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Date:   Sat, 14 Nov 2020 12:57:24 +0000 (GMT)
From:   Daire Byrne <daire@dneg.com>
To:     bfields <bfields@fieldses.org>
Cc:     Trond Myklebust <trondmy@hammerspace.com>,
        linux-cachefs <linux-cachefs@redhat.com>,
        linux-nfs <linux-nfs@vger.kernel.org>
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Subject: Re: Adventures in NFS re-exporting
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Thread-Topic: Adventures in NFS re-exporting
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----- On 13 Nov, 2020, at 22:26, bfields bfields@fieldses.org wrote:
> On Fri, Nov 13, 2020 at 09:50:50AM -0500, bfields wrote:
>> On Thu, Nov 12, 2020 at 11:05:57PM +0000, Daire Byrne wrote:
>> > So, I can't lay claim to identifying the exact optimisation/hack that
>> > improves the retention of the re-export server's client cache when
>> > re-exporting an NFSv3 server (which is then read by many clients). We
>> > were working with an engineer at the time who showed an interest in
>> > our use case and after we supplied a reproducer he suggested modifying
>> > the nfs/inode.c
>> > 
>> > -		if (!inode_eq_iversion_raw(inode, fattr->change_attr)) {
>> > +		if (inode_peek_iversion_raw(inode) < fattr->change_attr)
>> > {
>> > 
>> > His reasoning at the time was:
>> > 
>> > "Fixes inode invalidation caused by read access. The least important
>> > bit is ORed with 1 and causes the inode version to differ from the one
>> > seen on the NFS share. This in turn causes unnecessary re-download
>> > impacting the performance significantly. This fix makes it only
>> > re-fetch file content if inode version seen on the server is newer
>> > than the one on the client."
>> > 
>> > But I've always been puzzled by why this only seems to be the case
>> > when using knfsd to re-export the (NFSv3) client mount. Using multiple
>> > processes on a standard client mount never causes any similar
>> > re-validations. And this happens with a completely read-only share
>> > which is why I started to think it has something to do with atimes as
>> > that could perhaps still cause a "write" modification even when
>> > read-only?
>> 
>> Ah-hah!  So, it's inode_query_iversion() that's modifying a nfs inode's
>> i_version.  That's a special thing that only nfsd would do.
>> 
>> I think that's totally fixable, we'll just have to think a little about
>> how....
> 
> I wonder if something like this helps?--b.
> 
> commit 0add88a9ccc5
> Author: J. Bruce Fields <bfields@redhat.com>
> Date:   Fri Nov 13 17:03:04 2020 -0500
> 
>    nfs: don't mangle i_version on NFS
>    
>    The i_version on NFS has pretty much opaque to the client, so we don't
>    want to give the low bit any special interpretation.
>    
>    Define a new FS_PRIVATE_I_VERSION flag for filesystems that manage the
>    i_version on their own.
>    
>    Signed-off-by: J. Bruce Fields <bfields@redhat.com>
> 
> diff --git a/fs/nfs/fs_context.c b/fs/nfs/fs_context.c
> index 29ec8b09a52d..9b8dd5b713a7 100644
> --- a/fs/nfs/fs_context.c
> +++ b/fs/nfs/fs_context.c
> @@ -1488,7 +1488,8 @@ struct file_system_type nfs_fs_type = {
> 	.init_fs_context	= nfs_init_fs_context,
> 	.parameters		= nfs_fs_parameters,
> 	.kill_sb		= nfs_kill_super,
> -	.fs_flags		= FS_RENAME_DOES_D_MOVE|FS_BINARY_MOUNTDATA,
> +	.fs_flags		= FS_RENAME_DOES_D_MOVE|FS_BINARY_MOUNTDATA|
> +				  FS_PRIVATE_I_VERSION,
> };
> MODULE_ALIAS_FS("nfs");
> EXPORT_SYMBOL_GPL(nfs_fs_type);
> @@ -1500,7 +1501,8 @@ struct file_system_type nfs4_fs_type = {
> 	.init_fs_context	= nfs_init_fs_context,
> 	.parameters		= nfs_fs_parameters,
> 	.kill_sb		= nfs_kill_super,
> -	.fs_flags		= FS_RENAME_DOES_D_MOVE|FS_BINARY_MOUNTDATA,
> +	.fs_flags		= FS_RENAME_DOES_D_MOVE|FS_BINARY_MOUNTDATA|
> +				  FS_PRIVATE_I_VERSION,
> };
> MODULE_ALIAS_FS("nfs4");
> MODULE_ALIAS("nfs4");
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 21cc971fd960..c5bb4268228b 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -2217,6 +2217,7 @@ struct file_system_type {
> #define FS_HAS_SUBTYPE		4
> #define FS_USERNS_MOUNT		8	/* Can be mounted by userns root */
> #define FS_DISALLOW_NOTIFY_PERM	16	/* Disable fanotify permission events */
> +#define FS_PRIVATE_I_VERSION	32	/* i_version managed by filesystem */
> #define FS_THP_SUPPORT		8192	/* Remove once all fs converted */
> #define FS_RENAME_DOES_D_MOVE	32768	/* FS will handle d_move() during rename()
> internally. */
> 	int (*init_fs_context)(struct fs_context *);
> diff --git a/include/linux/iversion.h b/include/linux/iversion.h
> index 2917ef990d43..52c790a847de 100644
> --- a/include/linux/iversion.h
> +++ b/include/linux/iversion.h
> @@ -307,6 +307,8 @@ inode_query_iversion(struct inode *inode)
> 	u64 cur, old, new;
> 
> 	cur = inode_peek_iversion_raw(inode);
> +	if (inode->i_sb->s_type->fs_flags & FS_PRIVATE_I_VERSION)
> +		return cur;
> 	for (;;) {
> 		/* If flag is already set, then no need to swap */
>  		if (cur & I_VERSION_QUERIED) {

Yes, I can confirm that this absolutely helps! I replaced our (brute force) iversion patch with this (much nicer) patch and we got the same improvement; nfsd and it's clients no longer cause the re-export server's client cache to constantly be re-validated. The re-export server can now serve the same results to many clients from cache. Thanks so much for spending the time to track this down. If merged, future (crazy) NFS re-exporters will benefit from the metadata performance improvement/acceleration!

Now if anyone has any ideas why all the read calls to the originating server are limited to a maximum of 128k (with rsize=1M) when coming via the re-export server's nfsd threads, I see that as the next biggest performance issue. Reading directly on the re-export server with a userspace process issues 1MB reads as expected. It doesn't happen for writes (wsize=1MB all the way through) but I'm not sure if that has more to do with async and write back caching helping to build up the size before commit?

I figure the other remaining items on my (wish) list are probably more in the "won't fix" or "can't fix" category (except maybe the NFSv4.0 input/output errors?).

Daire