrdma_alloc_sparse_pages(struct xdr_buf *buf)
>>  	return 0;
>>  }
>>  -/* Split @vec on page boundaries into SGEs. FMR registers pages, not
>> - * a byte range. Other modes coalesce these SGEs into a single MR
>> - * when they can.
>> +/* Convert @vec to a single SGL element.
>>   *
>>   * Returns pointer to next available SGE, and bumps the total number
>>   * of SGEs consumed.
>> @@ -215,21 +213,12 @@ static struct rpcrdma_mr_seg *
>>  rpcrdma_convert_kvec(struct kvec *vec, struct rpcrdma_mr_seg *seg,
>>  		     unsigned int *n)
>>  {
>> -	u32 remaining, page_offset;
>> -	char *base;
>> -
>> -	base = vec->iov_base;
>> -	page_offset = offset_in_page(base);
>> -	remaining = vec->iov_len;
>> -	while (remaining) {
>> -		seg->mr_page = NULL;
>> -		seg->mr_offset = base;
>> -		seg->mr_len = min_t(u32, PAGE_SIZE - page_offset, remaining);
>> -		remaining -= seg->mr_len;
>> -		base += seg->mr_len;
>> +	if (vec->iov_len) {
>> +		seg->mr_page = virt_to_page(vec->iov_base);
>> +		seg->mr_offset = vec->iov_base;
>> +		seg->mr_len = vec->iov_len;
>>  		++seg;
>>  		++(*n);
>> -		page_offset = 0;
>>  	}
>>  	return seg;
>>  }
>> diff --git a/net/sunrpc/xprtrdma/xprt_rdma.h b/net/sunrpc/xprtrdma/xprt_rdma.h
>> index 94b28657aeeb..4a9fe6592795 100644
>> --- a/net/sunrpc/xprtrdma/xprt_rdma.h
>> +++ b/net/sunrpc/xprtrdma/xprt_rdma.h
>> @@ -285,7 +285,7 @@ enum {
>>    struct rpcrdma_mr_seg {		/* chunk descriptors */
>>  	u32		mr_len;		/* length of chunk or segment */
>> -	struct page	*mr_page;	/* owning page, if any */
>> +	struct page	*mr_page;	/* underlying struct page */
>>  	char		*mr_offset;	/* kva if no page, else offset */
> 
> Is this comment ("kva if no page") actually correct? The hunk just
> above is an example of a case where mr_page is set, yet mr_offset
> is an iov_base. Is iov_base not a kva?

Ah, well the "if no page" part is now obsolete.

I suppose it should be set to "offset_in_page(vec->iov_base)" ?


--
Chuck Lever







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