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Message-ID: <20240930155520.GM21853@frogsfrogsfrogs>
Date: Mon, 30 Sep 2024 08:55:20 -0700
From: "Darrick J. Wong" <djwong@...nel.org>
To: John Garry <john.g.garry@...cle.com>
Cc: axboe@...nel.dk, brauner@...nel.org, viro@...iv.linux.org.uk,
jack@...e.cz, dchinner@...hat.com, hch@....de, cem@...nel.org,
linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
hare@...e.de, martin.petersen@...cle.com,
catherine.hoang@...cle.com, mcgrof@...nel.org,
ritesh.list@...il.com, ojaswin@...ux.ibm.com
Subject: Re: [PATCH v6 3/7] fs: iomap: Atomic write support
On Mon, Sep 30, 2024 at 12:54:34PM +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@...cle.com>
> ---
> Maybe we should also enforce that a mapping is in written state, as it
> avoids issues later with forcealign and writing mappings which cover
> multiple extents in different written/unwritten state.
>
> fs/iomap/direct-io.c | 26 +++++++++++++++++++++++---
> fs/iomap/trace.h | 3 ++-
> include/linux/iomap.h | 1 +
> 3 files changed, 26 insertions(+), 4 deletions(-)
>
> diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> index f637aa0706a3..9401c05cd2c0 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 (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;
> 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 4ad12a3c8bae..c7644bdcfca3 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)
This new flag needs a documentation update. What do you think of this?
diff --git a/Documentation/filesystems/iomap/operations.rst b/Documentation/filesystems/iomap/operations.rst
index 8e6c721d23301..279db993be7fa 100644
--- a/Documentation/filesystems/iomap/operations.rst
+++ b/Documentation/filesystems/iomap/operations.rst
@@ -513,6 +513,16 @@ 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 must be persisted in its entirety or
+ not at all.
+ The write must not be split into multiple I/O requests.
+ 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.
--D
>
> struct iomap_ops {
> /*
> --
> 2.31.1
>
>
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