linux-kernel - RE: Cleancache and shared filesystems

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Date:	Fri, 27 May 2011 08:31:40 -0700 (PDT)
From:	Dan Magenheimer <dan.magenheimer@...cle.com>
To:	Steven Whitehouse <swhiteho@...hat.com>
Cc:	linux-kernel@...r.kernel.org, sunil.mushran@...cle.com
Subject: RE: Cleancache and shared filesystems

> From: Steven Whitehouse [mailto:swhiteho@...hat.com]
> Sent: Friday, May 27, 2011 7:52 AM
> Cc: linux-kernel@...r.kernel.org
> Subject: Cleancache and shared filesystems
> 
> Hi,
> 
> I'm trying to figure out what I would need to do in order to get GFS2
> to
> work with cleancache. Looking at the OCFS2 implementation leaves me
> with
> some questions about how this is supposed to work. The docs say that
> the
> cleancache_init_shared_fs() function is supposed to take a 128 bit UUID
> plus the sb.
> 
> In OCFS2 it is passed a pointer to a 32 bit little endian quantity as
> the UUID:
> 
> __le32 uuid_net_key;
> 
> ...
> 
> memcpy(&uuid_net_key, di->id2.i_super.s_uuid, sizeof(uuid_net_key));
> 
> ...
> 
> cleancache_init_shared_fs((char *)&uuid_net_key, sb);
> 
> and in the Xen backend driver this then appears to be dereferenced as
> if
> its two 64 bit values, which doesn't look right to me.

Hi Steve --

Thanks for looking at cleancache!

Hmmm... yes... it looks like you are right and the cleancache
interface code for ocfs2 has bit-rotted over time and a bad value
is being used for the uuid.  This would result in pages not being
shared between clustered VMs on the same physical machine, but
would only result in a lower performance advantage, not any
other visible effects, which would explain why it has gone
undiscovered for so long.

Thanks for pointing this out as it is definitely a bug!

> Also, since the sb has a UUID field in it anyway, is there some reason
> why that cannot be used directly rather than passing the uuid as a
> separate variable?

Forgive me but I am not a clustered FS expert (even for ocfs2):
If the UUID field in the sb is always 128-bits and is always
identical for all cluster nodes, and this fact is consistent
across all clustered FS's at mount time, I agree that there is
no need to pass the uuid as a parameter in
cleancache_init_shared_fs as it can be derived in the body of
cleancache_init_shared_fs and then passed to
__cleancache_init_shared_fs (which only cares that it gets
128-bits and probably has no business digging through a
superblock).  OTOH, this call is made only once per mount
so there's no significant advantage in changing this... or am
I missing something?

Thanks,
Dan

====

Thanks... for the memory!
I really could use more / my throughput's on the floor
The balloon is flat / my swap disk's fat / I've OOM's in store
Overcommitted so much
(with apologies to Bob Hope)

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