linux-kernel - Re: [RFC PATCH] NFS: Save 4 bytes when re-exporting

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Date:   Sun, 12 Dec 2021 22:51:13 +0100 (CET)
From:   Richard Weinberger <richard@....at>
To:     Trond Myklebust <trondmy@...merspace.com>
Cc:     linux-nfs <linux-nfs@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        anna schumaker <anna.schumaker@...app.com>,
        david <david@...ma-star.at>
Subject: Re: [RFC PATCH] NFS: Save 4 bytes when re-exporting

----- Ursprüngliche Mail -----
> Von: "richard" <richard@....at>
> ----- Ursprüngliche Mail -----
>> Von: "Trond Myklebust" <trondmy@...merspace.com>
>> On Sun, 2021-12-12 at 22:00 +0100, Richard Weinberger wrote:
>>> When re-exporting, the whole struct nfs_fh is embedded in the new
>>> fhandle.
>>> But we need only nfs_fh->data[], nfs_fh->size is not needed.
>>> So skip fs_fh->size and save a full word (4 bytes).
>>> The downside is the extra memcpy() in nfs_fh_to_dentry().
>>> 
>>> Signed-off-by: Richard Weinberger <richard@....at>
>>> ---
>>> While investigating into improving NFS re-export I noticed that
>>> we can already save 4 bytes of overhead.
>>> I don't think we need to embedd the full struct nfs_fh and
>>> can skip ->size.
>>> 
>> 
>> NACK. This will break existing running clients. Any code to change the
>> filehandle format must be accompanied with code to detect and service
>> filehandles that are presented in the old format.
> 
> One possible way to distinguish between old and new formats
> is looking at the length of the data.
> 2 * XDR_UNIT = new format (without ->size).
> 3 * XDR_UNIT = old format.

Never mind. This won't work.

Thanks,
//richard