[PATCH 084/119] xfs: implement CoW for directio writes

Darrick J. Wong
2016-06-17T01:26:58Z

For O_DIRECT writes to shared blocks, we have to CoW them just like
we would with buffered writes. For writes that are not block-aligned,
just bounce them to the page cache.

For block-aligned writes, however, we can do better than that. Use
the same mechanisms that we employ for buffered CoW to set up a
delalloc reservation, allocate all the blocks at once, issue the
writes against the new blocks and use the same ioend functions to
remap the blocks after the write. This should be fairly performant.

v2: Turns out that there's no way for xfs_end_io_direct_write to know
if the write completed successfully. Therefore, do /not/ use the
ioend for dio cow post-processing; instead, move it to xfs_vm_do_dio
where we *can* tell if the write succeeded or not.

v3: Update the file size if we do a directio CoW across EOF. This
can happen if the last block is shared, the cowextsize hint is set,
and we do a dio write past the end of the file.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
fs/xfs/xfs_aops.c | 112 +++++++++++++++++++++++++++++++++++++++++++++++---
fs/xfs/xfs_file.c | 12 ++++-
fs/xfs/xfs_reflink.c | 105 +++++++++++++++++++++++++++++++++++++++++++++++
fs/xfs/xfs_reflink.h | 5 ++
4 files changed, 225 insertions(+), 9 deletions(-)

diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
index 232039c..31318b3 100644
--- a/fs/xfs/xfs_aops.c
+++ b/fs/xfs/xfs_aops.c
@@ -40,6 +40,7 @@
/* flags for direct write completions */
#define XFS_DIO_FLAG_UNWRITTEN (1 << 0)
#define XFS_DIO_FLAG_APPEND (1 << 1)
+#define XFS_DIO_FLAG_COW (1 << 2)

/*
* structure owned by writepages passed to individual writepage calls
@@ -1130,18 +1131,24 @@ xfs_map_direct(
struct inode *inode,
struct buffer_head *bh_result,
struct xfs_bmbt_irec *imap,
- xfs_off_t offset)
+ xfs_off_t offset,
+ bool is_cow)
{
uintptr_t *flags = (uintptr_t *)&bh_result->b_private;
xfs_off_t size = bh_result->b_size;

trace_xfs_get_blocks_map_direct(XFS_I(inode), offset, size,
- ISUNWRITTEN(imap) ? XFS_IO_UNWRITTEN : XFS_IO_OVERWRITE, imap);
+ ISUNWRITTEN(imap) ? XFS_IO_UNWRITTEN : is_cow ? XFS_IO_COW :
+ XFS_IO_OVERWRITE, imap);

if (ISUNWRITTEN(imap)) {
*flags |= XFS_DIO_FLAG_UNWRITTEN;
set_buffer_defer_completion(bh_result);
- } else if (offset + size > i_size_read(inode) || offset + size < 0) {
+ } else if (is_cow) {
+ *flags |= XFS_DIO_FLAG_COW;
+ set_buffer_defer_completion(bh_result);
+ }
+ if (offset + size > i_size_read(inode) || offset + size < 0) {
*flags |= XFS_DIO_FLAG_APPEND;
set_buffer_defer_completion(bh_result);
}
@@ -1187,6 +1194,43 @@ xfs_map_trim_size(
bh_result->b_size = mapping_size;
}

+/* Bounce unaligned directio writes to the page cache. */
+static int
+xfs_bounce_unaligned_dio_write(
+ struct xfs_inode *ip,
+ xfs_fileoff_t offset_fsb,
+ struct xfs_bmbt_irec *imap)
+{
+ bool shared;
+ struct xfs_bmbt_irec irec;
+ xfs_fileoff_t delta;
+ int error;
+
+ irec = *imap;
+ if (offset_fsb > irec.br_startoff) {
+ delta = offset_fsb - irec.br_startoff;
+ irec.br_blockcount -= delta;
+ irec.br_startblock += delta;
+ irec.br_startoff = offset_fsb;
+ }
+ error = xfs_reflink_irec_is_shared(ip, &irec, &shared);
+ if (error)
+ return error;
+ /*
+ * Are we doing a DIO write to a shared block? In
+ * the ideal world we at least would fork full blocks,
+ * but for now just fall back to buffered mode. Yuck.
+ * Use -EREMCHG ("remote address changed") to signal
+ * this, since in general XFS doesn't do this sort of
+ * fallback.
+ */
+ if (shared) {
+ trace_xfs_reflink_bounce_dio_write(ip, imap);
+ return -EREMCHG;
+ }
+ return 0;
+}
+
STATIC int
__xfs_get_blocks(
struct inode *inode,
@@ -1206,6 +1250,8 @@ __xfs_get_blocks(
xfs_off_t offset;
ssize_t size;
int new = 0;
+ bool is_cow = false;
+ bool need_alloc = false;

if (XFS_FORCED_SHUTDOWN(mp))
return -EIO;
@@ -1237,8 +1283,27 @@ __xfs_get_blocks(
end_fsb = XFS_B_TO_FSB(mp, (xfs_ufsize_t)offset + size);
offset_fsb = XFS_B_TO_FSBT(mp, offset);

- error = xfs_bmapi_read(ip, offset_fsb, end_fsb - offset_fsb,
- &imap, &nimaps, XFS_BMAPI_ENTIRE);
+ if (create && direct)
+ is_cow = xfs_reflink_is_cow_pending(ip, offset);
+ if (is_cow)
+ error = xfs_reflink_find_cow_mapping(ip, offset, &imap,
+ &need_alloc);
+ else {
+ error = xfs_bmapi_read(ip, offset_fsb, end_fsb - offset_fsb,
+ &imap, &nimaps, XFS_BMAPI_ENTIRE);
+ /*
+ * Truncate an overwrite extent if there's a pending CoW
+ * reservation before the end of this extent. This forces us
+ * to come back to writepage to take care of the CoW.
+ */
+ if (create && direct && nimaps &&
+ imap.br_startblock != HOLESTARTBLOCK &&
+ imap.br_startblock != DELAYSTARTBLOCK &&
+ !ISUNWRITTEN(&imap))
+ xfs_reflink_trim_irec_to_next_cow(ip, offset_fsb,
+ &imap);
+ }
+ ASSERT(!need_alloc);
if (error)
goto out_unlock;

@@ -1310,6 +1375,13 @@ __xfs_get_blocks(
if (imap.br_startblock != HOLESTARTBLOCK &&
imap.br_startblock != DELAYSTARTBLOCK &&
(create || !ISUNWRITTEN(&imap))) {
+ if (create && direct && !is_cow) {
+ error = xfs_bounce_unaligned_dio_write(ip, offset_fsb,
+ &imap);
+ if (error)
+ return error;
+ }
+
xfs_map_buffer(inode, bh_result, &imap, offset);
if (ISUNWRITTEN(&imap))
set_buffer_unwritten(bh_result);
@@ -1318,7 +1390,8 @@ __xfs_get_blocks(
if (dax_fault)
ASSERT(!ISUNWRITTEN(&imap));
else
- xfs_map_direct(inode, bh_result, &imap, offset);
+ xfs_map_direct(inode, bh_result, &imap, offset,
+ is_cow);
}
}

@@ -1452,7 +1525,11 @@ xfs_end_io_direct_write(
trace_xfs_end_io_direct_write_unwritten(ip, offset, size);

error = xfs_iomap_write_unwritten(ip, offset, size);
- } else if (flags & XFS_DIO_FLAG_APPEND) {
+ }
+ if (flags & XFS_DIO_FLAG_COW) {
+ error = xfs_reflink_end_cow(ip, offset, size);
+ }
+ if (flags & XFS_DIO_FLAG_APPEND) {
struct xfs_trans *tp;

trace_xfs_end_io_direct_write_append(ip, offset, size);
@@ -1475,6 +1552,27 @@ xfs_vm_direct_IO(
dio_iodone_t *endio = NULL;
int flags = 0;
struct block_device *bdev;
+ loff_t end;
+ loff_t block_mask;
+ bool dio_cow = false;
+ int error;
+
+ /* If this is a block-aligned directio CoW, remap immediately. */
+ end = iocb->ki_pos + iov_iter_count(iter);
+ block_mask = (1 << inode->i_blkbits) - 1;
+ if (iov_iter_rw(iter) == WRITE &&
+ xfs_is_reflink_inode(XFS_I(inode)) &&
+ !((iocb->ki_pos | end) & block_mask)) {
+ dio_cow = true;
+ error = xfs_reflink_reserve_cow_range(XFS_I(inode),
+ iocb->ki_pos, iov_iter_count(iter));
+ if (error)
+ return error;
+ error = xfs_reflink_allocate_cow_range(XFS_I(inode),
+ iocb->ki_pos, iov_iter_count(iter));
+ if (error)
+ return error;
+ }

if (iov_iter_rw(iter) == WRITE) {
endio = xfs_end_io_direct_write;
diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
index 148d0b3..b979f01 100644
--- a/fs/xfs/xfs_file.c
+++ b/fs/xfs/xfs_file.c
@@ -896,10 +896,18 @@ xfs_file_write_iter(
if (XFS_FORCED_SHUTDOWN(ip->i_mount))
return -EIO;

- if ((iocb->ki_flags & IOCB_DIRECT) || IS_DAX(inode))
+ /*
+ * Allow DIO to fall back to buffered *only* in the case that we're
+ * doing a reflink CoW.
+ */
+ if ((iocb->ki_flags & IOCB_DIRECT) || IS_DAX(inode)) {
ret = xfs_file_dio_aio_write(iocb, from);
- else
+ if (ret == -EREMCHG)
+ goto buffered;
+ } else {
+buffered:
ret = xfs_file_buffered_aio_write(iocb, from);
+ }

if (ret > 0) {
XFS_STATS_ADD(ip->i_mount, xs_write_bytes, ret);
diff --git a/fs/xfs/xfs_reflink.c b/fs/xfs/xfs_reflink.c
index 59c8e86..113f333 100644
--- a/fs/xfs/xfs_reflink.c
+++ b/fs/xfs/xfs_reflink.c
@@ -146,6 +146,51 @@ xfs_trim_extent(
}
}

+/*
+ * Determine if any of the blocks in this mapping are shared.
+ */
+int
+xfs_reflink_irec_is_shared(
+ struct xfs_inode *ip,
+ struct xfs_bmbt_irec *irec,
+ bool *shared)
+{
+ xfs_agnumber_t agno;
+ xfs_agblock_t agbno;
+ xfs_extlen_t aglen;
+ xfs_agblock_t fbno;
+ xfs_extlen_t flen;
+ int error = 0;
+
+ /* Holes, unwritten, and delalloc extents cannot be shared */
+ if (!xfs_is_reflink_inode(ip) ||
+ ISUNWRITTEN(irec) ||
+ irec->br_startblock == HOLESTARTBLOCK ||
+ irec->br_startblock == DELAYSTARTBLOCK) {
+ *shared = false;
+ return 0;
+ }
+
+ trace_xfs_reflink_irec_is_shared(ip, irec);
+
+ agno = XFS_FSB_TO_AGNO(ip->i_mount, irec->br_startblock);
+ agbno = XFS_FSB_TO_AGBNO(ip->i_mount, irec->br_startblock);
+ aglen = irec->br_blockcount;
+
+ /* Are there any shared blocks here? */
+ error = xfs_refcount_find_shared(ip->i_mount, agno, agbno,
+ aglen, &fbno, &flen, false);
+ if (error)
+ return error;
+ if (flen == 0) {
+ *shared = false;
+ return 0;
+ }
+
+ *shared = true;
+ return 0;
+}
+
/* Find the shared ranges under an irec, and set up delalloc extents. */
static int
xfs_reflink_reserve_cow_extent(
@@ -273,6 +318,66 @@ xfs_reflink_reserve_cow_range(
}

/*
+ * Allocate blocks to all CoW reservations within a byte range of a file.
+ */
+int
+xfs_reflink_allocate_cow_range(
+ struct xfs_inode *ip,
+ xfs_off_t pos,
+ xfs_off_t len)
+{
+ struct xfs_ifork *ifp;
+ struct xfs_bmbt_rec_host *gotp;
+ struct xfs_bmbt_irec imap;
+ int error = 0;
+ xfs_fileoff_t start_lblk;
+ xfs_fileoff_t end_lblk;
+ xfs_extnum_t idx;
+
+ if (!xfs_is_reflink_inode(ip))
+ return 0;
+
+ trace_xfs_reflink_allocate_cow_range(ip, len, pos, 0);
+
+ start_lblk = XFS_B_TO_FSBT(ip->i_mount, pos);
+ end_lblk = XFS_B_TO_FSB(ip->i_mount, pos + len);
+ ifp = XFS_IFORK_PTR(ip, XFS_COW_FORK);
+ xfs_ilock(ip, XFS_ILOCK_EXCL);
+
+ gotp = xfs_iext_bno_to_ext(ifp, start_lblk, &idx);
+ while (gotp) {
+ xfs_bmbt_get_all(gotp, &imap);
+
+ if (imap.br_startoff >= end_lblk)
+ break;
+ if (!isnullstartblock(imap.br_startblock))
+ goto advloop;
+ xfs_trim_extent(&imap, start_lblk, end_lblk - start_lblk);
+ trace_xfs_reflink_allocate_cow_extent(ip, &imap);
+
+ xfs_iunlock(ip, XFS_ILOCK_EXCL);
+ error = xfs_iomap_write_allocate(ip, XFS_COW_FORK,
+ XFS_FSB_TO_B(ip->i_mount, imap.br_startoff +
+ imap.br_blockcount - 1), &imap);
+ xfs_ilock(ip, XFS_ILOCK_EXCL);
+ if (error)
+ break;
+advloop:
+ /* Roll on... */
+ idx++;
+ if (idx >= ifp->if_bytes / sizeof(xfs_bmbt_rec_t))
+ break;
+ gotp = xfs_iext_get_ext(ifp, idx);
+ }
+
+ xfs_iunlock(ip, XFS_ILOCK_EXCL);
+
+ if (error)
+ trace_xfs_reflink_allocate_cow_range_error(ip, error, _RET_IP_);
+ return error;
+}
+
+/*
* Determine if there's a CoW reservation at a byte offset of an inode.
*/
bool
diff --git a/fs/xfs/xfs_reflink.h b/fs/xfs/xfs_reflink.h
index 27ae6c0..fb128dd 100644
--- a/fs/xfs/xfs_reflink.h
+++ b/fs/xfs/xfs_reflink.h
@@ -20,8 +20,13 @@
#ifndef __XFS_REFLINK_H
#define __XFS_REFLINK_H 1

+extern int xfs_reflink_irec_is_shared(struct xfs_inode *ip,
+ struct xfs_bmbt_irec *imap, bool *shared);
+
extern int xfs_reflink_reserve_cow_range(struct xfs_inode *ip, xfs_off_t pos,
xfs_off_t len);
+extern int xfs_reflink_allocate_cow_range(struct xfs_inode *ip, xfs_off_t pos,
+ xfs_off_t len);
extern bool xfs_reflink_is_cow_pending(struct xfs_inode *ip, xfs_off_t offset);
extern int xfs_reflink_find_cow_mapping(struct xfs_inode *ip, xfs_off_t offset,
struct xfs_bmbt_irec *imap, bool *need_alloc);