Re: [PATCH v1 4/4] svcrdma: use bvec-based RDMA read/write API

[Christoph Hellwig <hch@lst.de>]

On Thu, Jan 15, 2026 at 04:53:23PM -0500, Chuck Lever wrote:
> These are contiguous because the xdr_buf "pages" field is an array
> of "struct page *" pointers. So these don't have per-entry offsets.
> There is one "page_offset" field in the xdr_buf that applies only
> to the first entry in that array.
> 
> Therefore the payload buffer starts at an offset of zero or greater
> in the first page in that array, but after that, the buffer continues
> across the boundaries of each page from offset 4095 on page N to
> byte 0 of page N+1, for all N.
> 
> The comment is a little misleading -- it documents an assumption
> that is due to each entry of the xdr_buf pages array being "struct
> page *" and there not being an offset field for each entry.

Yeah.  I also realized both the classic RDMA memory registrations,
and the IOVA based DMA mapping requires the subsequent entries to
be at least page aligned.  There is a Mellanox-specific MR type that
doesn't require that, but that obviously doesn't help iWarp, and the
IOVA coalescing is nice to have as well.