Re: [PATCH v2] BTRFS/NFSD: provide more unique inode number for btrfs export

["J. Bruce Fields" <bfields@fieldses.org>]

On Thu, Aug 26, 2021 at 04:03:04PM +1000, NeilBrown wrote:
> 
> [[ Hi Bruce and Chuck,
>    I've rebased this patch on the earlier patch I sent which allows
>    me to use the name "fh_flags".  I've also added a missing #include.
>    I've added the 'Acked-by' which Joesf provided earlier for the
>    btrfs-part.  I don't have an 'ack' for the  stat.h part, but no-one
>    has complained wither.
>    I think it is as ready as it can be, and am keen to know what you
>    think.
>    I'm not *very* keen on testing s_magic in nfsd code (though we
>    already have a couple of such tests in nfs3proc.c), but it does have
>    the advantage of ensuring that no other filesystem can use this
>    functionality without landing a patch in fs/nfsd/.
>  
>    Thanks for any review that you can provide,
>    NeilBrown
> ]]

This seems hairy, but *somebody* has hated every single solution
proposed, so, argh, I don't know, maybe it's best.

There was a ton of "but why can't we just..." in previous threads, could
we include URLs for those and/or the lwn articles?  E.g.:

	https://lore.kernel.org/linux-nfs/162742539595.32498.13687924366155737575.stgit@noble.brown/#b
	https://lwn.net/Articles/866709/

> BTRFS does not provide unique inode numbers across a filesystem.
> It only provides unique inode numbers within a subvolume and
> uses synthetic device numbers for different subvolumes to ensure
> uniqueness for device+inode.
> 
> nfsd cannot use these varying synthetic device numbers.  If nfsd were to
> synthesise different stable filesystem ids to give to the client, that
> would cause subvolumes to appear in the mount table on the client, even
> though they don't appear in the mount table on the server.  Also, NFSv3
> doesn't support changing the filesystem id without a new explicit mount
> on the client (this is partially supported in practice, but violates the
> protocol specification and has problems in some edge cases).
> 
> So currently, the roots of all subvolumes report the same inode number
> in the same filesystem to NFS clients and tools like 'find' notice that
> a directory has the same identity as an ancestor, and so refuse to
> enter that directory.
> 
> This patch allows btrfs (or any filesystem) to provide a 64bit number
> that can be xored with the inode number to make the number more unique.
> Rather than the client being certain to see duplicates, with this patch
> it is possible but extremely rare.
> 
> The number that btrfs provides is a swab64() version of the subvolume
> identifier.  This has most entropy in the high bits (the low bits of the
> subvolume identifer), while the inode has most entropy in the low bits.
> The result will always be unique within a subvolume, and will almost
> always be unique across the filesystem.
> 
> If an upgrade of the NFS server caused all inode numbers in an exportfs
> BTRFS filesystem to appear to the client to change, the client may not
> handle this well.  The Linux client will cause any open files to become
> 'stale'.  If the mount point changed inode number, the whole mount would
> become inaccessible.
> 
> To avoid this, an unused byte in the filehandle (fh_auth) has been
> repurposed as "fh_flags".  The new behaviour of uniquifying inode number
> is only activated when a new flag is set.
> 
> NFSD will only set this flag in filehandles it reports if the filehandle
> of the parent (provided by the client) contains the bit, or if
>  - the filehandle for the parent is not provided or is for a different
>    export and
>  - the filehandle refers to a BTRFS filesystem.
> 
> Thus if you have a BTRFS filesystem originally mounted from a server
> without this patch, the flag will never be set and the current behaviour
> will continue.  Only once you re-mount the filesystem (or the filesystem
> is re-auto-mounted) will the inode numbers change.  When that happens,
> it is likely that the filesystem st_dev number seen on the client will
> change anyway.
> 
> Acked-by: Josef Bacik <josef@toxicpanda.com> (for BTFS change)

s/BTFS/BTRFS/.

> Signed-off-by: NeilBrown <neilb@suse.de>
> ---
>  fs/btrfs/inode.c     |  4 ++++
>  fs/nfsd/nfs3xdr.c    | 15 ++++++++++++++-
>  fs/nfsd/nfs4xdr.c    |  7 ++++---
>  fs/nfsd/nfsfh.c      | 14 ++++++++++++--
>  fs/nfsd/nfsfh.h      | 34 +++++++++++++++++++++++++++++++---
>  fs/nfsd/xdr3.h       |  2 ++
>  include/linux/stat.h | 18 ++++++++++++++++++
>  7 files changed, 85 insertions(+), 9 deletions(-)
> 
> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> index 06f9f167222b..d35e2a30f25f 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -9195,6 +9195,10 @@ static int btrfs_getattr(struct user_namespace *mnt_userns,
>  	generic_fillattr(&init_user_ns, inode, stat);
>  	stat->dev = BTRFS_I(inode)->root->anon_dev;
>  
> +	if (BTRFS_I(inode)->root->root_key.objectid != BTRFS_FS_TREE_OBJECTID)
> +		stat->ino_uniquifier =
> +			swab64(BTRFS_I(inode)->root->root_key.objectid);
> +
>  	spin_lock(&BTRFS_I(inode)->lock);
>  	delalloc_bytes = BTRFS_I(inode)->new_delalloc_bytes;
>  	inode_bytes = inode_get_bytes(inode);
> diff --git a/fs/nfsd/nfs3xdr.c b/fs/nfsd/nfs3xdr.c
> index 3d37923afb06..5e2d5c352ecd 100644
> --- a/fs/nfsd/nfs3xdr.c
> +++ b/fs/nfsd/nfs3xdr.c
> @@ -340,6 +340,7 @@ svcxdr_encode_fattr3(struct svc_rqst *rqstp, struct xdr_stream *xdr,
>  {
>  	struct user_namespace *userns = nfsd_user_namespace(rqstp);
>  	__be32 *p;
> +	u64 ino;
>  	u64 fsid;
>  
>  	p = xdr_reserve_space(xdr, XDR_UNIT * 21);
> @@ -377,7 +378,8 @@ svcxdr_encode_fattr3(struct svc_rqst *rqstp, struct xdr_stream *xdr,
>  	p = xdr_encode_hyper(p, fsid);
>  
>  	/* fileid */
> -	p = xdr_encode_hyper(p, stat->ino);
> +	ino = nfsd_uniquify_ino(fhp, stat);
> +	p = xdr_encode_hyper(p, ino);
>  
>  	p = encode_nfstime3(p, &stat->atime);
>  	p = encode_nfstime3(p, &stat->mtime);
> @@ -1151,6 +1153,17 @@ svcxdr_encode_entry3_common(struct nfsd3_readdirres *resp, const char *name,
>  	if (xdr_stream_encode_item_present(xdr) < 0)
>  		return false;
>  	/* fileid */
> +	if (!resp->dir_have_uniquifier) {
> +		struct kstat stat;
> +		if (fh_getattr(&resp->fh, &stat) == nfs_ok)
> +			resp->dir_ino_uniquifier =
> +				nfsd_ino_uniquifier(&resp->fh, &stat);
> +		else
> +			resp->dir_ino_uniquifier = 0;
> +		resp->dir_have_uniquifier = true;

This took me a minute.  So we're assuming the uniquifier stays the same
across a directory and its children (because you can't hard link across
subvolumes), and this code is just caching the uniquifier for use across
the directory--is that right?

> +	}
> +	if (resp->dir_ino_uniquifier != ino)
> +		ino ^= resp->dir_ino_uniquifier;

I guess this check (here and in nfsd_uniquify_ino) is just to prevent
returning inode number zero?

--b.

>  	if (xdr_stream_encode_u64(xdr, ino) < 0)
>  		return false;
>  	/* name */
> diff --git a/fs/nfsd/nfs4xdr.c b/fs/nfsd/nfs4xdr.c
> index a54b2845473b..6e31f6286e0b 100644
> --- a/fs/nfsd/nfs4xdr.c
> +++ b/fs/nfsd/nfs4xdr.c
> @@ -3114,10 +3114,11 @@ nfsd4_encode_fattr(struct xdr_stream *xdr, struct svc_fh *fhp,
>  					fhp->fh_handle.fh_size);
>  	}
>  	if (bmval0 & FATTR4_WORD0_FILEID) {
> +		u64 ino = nfsd_uniquify_ino(fhp, &stat);
>  		p = xdr_reserve_space(xdr, 8);
>  		if (!p)
>  			goto out_resource;
> -		p = xdr_encode_hyper(p, stat.ino);
> +		p = xdr_encode_hyper(p, ino);
>  	}
>  	if (bmval0 & FATTR4_WORD0_FILES_AVAIL) {
>  		p = xdr_reserve_space(xdr, 8);
> @@ -3274,7 +3275,7 @@ nfsd4_encode_fattr(struct xdr_stream *xdr, struct svc_fh *fhp,
>  
>  		p = xdr_reserve_space(xdr, 8);
>  		if (!p)
> -                	goto out_resource;
> +			goto out_resource;
>  		/*
>  		 * Get parent's attributes if not ignoring crossmount
>  		 * and this is the root of a cross-mounted filesystem.
> @@ -3284,7 +3285,7 @@ nfsd4_encode_fattr(struct xdr_stream *xdr, struct svc_fh *fhp,
>  			err = get_parent_attributes(exp, &parent_stat);
>  			if (err)
>  				goto out_nfserr;
> -			ino = parent_stat.ino;
> +			ino = nfsd_uniquify_ino(fhp, &parent_stat);
>  		}
>  		p = xdr_encode_hyper(p, ino);
>  	}
> diff --git a/fs/nfsd/nfsfh.c b/fs/nfsd/nfsfh.c
> index 7695c0f1eefe..18bb139f8bfe 100644
> --- a/fs/nfsd/nfsfh.c
> +++ b/fs/nfsd/nfsfh.c
> @@ -9,6 +9,7 @@
>   */
>  
>  #include <linux/exportfs.h>
> +#include <linux/magic.h>
>  
>  #include <linux/sunrpc/svcauth_gss.h>
>  #include "nfsd.h"
> @@ -173,7 +174,7 @@ static __be32 nfsd_set_fh_dentry(struct svc_rqst *rqstp, struct svc_fh *fhp)
>  
>  	if (--data_left < 0)
>  		return error;
> -	if (fh->fh_auth_type != 0)
> +	if ((fh->fh_flags & ~NFSD_FH_FLAG_ALL) != 0)
>  		return error;
>  	len = key_len(fh->fh_fsid_type) / 4;
>  	if (len == 0)
> @@ -532,6 +533,7 @@ fh_compose(struct svc_fh *fhp, struct svc_export *exp, struct dentry *dentry,
>  
>  	struct inode * inode = d_inode(dentry);
>  	dev_t ex_dev = exp_sb(exp)->s_dev;
> +	u8 flags = 0;
>  
>  	dprintk("nfsd: fh_compose(exp %02x:%02x/%ld %pd2, ino=%ld)\n",
>  		MAJOR(ex_dev), MINOR(ex_dev),
> @@ -548,6 +550,14 @@ fh_compose(struct svc_fh *fhp, struct svc_export *exp, struct dentry *dentry,
>  	/* If we have a ref_fh, then copy the fh_no_wcc setting from it. */
>  	fhp->fh_no_wcc = ref_fh ? ref_fh->fh_no_wcc : false;
>  
> +	if (ref_fh && ref_fh->fh_export == exp) {
> +		flags = ref_fh->fh_handle.fh_flags;
> +	} else {
> +		/* Set flags as needed */
> +		if (exp->ex_path.mnt->mnt_sb->s_magic == BTRFS_SUPER_MAGIC)
> +			flags |= NFSD_FH_FLAG_INO_UNIQUIFY;
> +	}
> +
>  	if (ref_fh == fhp)
>  		fh_put(ref_fh);
>  
> @@ -565,7 +575,7 @@ fh_compose(struct svc_fh *fhp, struct svc_export *exp, struct dentry *dentry,
>  
>  	fhp->fh_handle.fh_size =
>  		key_len(fhp->fh_handle.fh_fsid_type) + 4;
> -	fhp->fh_handle.fh_auth_type = 0;
> +	fhp->fh_handle.fh_flags = flags;
>  
>  	mk_fsid(fhp->fh_handle.fh_fsid_type,
>  		fhp->fh_handle.fh_fsid,
> diff --git a/fs/nfsd/nfsfh.h b/fs/nfsd/nfsfh.h
> index f36234c474dc..bbc7ddd34143 100644
> --- a/fs/nfsd/nfsfh.h
> +++ b/fs/nfsd/nfsfh.h
> @@ -18,11 +18,17 @@
>   * The file handle starts with a sequence of four-byte words.
>   * The first word contains a version number (1) and three descriptor bytes
>   * that tell how the remaining 3 variable length fields should be handled.
> - * These three bytes are auth_type, fsid_type and fileid_type.
> + * These three bytes are flags, fsid_type and fileid_type.
>   *
>   * All four-byte values are in host-byte-order.
>   *
> - * The auth_type field is deprecated and must be set to 0.
> + * The flags field (previously auth_type) can be used when nfsd behaviour
> + * needs to change in a non-compatible way, usually for some specific
> + * filesystem.  Flags should only be set in filehandles for filesystems which
> + * need them.
> + * Current values:
> + *   1  -  BTRFS only.  Cause stat->ino_uniquifier to be used to improve inode
> + *         number uniqueness.
>   *
>   * The fsid_type identifies how the filesystem (or export point) is
>   *    encoded.
> @@ -53,7 +59,7 @@ struct knfsd_fh {
>  		__u32			fh_raw[NFS4_FHSIZE/4];
>  		struct {
>  			__u8		fh_version;	/* == 1 */
> -			__u8		fh_auth_type;	/* deprecated */
> +			__u8		fh_flags;
>  			__u8		fh_fsid_type;
>  			__u8		fh_fileid_type;
>  			__u32		fh_fsid[]; /* flexible-array member */
> @@ -131,6 +137,28 @@ enum fsid_source {
>  };
>  extern enum fsid_source fsid_source(const struct svc_fh *fhp);
>  
> +enum nfsd_fh_flags {
> +	NFSD_FH_FLAG_INO_UNIQUIFY = 1,	/* BTRFS only */
> +
> +	NFSD_FH_FLAG_ALL = 1
> +};
> +
> +static inline u64 nfsd_ino_uniquifier(const struct svc_fh *fhp,
> +				      const struct kstat *stat)
> +{
> +	if (fhp->fh_handle.fh_flags & NFSD_FH_FLAG_INO_UNIQUIFY)
> +		return stat->ino_uniquifier;
> +	return 0;
> +}
> +
> +static inline u64 nfsd_uniquify_ino(const struct svc_fh *fhp,
> +				    const struct kstat *stat)
> +{
> +	u64 u = nfsd_ino_uniquifier(fhp, stat);
> +	if (u != stat->ino)
> +		return stat->ino ^ u;
> +	return stat->ino;
> +}
>  
>  /*
>   * This might look a little large to "inline" but in all calls except
> diff --git a/fs/nfsd/xdr3.h b/fs/nfsd/xdr3.h
> index 933008382bbe..d9b6c8314bbb 100644
> --- a/fs/nfsd/xdr3.h
> +++ b/fs/nfsd/xdr3.h
> @@ -179,6 +179,8 @@ struct nfsd3_readdirres {
>  	struct xdr_buf		dirlist;
>  	struct svc_fh		scratch;
>  	struct readdir_cd	common;
> +	u64			dir_ino_uniquifier;
> +	bool			dir_have_uniquifier;
>  	unsigned int		cookie_offset;
>  	struct svc_rqst *	rqstp;
>  
> diff --git a/include/linux/stat.h b/include/linux/stat.h
> index fff27e603814..0f3f74d302f8 100644
> --- a/include/linux/stat.h
> +++ b/include/linux/stat.h
> @@ -46,6 +46,24 @@ struct kstat {
>  	struct timespec64 btime;			/* File creation time */
>  	u64		blocks;
>  	u64		mnt_id;
> +	/*
> +	 * BTRFS does not provide unique inode numbers within a filesystem,
> +	 * depending on a synthetic 'dev' to provide uniqueness.
> +	 * NFSd cannot make use of this 'dev' number so clients often see
> +	 * duplicate inode numbers.
> +	 * For BTRFS, 'ino' is unlikely to use the high bits until the filesystem
> +	 * has created a great many inodes.
> +	 * It puts another number in ino_uniquifier which:
> +	 * - has most entropy in the high bits
> +	 * - is different precisely when 'dev' is different
> +	 * - is stable across unmount/remount
> +	 * NFSd can xor this with 'ino' to get a substantially more unique
> +	 * number for reporting to the client.
> +	 * The ino_uniquifier for a directory can reasonably be applied
> +	 * to inode numbers reported by the readdir filldir callback.
> +	 * It is NOT currently exported to user-space.
> +	 */
> +	u64		ino_uniquifier;
>  };
>  
>  #endif
> -- 
> 2.32.0
> 
>