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Re: Cleancache support in XFS

To: Dave Chinner <david@xxxxxxxxxxxxx>, Ben Myers <bpm@xxxxxxx>, <konrad.wilk@xxxxxxxxxx>
Subject: Re: Cleancache support in XFS
From: James Dingwall <james.dingwall@xxxxxxxxxxx>
Date: Thu, 2 May 2013 09:24:51 +0100
Cc: <xfs@xxxxxxxxxxx>
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20130501223022.GQ10481@dastard>
References: <51810CED.4080003@xxxxxxxxxxx> <20130501162044.GN29359@xxxxxxx> <20130501223022.GQ10481@dastard>
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Dave Chinner wrote:
On Wed, May 01, 2013 at 11:20:44AM -0500, Ben Myers wrote:
Hi James,

On Wed, May 01, 2013 at 01:39:09PM +0100, James Dingwall wrote:
In reference to: http://oss.sgi.com/archives/xfs/2012-05/msg00046.html

$ grep -r cleancache fs/xfs
on the 3.9 kernel source suggests that no patch was submitted to
enable cleancache for the XFS filesystem.  Since it was suggested
that this could be a one liner I've had a go and my first effort is
inline below.  While this seems to compile OK I have no experience
in filesystems so I would appreciate it if anyone can point out that
it is obviously wrong and likely to eat my data before I try booting
the kernel.

If it seems a reasonable attempt what would be the best way to check
that it isn't doing nasty things?
Hrm.. Looks like there is a doc in Documentation/vm/cleancache.txt which
includes a list of attributes the filesystem needs to have to work properly
with cleancache.
So, those points are:
I had started to look at these too but I feel very out of my depth! I had similar conclusions to what Dave wrote but I don't think my thoughts should carry very much (any) weight. Anyway I gambled and booted my xen domU with this patch and so far so good... xen top shows that tmem is now being used where previously it wasn't. I'll try running the xfstests at the weekend after a couple more days up time to see what happens.

| Some points for a filesystem to consider:
| - The FS should be block-device-based (e.g. a ram-based FS such
|  as tmpfs should not enable cleancache)


|- To ensure coherency/correctness, the FS must ensure that all
|  file removal or truncation operations either go through VFS or
|  add hooks to do the equivalent cleancache "invalidate" operations

There be dragons - do all the XFS ioctls do the right thing?
vfs_unlink() calls *dir->i_op->unlink, in xfs_iops.c for S_IFDIR there is:

if (xfs_sb_version_hasasciici(&XFS_M(inode->i_sb)->m_sb))
        inode->i_op = &xfs_dir_ci_inode_operations;
        inode->i_op = &xfs_dir_inode_operations;

where .unlink in xfs_dir_inode_operations is xfs_vn_unlink() in either

I can't work out how to follow the vfs_truncate() in to the filesystem
code and perhaps there are other paths that would lead to file removal.

|- To ensure coherency/correctness, either inode numbers must
|  be unique across the lifetime of the on-disk file OR the
|  FS must provide an "encode_fh" function.


|- The FS must call the VFS superblock alloc and deactivate routines
|  or add hooks to do the equivalent cleancache calls done there.


|- To maximize performance, all pages fetched from the FS should
|  go through the do_mpag_readpage routine or the FS should add
|  hooks to do the equivalent (cf. btrfs)

xfs uses mpage_readpages() so should be fine.
I think there is a cleancache documentation bug since no other fs calls do_mpage_readpage().

|- Currently, the FS blocksize must be the same as PAGESIZE.  This
|  is not an architectural restriction, but no backends currently
|  support anything different.

Which means that we need hooks in the mount path to determine if
this is the case or not. I note that neither ext3/ext4 do this check
so I can't determine why this restriction is mentioned, and I'm not
sure if it has any relevance to btrfs.

IOWs, I'd like to know why this restriction exists - what does
cleancache care about how the filesystem maps blocks to the page in
the page cache - any way the filesystem does this it uses
page->private to hide this fact from the page cache....
+ Konrad (cleancache maintainer) for any opinion.

|- A clustered FS should invoke the "shared_init_fs" cleancache
|  hook to get best performance for some backends.

Not a problem.

So there's a couple of things that need to be explained and
explored, and a bunch of testing to be done....


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