linux-kernel - Re: [PATCH v2 6/6] staging: lustre: mdc: use large xattr buffers for old servers

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Date:   Fri, 01 Jun 2018 15:38:15 +1000
From:   NeilBrown <neilb@...e.com>
To:     "Dilger\, Andreas" <andreas.dilger@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     James Simmons <jsimmons@...radead.org>,
        "devel\@driverdev.osuosl.org" <devel@...verdev.osuosl.org>,
        "Drokin\, Oleg" <oleg.drokin@...el.com>,
        "Hammond\, John" <john.hammond@...el.com>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        Lustre Development List <lustre-devel@...ts.lustre.org>
Subject: Re: [PATCH v2 6/6] staging: lustre: mdc: use large xattr buffers for old servers

On Thu, May 31 2018, Dilger, Andreas wrote:

> On May 31, 2018, at 18:54, Greg Kroah-Hartman <gregkh@...uxfoundation.org> wrote:
>> 
>> On Tue, May 29, 2018 at 10:21:45AM -0400, James Simmons wrote:
>>> From: "John L. Hammond" <john.hammond@...el.com>
>>> 
>>> Pre 2.10.1 MDTs will crash when they receive a listxattr (MDS_GETXATTR
>>> with OBD_MD_FLXATTRLS) RPC for an orphan or dead object. So for
>>> clients connected to these older MDTs, try to avoid sending listxattr
>>> RPCs by making the bulk getxattr (MDS_GETXATTR with OBD_MD_FLXATTRALL)
>>> more likely to succeed and thereby reducing the chances of falling
>>> back to listxattr.
>>> 
>>> +#if LUSTRE_VERSION_CODE < OBD_OCD_VERSION(3, 0, 53, 0)
>> 
>> Why are you adding pointless version checks to mainline?  Please don't
>> add new ones of these, you need to be working on removing the existing
>> ones.
>
> These are not Linux kernel version checks, but rather Lustre release version
> checks.  This allows us to remove workarounds like this in the future when
> they are no longer needed, rather than accumulating cruft forever.  It's like
> the separation of NFSv2 vs NFSv3 vs NFSv4.

It looks very different to the separation of NFSv{2,3,4}.  Those are
conditionally compiled on a whole-file basis.
If we ever want to remove this code it should be hard to search for
occurances of OBD_OCD_VERSION() in the code, we don't need the C
preprocessor to be able to remove them for us.
In this particular example:

+	if (exp->exp_connect_data.ocd_version < OBD_OCD_VERSION(2, 10, 1, 0))
+		min_buf_size = exp->exp_connect_data.ocd_max_easize;

if you want to be able to compile without that one test, you could arrange
that OBD_OSC_VERSION(2, 10, 1, 0) evaluates to 0.
As ocd_version is unsigned, the comparison will always be false, and
the compiler will optimize the code away.

As a general rule, you need a very good reason to have #if or #ifdef in
.c files.  They are usually OK in .h files.

Thanks,
NeilBrown


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