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

Allison Henderson
2017-07-23T18:03:10Z

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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