Re: [PATCH v8 4/7] fs: iomap: Atomic write support
From: John Garry
Date: Tue Oct 15 2024 - 08:25:05 EST
On 15/10/2024 13:12, Christoph Hellwig wrote:
On Tue, Oct 15, 2024 at 09:01:39AM +0000, John Garry wrote:
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.
Signed-off-by: John Garry <john.g.garry@xxxxxxxxxx>
---
.../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 8e6c721d2330..fb95e99ca1a0 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..c968a0e2a60b 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))
Nit: no need for the inner braces here.
ok
+ if (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;
Do we want a WARN_ON_ONCE here because this is a condition that should be
impossible to hit?
I have no objection - as you said, it should not be hit. But it would be
nice to see why we are getting the -EINVAL if it were hit (so I can add it)
Otherwise looks good:
Reviewed-by: Christoph Hellwig <hch@xxxxxx>
cheers