[PATCH v9 5/8] fs: iomap: Atomic write support

John Garry
2024-10-16T10:04:14Z

Support direct I/O atomic writes by producing a single bio with REQ_ATOMIC
flag set.

Initially FSes (XFS) should only support writing a single FS block
atomically.

As with any atomic write, we should produce a single bio which covers the
complete write length.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: John Garry <john.g.garry@oracle.com>
---
.../filesystems/iomap/operations.rst | 11 ++++++
fs/iomap/direct-io.c | 38 +++++++++++++++++--
fs/iomap/trace.h | 3 +-
include/linux/iomap.h | 1 +
4 files changed, 48 insertions(+), 5 deletions(-)

diff --git a/Documentation/filesystems/iomap/operations.rst b/Documentation/filesystems/iomap/operations.rst
index b93115ab8748..5f382076db67 100644
--- a/Documentation/filesystems/iomap/operations.rst
+++ b/Documentation/filesystems/iomap/operations.rst
@@ -513,6 +513,17 @@ IOMAP_WRITE`` with any combination of the following enhancements:
if the mapping is unwritten and the filesystem cannot handle zeroing
the unaligned regions without exposing stale contents.

+ * ``IOMAP_ATOMIC``: This write is being issued with torn-write
+ protection. Only a single bio can be created for the write, and the
+ write must not be split into multiple I/O requests, i.e. flag
+ REQ_ATOMIC must be set.
+ The file range to write must be aligned to satisfy the requirements
+ of both the filesystem and the underlying block device's atomic
+ commit capabilities.
+ If filesystem metadata updates are required (e.g. unwritten extent
+ conversion or copy on write), all updates for the entire file range
+ must be committed atomically as well.
+
Callers commonly hold ``i_rwsem`` in shared or exclusive mode before
calling this function.

diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
index f637aa0706a3..ed4764e3b8f0 100644
--- a/fs/iomap/direct-io.c
+++ b/fs/iomap/direct-io.c
@@ -271,7 +271,7 @@ static int iomap_dio_zero(const struct iomap_iter *iter, struct iomap_dio *dio,
* clearing the WRITE_THROUGH flag in the dio request.
*/
static inline blk_opf_t iomap_dio_bio_opflags(struct iomap_dio *dio,
- const struct iomap *iomap, bool use_fua)
+ const struct iomap *iomap, bool use_fua, bool atomic)
{
blk_opf_t opflags = REQ_SYNC | REQ_IDLE;

@@ -283,6 +283,8 @@ static inline blk_opf_t iomap_dio_bio_opflags(struct iomap_dio *dio,
opflags |= REQ_FUA;
else
dio->flags &= ~IOMAP_DIO_WRITE_THROUGH;
+ if (atomic)
+ opflags |= REQ_ATOMIC;

return opflags;
}
@@ -293,7 +295,8 @@ static loff_t iomap_dio_bio_iter(const struct iomap_iter *iter,
const struct iomap *iomap = &iter->iomap;
struct inode *inode = iter->inode;
unsigned int fs_block_size = i_blocksize(inode), pad;
- loff_t length = iomap_length(iter);
+ const loff_t length = iomap_length(iter);
+ bool atomic = iter->flags & IOMAP_ATOMIC;
loff_t pos = iter->pos;
blk_opf_t bio_opf;
struct bio *bio;
@@ -303,6 +306,9 @@ static loff_t iomap_dio_bio_iter(const struct iomap_iter *iter,
size_t copied = 0;
size_t orig_count;

+ if (atomic && length != fs_block_size)
+ return -EINVAL;
+
if ((pos | length) & (bdev_logical_block_size(iomap->bdev) - 1) ||
!bdev_iter_is_aligned(iomap->bdev, dio->submit.iter))
return -EINVAL;
@@ -382,7 +388,7 @@ static loff_t iomap_dio_bio_iter(const struct iomap_iter *iter,
* can set up the page vector appropriately for a ZONE_APPEND
* operation.
*/
- bio_opf = iomap_dio_bio_opflags(dio, iomap, use_fua);
+ bio_opf = iomap_dio_bio_opflags(dio, iomap, use_fua, atomic);

nr_pages = bio_iov_vecs_to_alloc(dio->submit.iter, BIO_MAX_VECS);
do {
@@ -415,6 +421,17 @@ static loff_t iomap_dio_bio_iter(const struct iomap_iter *iter,
}

n = bio->bi_iter.bi_size;
+ if (WARN_ON_ONCE(atomic && n != length)) {
+ /*
+ * This bio should have covered the complete length,
+ * which it doesn't, so error. We may need to zero out
+ * the tail (complete FS block), similar to when
+ * bio_iov_iter_get_pages() returns an error, above.
+ */
+ ret = -EINVAL;
+ bio_put(bio);
+ goto zero_tail;
+ }
if (dio->flags & IOMAP_DIO_WRITE) {
task_io_account_write(n);
} else {
@@ -598,6 +615,9 @@ __iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
if (iocb->ki_flags & IOCB_NOWAIT)
iomi.flags |= IOMAP_NOWAIT;

+ if (iocb->ki_flags & IOCB_ATOMIC)
+ iomi.flags |= IOMAP_ATOMIC;
+
if (iov_iter_rw(iter) == READ) {
/* reads can always complete inline */
dio->flags |= IOMAP_DIO_INLINE_COMP;
@@ -659,7 +679,17 @@ __iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
if (ret != -EAGAIN) {
trace_iomap_dio_invalidate_fail(inode, iomi.pos,
iomi.len);
- ret = -ENOTBLK;
+ if (iocb->ki_flags & IOCB_ATOMIC) {
+ /*
+ * folio invalidation failed, maybe
+ * this is transient, unlock and see if
+ * the caller tries again.
+ */
+ ret = -EAGAIN;
+ } else {
+ /* fall back to buffered write */
+ ret = -ENOTBLK;
+ }
}
goto out_free_dio;
}
diff --git a/fs/iomap/trace.h b/fs/iomap/trace.h
index 0a991c4ce87d..4118a42cdab0 100644
--- a/fs/iomap/trace.h
+++ b/fs/iomap/trace.h
@@ -98,7 +98,8 @@ DEFINE_RANGE_EVENT(iomap_dio_rw_queued);
{ IOMAP_REPORT, "REPORT" }, \
{ IOMAP_FAULT, "FAULT" }, \
{ IOMAP_DIRECT, "DIRECT" }, \
- { IOMAP_NOWAIT, "NOWAIT" }
+ { IOMAP_NOWAIT, "NOWAIT" }, \
+ { IOMAP_ATOMIC, "ATOMIC" }

#define IOMAP_F_FLAGS_STRINGS \
{ IOMAP_F_NEW, "NEW" }, \
diff --git a/include/linux/iomap.h b/include/linux/iomap.h
index d0420e962ffd..84282db3e4c1 100644
--- a/include/linux/iomap.h
+++ b/include/linux/iomap.h
@@ -178,6 +178,7 @@ struct iomap_folio_ops {
#else
#define IOMAP_DAX 0
#endif /* CONFIG_FS_DAX */
+#define IOMAP_ATOMIC (1 << 9)

struct iomap_ops {
/*
--
2.31.1