Re: [PATCH 20/22] xfs: scrub parent pointers

[Allison Henderson <allison.henderson@oracle.com>]

On 7/20/2017 9:40 PM, Darrick J. Wong wrote:
> From: Darrick J. Wong <darrick.wong@oracle.com>
>
> Scrub parent pointers, sort of.  For directories, we can ride the
> '..' entry up to the parent to confirm that there's at most one
> dentry that points back to this directory.
>
> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> ---
>  fs/xfs/Makefile        |    1
>  fs/xfs/libxfs/xfs_fs.h |    3 -
>  fs/xfs/scrub/common.c  |    4 +
>  fs/xfs/scrub/common.h  |    2
>  fs/xfs/scrub/parent.c  |  252 ++++++++++++++++++++++++++++++++++++++++++++++++
>  5 files changed, 261 insertions(+), 1 deletion(-)
>  create mode 100644 fs/xfs/scrub/parent.c
>
>
> diff --git a/fs/xfs/Makefile b/fs/xfs/Makefile
> index 3d862b9..e73cdc2 100644
> --- a/fs/xfs/Makefile
> +++ b/fs/xfs/Makefile
> @@ -151,6 +151,7 @@ xfs-y				+= $(addprefix scrub/, \
>  				   ialloc.o \
>  				   inode.o \
>  				   metabufs.o \
> +				   parent.o \
>  				   refcount.o \
>  				   rmap.o \
>  				   symlink.o \
> diff --git a/fs/xfs/libxfs/xfs_fs.h b/fs/xfs/libxfs/xfs_fs.h
> index 95d9ce9..4ad3056 100644
> --- a/fs/xfs/libxfs/xfs_fs.h
> +++ b/fs/xfs/libxfs/xfs_fs.h
> @@ -500,7 +500,8 @@ struct xfs_scrub_metadata {
>  #define XFS_SCRUB_TYPE_DIR	16	/* directory */
>  #define XFS_SCRUB_TYPE_XATTR	17	/* extended attribute */
>  #define XFS_SCRUB_TYPE_SYMLINK	18	/* symbolic link */
> -#define XFS_SCRUB_TYPE_MAX	18
> +#define XFS_SCRUB_TYPE_PARENT	19	/* parent pointers */
> +#define XFS_SCRUB_TYPE_MAX	19
>
>  /* i: repair this metadata */
>  #define XFS_SCRUB_FLAG_REPAIR		(1 << 0)
> diff --git a/fs/xfs/scrub/common.c b/fs/xfs/scrub/common.c
> index 4003c2f..6f701c6 100644
> --- a/fs/xfs/scrub/common.c
> +++ b/fs/xfs/scrub/common.c
> @@ -856,6 +856,10 @@ static const struct xfs_scrub_meta_fns meta_scrub_fns[] = {
>  		.setup	= xfs_scrub_setup_symlink,
>  		.scrub	= xfs_scrub_symlink,
>  	},
> +	{ /* parent pointers */
> +		.setup	= xfs_scrub_setup_parent,
> +		.scrub	= xfs_scrub_parent,
> +	},
>  };
>
>  /* Dispatch metadata scrubbing. */
> diff --git a/fs/xfs/scrub/common.h b/fs/xfs/scrub/common.h
> index 6d02a64..1873a31 100644
> --- a/fs/xfs/scrub/common.h
> +++ b/fs/xfs/scrub/common.h
> @@ -223,6 +223,7 @@ SETUP_FN(xfs_scrub_setup_inode_bmap_data);
>  SETUP_FN(xfs_scrub_setup_inode_bmap);
>  SETUP_FN(xfs_scrub_setup_directory);
>  SETUP_FN(xfs_scrub_setup_xattr);
> +SETUP_FN(xfs_scrub_setup_parent);
>  SETUP_FN(xfs_scrub_setup_symlink);
>  #undef SETUP_FN
>
> @@ -247,6 +248,7 @@ SCRUB_FN(xfs_scrub_bmap_attr);
>  SCRUB_FN(xfs_scrub_bmap_cow);
>  SCRUB_FN(xfs_scrub_directory);
>  SCRUB_FN(xfs_scrub_xattr);
> +SCRUB_FN(xfs_scrub_parent);
>  SCRUB_FN(xfs_scrub_symlink);
>  #undef SCRUB_FN
>
> diff --git a/fs/xfs/scrub/parent.c b/fs/xfs/scrub/parent.c
> new file mode 100644
> index 0000000..e604885
> --- /dev/null
> +++ b/fs/xfs/scrub/parent.c
> @@ -0,0 +1,252 @@
> +/*
> + * Copyright (C) 2017 Oracle.  All Rights Reserved.
> + *
> + * Author: Darrick J. Wong <darrick.wong@oracle.com>
> + *
> + * This program is free software; you can redistribute it and/or
> + * modify it under the terms of the GNU General Public License
> + * as published by the Free Software Foundation; either version 2
> + * of the License, or (at your option) any later version.
> + *
> + * This program is distributed in the hope that it would be useful,
> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> + * GNU General Public License for more details.
> + *
> + * You should have received a copy of the GNU General Public License
> + * along with this program; if not, write the Free Software Foundation,
> + * Inc.,  51 Franklin St, Fifth Floor, Boston, MA  02110-1301, USA.
> + */
> +#include "xfs.h"
> +#include "xfs_fs.h"
> +#include "xfs_shared.h"
> +#include "xfs_format.h"
> +#include "xfs_trans_resv.h"
> +#include "xfs_mount.h"
> +#include "xfs_defer.h"
> +#include "xfs_btree.h"
> +#include "xfs_bit.h"
> +#include "xfs_log_format.h"
> +#include "xfs_trans.h"
> +#include "xfs_trace.h"
> +#include "xfs_sb.h"
> +#include "xfs_inode.h"
> +#include "xfs_icache.h"
> +#include "xfs_dir2.h"
> +#include "xfs_dir2_priv.h"
> +#include "scrub/common.h"
> +
> +/* Set us up to scrub parents. */
> +int
> +xfs_scrub_setup_parent(
> +	struct xfs_scrub_context	*sc,
> +	struct xfs_inode		*ip)
> +{
> +	return xfs_scrub_setup_inode_contents(sc, ip, 0);
> +}
> +
> +/* Parent pointers */
> +
> +/* Look for an entry in a parent pointing to this inode. */
> +
> +struct xfs_scrub_parent_ctx {
> +	struct dir_context		dc;
> +	xfs_ino_t			ino;
> +	xfs_nlink_t			nr;
> +};
> +
> +/* Look for a single entry in a directory pointing to an inode. */
> +STATIC int
> +xfs_scrub_parent_actor(
> +	struct dir_context		*dc,
> +	const char			*name,
> +	int				namelen,
> +	loff_t				pos,
> +	u64				ino,
> +	unsigned			type)
> +{
> +	struct xfs_scrub_parent_ctx	*spc;
> +
> +	spc = container_of(dc, struct xfs_scrub_parent_ctx, dc);
> +	if (spc->ino == ino)
> +		spc->nr++;
> +	return 0;
> +}
> +
> +/* Count the number of dentries in the parent dir that point to this inode. */
> +STATIC int
> +xfs_scrub_parent_count_parent_dentries(
> +	struct xfs_scrub_context	*sc,
> +	struct xfs_inode		*parent,
> +	xfs_nlink_t			*nr)
> +{
> +	struct xfs_scrub_parent_ctx	spc = {
> +		.dc.actor = xfs_scrub_parent_actor,
> +		.dc.pos = 0,
> +		.ino = sc->ip->i_ino,
> +		.nr = 0,
> +	};
> +	struct xfs_ifork		*ifp;
> +	size_t				bufsize;
> +	loff_t				oldpos;
> +	uint				lock_mode;
> +	int				error;
> +
> +	/*
> +	 * Load the parent directory's extent map.  A regular directory
> +	 * open would start readahead (and thus load the extent map)
> +	 * before we even got to a readdir call, but this isn't
> +	 * guaranteed here.
> +	 */
> +	lock_mode = xfs_ilock_data_map_shared(parent);
> +	ifp = XFS_IFORK_PTR(parent, XFS_DATA_FORK);
> +	if (XFS_IFORK_FORMAT(parent, XFS_DATA_FORK) == XFS_DINODE_FMT_BTREE &&
> +	    !(ifp->if_flags & XFS_IFEXTENTS)) {
> +		error = xfs_iread_extents(sc->tp, parent, XFS_DATA_FORK);
> +		if (error) {
> +			xfs_iunlock(parent, lock_mode);
> +			return error;
> +		}
> +	}
> +	xfs_iunlock(parent, lock_mode);
> +
> +	/*
> +	 * Iterate the parent dir to confirm that there is
> +	 * exactly one entry pointing back to the inode being
> +	 * scanned.
> +	 */
> +	bufsize = (size_t)min_t(loff_t, 32768, parent->i_d.di_size);
Where does the 32768 come from?  A max number of entries for the parent 
maybe? Rest looks good otherwise.

Reviewed by: Allison Henderson <allison.henderson@oracle.com>

> +	oldpos = 0;
> +	while (true) {
> +		error = xfs_readdir(sc->tp, parent, &spc.dc, bufsize);
> +		if (error)
> +			goto out;
> +		if (oldpos == spc.dc.pos)
> +			break;
> +		oldpos = spc.dc.pos;
> +	}
> +	*nr = spc.nr;
> +out:
> +	return error;
> +}
> +
> +/* Scrub a parent pointer. */
> +#define XFS_SCRUB_PARENT_CHECK(fs_ok) \
> +	XFS_SCRUB_DATA_CHECK(sc, XFS_DATA_FORK, 0, "parent", fs_ok)
> +#define XFS_SCRUB_PARENT_GOTO(fs_ok, label) \
> +	XFS_SCRUB_DATA_GOTO(sc, XFS_DATA_FORK, 0, "parent", fs_ok, label)
> +#define XFS_SCRUB_PARENT_OP_ERROR_GOTO(label) \
> +	XFS_SCRUB_FILE_OP_ERROR_GOTO(sc, XFS_DATA_FORK, 0, "parent", \
> +		&error, label)
> +int
> +xfs_scrub_parent(
> +	struct xfs_scrub_context	*sc)
> +{
> +	struct xfs_mount		*mp = sc->mp;
> +	struct xfs_inode		*dp = NULL;
> +	xfs_ino_t			dnum;
> +	xfs_nlink_t			nr;
> +	int				tries = 0;
> +	int				error;
> +
> +	/*
> +	 * If we're a directory, check that the '..' link points up to
> +	 * a directory that has one entry pointing to us.
> +	 */
> +	if (!S_ISDIR(VFS_I(sc->ip)->i_mode))
> +		return -ENOENT;
> +
> +	/*
> +	 * The VFS grabs a read or write lock via i_rwsem before it reads
> +	 * or writes to a directory.  If we've gotten this far we've
> +	 * already obtained IOLOCK_EXCL, which (since 4.10) is the same as
> +	 * getting a write lock on i_rwsem.  Therefore, it is safe for us
> +	 * to drop the ILOCK here in order to do directory lookups.
> +	 */
> +	sc->ilock_flags &= ~(XFS_ILOCK_EXCL | XFS_MMAPLOCK_EXCL);
> +	xfs_iunlock(sc->ip, XFS_ILOCK_EXCL | XFS_MMAPLOCK_EXCL);
> +
> +	/* Look up '..' */
> +	error = xfs_dir_lookup(sc->tp, sc->ip, &xfs_name_dotdot, &dnum, NULL);
> +	XFS_SCRUB_PARENT_OP_ERROR_GOTO(out);
> +
> +	/* Is this the root dir?  Then '..' must point to itself. */
> +	if (sc->ip == mp->m_rootip) {
> +		XFS_SCRUB_PARENT_CHECK(sc->ip->i_ino == mp->m_sb.sb_rootino);
> +		XFS_SCRUB_PARENT_CHECK(dnum == sc->ip->i_ino);
> +		return 0;
> +	}
> +
> +try_again:
> +	/* Otherwise, '..' must not point to ourselves. */
> +	XFS_SCRUB_PARENT_GOTO(sc->ip->i_ino != dnum, out);
> +
> +	error = xfs_iget(mp, sc->tp, dnum, 0, 0, &dp);
> +	XFS_SCRUB_PARENT_OP_ERROR_GOTO(out);
> +	XFS_SCRUB_PARENT_GOTO(dp != sc->ip, out_rele);
> +
> +	/*
> +	 * We prefer to keep the inode locked while we lock and search
> +	 * its alleged parent for a forward reference.  However, this
> +	 * child -> parent scheme can deadlock with the parent -> child
> +	 * scheme that is normally used.  Therefore, if we can lock the
> +	 * parent, just validate the references and get out.
> +	 */
> +	if (xfs_ilock_nowait(dp, XFS_IOLOCK_SHARED)) {
> +		error = xfs_scrub_parent_count_parent_dentries(sc, dp, &nr);
> +		XFS_SCRUB_PARENT_OP_ERROR_GOTO(out_unlock);
> +		XFS_SCRUB_PARENT_CHECK(nr == 1);
> +		goto out_unlock;
> +	}
> +
> +	/*
> +	 * The game changes if we get here.  We failed to lock the parent,
> +	 * so we're going to try to verify both pointers while only holding
> +	 * one lock so as to avoid deadlocking with something that's actually
> +	 * trying to traverse down the directory tree.
> +	 */
> +	xfs_iunlock(sc->ip, sc->ilock_flags);
> +	sc->ilock_flags = 0;
> +	xfs_ilock(dp, XFS_IOLOCK_SHARED);
> +
> +	/* Go looking for our dentry. */
> +	error = xfs_scrub_parent_count_parent_dentries(sc, dp, &nr);
> +	XFS_SCRUB_PARENT_OP_ERROR_GOTO(out_unlock);
> +
> +	/* Drop the parent lock, relock this inode. */
> +	xfs_iunlock(dp, XFS_IOLOCK_SHARED);
> +	sc->ilock_flags = XFS_IOLOCK_EXCL;
> +	xfs_ilock(sc->ip, sc->ilock_flags);
> +
> +	/* Look up '..' to see if the inode changed. */
> +	error = xfs_dir_lookup(sc->tp, sc->ip, &xfs_name_dotdot, &dnum, NULL);
> +	XFS_SCRUB_PARENT_OP_ERROR_GOTO(out_rele);
> +
> +	/* Drat, parent changed.  Try again! */
> +	if (dnum != dp->i_ino) {
> +		IRELE(dp);
> +		tries++;
> +		if (tries < 20)
> +			goto try_again;
> +		XFS_SCRUB_INCOMPLETE(sc, "parent", false);
> +		goto out;
> +	}
> +	IRELE(dp);
> +
> +	/*
> +	 * '..' didn't change, so check that there was only one entry
> +	 * for us in the parent.
> +	 */
> +	XFS_SCRUB_PARENT_CHECK(nr == 1);
> +	goto out;
> +
> +out_unlock:
> +	xfs_iunlock(dp, XFS_IOLOCK_SHARED);
> +out_rele:
> +	IRELE(dp);
> +out:
> +	return error;
> +}
> +#undef XFS_SCRUB_PARENT_OP_ERROR_GOTO
> +#undef XFS_SCRUB_PARENT_GOTO
> +#undef XFS_SCRUB_PARENT_CHECK
>
> --
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