[LU-15092] Fix logic for unaligned transfer with o2iblnd

• Type: Bug
• Resolution: Fixed
• Priority: Minor
• Fix Version/s: Lustre 2.15.0
• Severity: 3
• Assignee: Chris Horn
• Reporter: Chris Horn

Description

It's possible for there to be an offset for the first page of a
transfer. However, there are two bugs with this code in o2iblnd.

The first is that this use-case will require LNET_MAX_IOV + 1 local
RDMA fragments, but we do not specify the correct corresponding values
for the max page list to ib_alloc_fast_reg_page_list(),
ib_alloc_fast_reg_mr(), etc.

The second issue is that the logic in kiblnd_setup_rd_iov() and
kiblnd_setup_rd_kiov() attempts to obtain one more scatterlist entry
than is actually needed. This causes the transfer to fail with -EFAULT.

Issue Links

• is related to: LU-13802 New i/o path: Buffered i/o as DIO

Comments

[Patrick Farrell]

ashehata , just FYI, Andreas' most recent comment is a much clearer statement of my question.

[Patrick Farrell]

I actually asked the same question - not stated as clearly - on https://jira.whamcloud.com/browse/LU-13805 

[Andreas Dilger]

Chris, Sereguei,
we were having a discussion related to LU-13802 (improving buffered read/write efficiency) about whether it is possible to have LNet do RDMA from "very" unaligned buffers on the client (i.e. page-relative memory offset does not match file-relative offset) into page+block-aligned buffers on the server?

For example, if an application allocates a 1MB buffer in userspace today with glibc malloc(), it is only guaranteed to be aligned on the word size (i.e. 8 bytes). If the client tries to write this unaligned 1MB buffer to a 1MB file-aligned offset, the kernel has to copy all of the data into aligned kernel page cache and then send those page cache pages to LNet for RDMA.

It would be ideal for large read/write operations if the client LNet could RDMA the unaligned userspace buffer directly into aligned server pages with O_DIRECT, but I don't know if this is a capability that LNet and/or IB/RoCE have, or they require the source/target page alignment to be the same? If this isn't possible, that is totally fine, and we are looking into other solutions to improve performance here, but when I saw this patch recently I just wanted to make sure that there isn't some easy "of course the data does not need to be page aligned" solution that we are missing.

[Peter Jones]

Landed for 2.15