Re: [RFC PATCH 0/2] some progress on rpcrdma bug [was: Re: [PATCH v8 5/7] NFSD: issue READs using O_DIRECT even if IO is misaligned]

[Mike Snitzer <snitzer@kernel.org>]

On Sat, Aug 30, 2025 at 01:38:50PM -0400, Mike Snitzer wrote:
> Hi Chuck,
> 
> Hopeful these 2 patches more clearly demonstrate what I'm finding
> needed when using RDMA with my NFSD misaligned DIO READ patch.
> 
> These patches build ontop of my v8 patchset. I've included quite a lot
> of context for the data mismatch seen by the NFS client, etc in the
> patch headers.
> 
> If I'm understanding you correctly, next step is to look closer at the
> rpcrdma code that would skip the throwaway front-pad page from being
> mapped to the start of the RDMA READ payload returned to the NFS
> client?
> 
> Such important adjustment code would need to know that the rq_bvec[]
> that reflects the READ payload doesn't include a bvec that points to
> the first page of rqstp->rq_pages (pointed to by rqstp->rq_next_page
> on entry to nfsd_iter_read) -- so it must skip past that memory in
> the READ payload's RDMA memory returned to NFS client?
> 
> Thanks,
> Mike
> 
> Mike Snitzer (2):
>   NFSD: fix misaligned DIO READ to not use a start_extra_page, exposes rpcrdma bug?
>   NFSD: use /end/ of rq_pages for front_pad page, simpler workaround for rpcrdma bug
> 
>  fs/nfsd/vfs.c | 27 ++++++++-------------------
>  1 file changed, 8 insertions(+), 19 deletions(-)
> 
> -- 
> 2.44.0
> 

Another important detail to mention, I've been working ontop of a
6.12.24 stable baseline that I've backported all NFS and NFSD changes
through 6.17-rc1 + (slightly stale) nfsd-testing to, please see:

https://git.kernel.org/pub/scm/linux/kernel/git/snitzer/linux.git/log/?h=kernel-6.12.24/nfsd-testing-snitm.18-testing

Not sure if rebasing to the latest nfsd-testing would show this
issue... I'm thinking yes, but cannot say for sure.  Needing the
ability to test/develop Hammerspace and pNFS stuff has prevented me
from simply rebasing.  But it should be doable without too much
effort.

Mike