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Subject: Re: [PATCH 1/1] NFSv4.2: fix LISTXATTR buffer receive size
From:   Chuck Lever <chuck.lever@oracle.com>
In-Reply-To: <CAN-5tyEJ4Lbf=Ht2P4gwd9y4EPvN=G6teAiaunL=Ayxox8MSdg@mail.gmail.com>
Date:   Mon, 23 Nov 2020 12:37:15 -0500
Cc:     Frank van der Linden <fllinden@amazon.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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To:     Olga Kornievskaia <olga.kornievskaia@gmail.com>
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> On Nov 23, 2020, at 11:42 AM, Olga Kornievskaia =
<olga.kornievskaia@gmail.com> wrote:
>=20
> Hi Frank, Chuck,
>=20
> 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.
>=20
> 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.
>=20
> 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.
>=20
> Question: Should there be code added to RDMA that will do something
> similar when it sees that flag set?

Isn't the logic in rpcrdma_convert_iovs() allocating those pages?


> Or, should LISTXATTR be re-written
> to be like READDIR which allocates pages before calling the code.

AIUI READDIR reads into the directory inode's page cache. I recall
that Frank couldn't do that for LISTXATTR because there's no
similar page cache associated with the xattr listing.

That said, I would prefer that the *XATTR procedures directly
allocate pages instead of relying on SPARSE_PAGES, which is a hack
IMO. I think it would have to use alloc_page() for that, and then
ensure those pages are released when the call has completed.

I'm not convinced this is the cause of the problem you're seeing,
though.

--
Chuck Lever