[PATCH] fs, dax: unify IOMAP_F_DIRTY read vs write handling policy in the dax core

Dan Williams
2017-11-14T01:36:09Z

While reviewing whether MAP_SYNC should strengthen its current guarantee
of syncing writes from the initiating process to also include
third-party readers observing dirty metadata, Dave pointed out that the
check of IOMAP_WRITE is misplaced.

The policy of what to with IOMAP_F_DIRTY should be separated from the
generic filesystem mechanism of reporting dirty metadata. Move this
policy to the fs-dax core to simplify the per-filesystem iomap handlers,
and further centralize code that implements the MAP_SYNC policy. This
otherwise should not change behavior, it just makes it easier to change
behavior in the future.

Cc: Jan Kara <jack@suse.cz>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Darrick J. Wong <darrick.wong@oracle.com>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Reported-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
---
This is an additional cleanup I want to include in the 4.15 merge
request for the MAP_SYNC feature. Please review, I'm looking to send the
pull request towards the end of the week.

fs/dax.c | 15 +++++++++++++--
fs/ext4/inode.c | 2 +-
fs/xfs/xfs_iomap.c | 6 +++---
3 files changed, 17 insertions(+), 6 deletions(-)

diff --git a/fs/dax.c b/fs/dax.c
index 78233c716757..27ba300660ff 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -1079,6 +1079,17 @@ static int dax_fault_return(int error)
return VM_FAULT_SIGBUS;
}

+/*
+ * MAP_SYNC on a dax mapping guarantees dirty metadata is
+ * flushed on write-faults (non-cow), but not read-faults.
+ */
+static bool dax_fault_is_synchronous(unsigned long flags,
+ struct vm_area_struct *vma, struct iomap *iomap)
+{
+ return (flags & IOMAP_WRITE) && (vma->vm_flags & VM_SYNC)
+ && (iomap->flags & IOMAP_F_DIRTY);
+}
+
static int dax_iomap_pte_fault(struct vm_fault *vmf, pfn_t *pfnp,
const struct iomap_ops *ops)
{
@@ -1170,7 +1181,7 @@ static int dax_iomap_pte_fault(struct vm_fault *vmf, pfn_t *pfnp,
goto finish_iomap;
}

- sync = (vma->vm_flags & VM_SYNC) && (iomap.flags & IOMAP_F_DIRTY);
+ sync = dax_fault_is_synchronous(flags, vma, &iomap);

switch (iomap.type) {
case IOMAP_MAPPED:
@@ -1390,7 +1401,7 @@ static int dax_iomap_pmd_fault(struct vm_fault *vmf, pfn_t *pfnp,
if (iomap.offset + iomap.length < pos + PMD_SIZE)
goto finish_iomap;

- sync = (vma->vm_flags & VM_SYNC) && (iomap.flags & IOMAP_F_DIRTY);
+ sync = dax_fault_is_synchronous(iomap_flags, vma, &iomap);

switch (iomap.type) {
case IOMAP_MAPPED:
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 13a198924a0f..ee4d907a4251 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -3479,7 +3479,7 @@ static int ext4_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
}

iomap->flags = 0;
- if ((flags & IOMAP_WRITE) && ext4_inode_datasync_dirty(inode))
+ if (ext4_inode_datasync_dirty(inode))
iomap->flags |= IOMAP_F_DIRTY;
iomap->bdev = inode->i_sb->s_bdev;
iomap->dax_dev = sbi->s_daxdev;
diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
index b43be199fbdf..888b60189983 100644
--- a/fs/xfs/xfs_iomap.c
+++ b/fs/xfs/xfs_iomap.c
@@ -1087,9 +1087,9 @@ xfs_file_iomap_begin(
trace_xfs_iomap_found(ip, offset, length, 0, &imap);
}

- if ((flags & IOMAP_WRITE) && xfs_ipincount(ip) &&
- (ip->i_itemp->ili_fsync_fields & ~XFS_ILOG_TIMESTAMP))
- iomap->flags |= IOMAP_F_DIRTY;
+ if (xfs_ipincount(ip))
+ if (ip->i_itemp->ili_fsync_fields & ~XFS_ILOG_TIMESTAMP)
+ iomap->flags |= IOMAP_F_DIRTY;

xfs_bmbt_to_iomap(ip, iomap, &imap);