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Subject: Re: [PATCH 1/1] NFSv4.2: fix LISTXATTR buffer receive size
Date:   Mon, 23 Nov 2020 17:38:02 +0000
From:   Frank van der Linden <fllinden@amazon.com>
To:     Olga Kornievskaia <olga.kornievskaia@gmail.com>
CC:     Chuck Lever <chuck.lever@oracle.com>,
        Trond Myklebust <trond.myklebust@hammerspace.com>,
        Anna Schumaker <anna.schumaker@netapp.com>,
        Linux NFS Mailing List <linux-nfs@vger.kernel.org>
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On Mon, Nov 23, 2020 at 11:42:46AM -0500, Olga Kornievskaia wrote:
> Hi Frank, Chuck,
> 
> I would like your option on how LISTXATTR is supposed to work over
> RDMA. Here's my current understanding of why the listxattr is not
> working over the RDMA.
> 
> This happens when the listxattr is called with a very small buffer
> size which RDMA wants to send an inline request. I really dont
> understand why, Chuck, you are not seeing any problems with hardware
> as far as I can tell it would have the same problem because the inline
> threshold size would still make this size inline.
> rcprdma_inline_fixup() is trying to write to pages that don't exist.
> 
> When LISTXATTR sets this flag XDRBUF_SPARSE_PAGES there is code that
> will allocate pages in xs_alloc_sparse_pages() but this is ONLY for
> TCP. RDMA doesn't have anything like that.
> 
> Question: Should there be code added to RDMA that will do something
> similar when it sees that flag set? Or, should LISTXATTR be re-written
> to be like READDIR which allocates pages before calling the code.

Hm.. so if the flag never worked for RDMA, was NFS_V3_ACL ever tested
over RDMA? That's the only other user.

If the flag simply doesn't work, I agree that it should either be fixed
or just removed.

It wouldn't be the end of the world to change LISTXATTRS (and GETXATTR)
to use preallocated pages. But, I didn't do that because I didn't want to
waste the max size (64k) every time, even though you usually just get
a few hundred bytes at most. So it seems like fixing XDRBUF_SPARSE_PAGES
is cleaner.

- Frank