Merge tag 'fs-atomic_2024-11-05' of ssh://gitolite.kernel.org/pub/scm/linux/kernel/git/djwong/xfs-linux into vfs.untorn.writes

Snapshot of untorn@fs-atomic#ritesh.list_ext4-do-not-fallback-to-buffered-io-for-dio-atomic-write at Tue Nov  5 16:20:51 PST 2024

Link: https://lore.kernel.org/r/20241106-zerkleinern-verzweifeln-7ec8173c56ad@brauner
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: brauner

Documentation/filesystems/iomap/operations.rst

@@ -513,6 +513,21 @@ 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.
+   Only one space mapping is allowed per untorn write.
+   Untorn writes must be aligned to, and must not be longer than, a
+   single file block.
+
 Callers commonly hold ``i_rwsem`` in shared or exclusive mode before
 calling this function.
 

block/fops.c

@@ -35,13 +35,10 @@ static blk_opf_t dio_bio_write_op(struct kiocb *iocb)
 	return opf;
 }
 
-static bool blkdev_dio_invalid(struct block_device *bdev, loff_t pos,
-				struct iov_iter *iter, bool is_atomic)
+static bool blkdev_dio_invalid(struct block_device *bdev, struct kiocb *iocb,
+				struct iov_iter *iter)
 {
-	if (is_atomic && !generic_atomic_write_valid(iter, pos))
-		return true;
-
-	return pos & (bdev_logical_block_size(bdev) - 1) ||
+	return iocb->ki_pos & (bdev_logical_block_size(bdev) - 1) ||
 		!bdev_iter_is_aligned(bdev, iter);
 }
 
@@ -368,13 +365,12 @@ static ssize_t __blkdev_direct_IO_async(struct kiocb *iocb,
 static ssize_t blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter)
 {
 	struct block_device *bdev = I_BDEV(iocb->ki_filp->f_mapping->host);
-	bool is_atomic = iocb->ki_flags & IOCB_ATOMIC;
 	unsigned int nr_pages;
 
 	if (!iov_iter_count(iter))
 		return 0;
 
-	if (blkdev_dio_invalid(bdev, iocb->ki_pos, iter, is_atomic))
+	if (blkdev_dio_invalid(bdev, iocb, iter))
 		return -EINVAL;
 
 	nr_pages = bio_iov_vecs_to_alloc(iter, BIO_MAX_VECS + 1);
@@ -383,7 +379,7 @@ static ssize_t blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter)
 			return __blkdev_direct_IO_simple(iocb, iter, bdev,
 							nr_pages);
 		return __blkdev_direct_IO_async(iocb, iter, bdev, nr_pages);
-	} else if (is_atomic) {
+	} else if (iocb->ki_flags & IOCB_ATOMIC) {
 		return -EINVAL;
 	}
 	return __blkdev_direct_IO(iocb, iter, bdev, bio_max_segs(nr_pages));
@@ -625,7 +621,7 @@ static int blkdev_open(struct inode *inode, struct file *filp)
 	if (!bdev)
 		return -ENXIO;
 
-	if (bdev_can_atomic_write(bdev) && filp->f_flags & O_DIRECT)
+	if (bdev_can_atomic_write(bdev))
 		filp->f_mode |= FMODE_CAN_ATOMIC_WRITE;
 
 	ret = bdev_open(bdev, mode, filp->private_data, NULL, filp);
@@ -700,6 +696,12 @@ static ssize_t blkdev_write_iter(struct kiocb *iocb, struct iov_iter *from)
 	if ((iocb->ki_flags & (IOCB_NOWAIT | IOCB_DIRECT)) == IOCB_NOWAIT)
 		return -EOPNOTSUPP;
 
+	if (iocb->ki_flags & IOCB_ATOMIC) {
+		ret = generic_atomic_write_valid(iocb, from);
+		if (ret)
+			return ret;
+	}
+
 	size -= iocb->ki_pos;
 	if (iov_iter_count(from) > size) {
 		shorted = iov_iter_count(from) - size;

fs/ext4/ext4.h

@@ -1729,6 +1729,10 @@ struct ext4_sb_info {
 	 */
 	struct work_struct s_sb_upd_work;
 
+	/* Atomic write unit values in bytes */
+	unsigned int s_awu_min;
+	unsigned int s_awu_max;
+
 	/* Ext4 fast commit sub transaction ID */
 	atomic_t s_fc_subtid;
 
@@ -3855,6 +3859,12 @@ static inline int ext4_buffer_uptodate(struct buffer_head *bh)
 	return buffer_uptodate(bh);
 }
 
+static inline bool ext4_inode_can_atomic_write(struct inode *inode)
+{
+
+	return S_ISREG(inode->i_mode) && EXT4_SB(inode->i_sb)->s_awu_min > 0;
+}
+
 extern int ext4_block_write_begin(handle_t *handle, struct folio *folio,
 				  loff_t pos, unsigned len,
 				  get_block_t *get_block);

fs/ext4/file.c

@@ -599,6 +599,13 @@ static ssize_t ext4_dio_write_iter(struct kiocb *iocb, struct iov_iter *from)
 		ssize_t err;
 		loff_t endbyte;
 
+		/*
+		 * There is no support for atomic writes on buffered-io yet,
+		 * we should never fallback to buffered-io for DIO atomic
+		 * writes.
+		 */
+		WARN_ON_ONCE(iocb->ki_flags & IOCB_ATOMIC);
+
 		offset = iocb->ki_pos;
 		err = ext4_buffered_write_iter(iocb, from);
 		if (err < 0)
@@ -692,6 +699,20 @@ ext4_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
 	if (IS_DAX(inode))
 		return ext4_dax_write_iter(iocb, from);
 #endif
+
+	if (iocb->ki_flags & IOCB_ATOMIC) {
+		size_t len = iov_iter_count(from);
+		int ret;
+
+		if (len < EXT4_SB(inode->i_sb)->s_awu_min ||
+		    len > EXT4_SB(inode->i_sb)->s_awu_max)
+			return -EINVAL;
+
+		ret = generic_atomic_write_valid(iocb, from);
+		if (ret)
+			return ret;
+	}
+
 	if (iocb->ki_flags & IOCB_DIRECT)
 		return ext4_dio_write_iter(iocb, from);
 	else
@@ -884,6 +905,9 @@ static int ext4_file_open(struct inode *inode, struct file *filp)
 			return ret;
 	}
 
+	if (ext4_inode_can_atomic_write(inode))
+		filp->f_mode |= FMODE_CAN_ATOMIC_WRITE;
+
 	filp->f_mode |= FMODE_NOWAIT | FMODE_CAN_ODIRECT;
 	return dquot_file_open(inode, filp);
 }

fs/ext4/inode.c

@@ -3444,17 +3444,34 @@ static int ext4_iomap_overwrite_begin(struct inode *inode, loff_t offset,
 	return ret;
 }
 
+static inline bool ext4_want_directio_fallback(unsigned flags, ssize_t written)
+{
+	/* must be a directio to fall back to buffered */
+	if ((flags & (IOMAP_WRITE | IOMAP_DIRECT)) !=
+		    (IOMAP_WRITE | IOMAP_DIRECT))
+		return false;
+
+	/* atomic writes are all-or-nothing */
+	if (flags & IOMAP_ATOMIC)
+		return false;
+
+	/* can only try again if we wrote nothing */
+	return written == 0;
+}
+
 static int ext4_iomap_end(struct inode *inode, loff_t offset, loff_t length,
 			  ssize_t written, unsigned flags, struct iomap *iomap)
 {
 	/*
 	 * Check to see whether an error occurred while writing out the data to
-	 * the allocated blocks. If so, return the magic error code so that we
-	 * fallback to buffered I/O and attempt to complete the remainder of
-	 * the I/O. Any blocks that may have been allocated in preparation for
-	 * the direct I/O will be reused during buffered I/O.
+	 * the allocated blocks. If so, return the magic error code for
+	 * non-atomic write so that we fallback to buffered I/O and attempt to
+	 * complete the remainder of the I/O.
+	 * For non-atomic writes, any blocks that may have been
+	 * allocated in preparation for the direct I/O will be reused during
+	 * buffered I/O. For atomic write, we never fallback to buffered-io.
 	 */
-	if (flags & (IOMAP_WRITE | IOMAP_DIRECT) && written == 0)
+	if (ext4_want_directio_fallback(flags, written))
 		return -ENOTBLK;
 
 	return 0;
@@ -5578,6 +5595,18 @@ int ext4_getattr(struct mnt_idmap *idmap, const struct path *path,
 		}
 	}
 
+	if ((request_mask & STATX_WRITE_ATOMIC) && S_ISREG(inode->i_mode)) {
+		struct ext4_sb_info *sbi = EXT4_SB(inode->i_sb);
+		unsigned int awu_min = 0, awu_max = 0;
+
+		if (ext4_inode_can_atomic_write(inode)) {
+			awu_min = sbi->s_awu_min;
+			awu_max = sbi->s_awu_max;
+		}
+
+		generic_fill_statx_atomic_writes(stat, awu_min, awu_max);
+	}
+
 	flags = ei->i_flags & EXT4_FL_USER_VISIBLE;
 	if (flags & EXT4_APPEND_FL)
 		stat->attributes |= STATX_ATTR_APPEND;

fs/ext4/super.c

@@ -4425,6 +4425,36 @@ static int ext4_handle_clustersize(struct super_block *sb)
 	return 0;
 }
 
+/*
+ * ext4_atomic_write_init: Initializes filesystem min & max atomic write units.
+ * @sb: super block
+ * TODO: Later add support for bigalloc
+ */
+static void ext4_atomic_write_init(struct super_block *sb)
+{
+	struct ext4_sb_info *sbi = EXT4_SB(sb);
+	struct block_device *bdev = sb->s_bdev;
+
+	if (!bdev_can_atomic_write(bdev))
+		return;
+
+	if (!ext4_has_feature_extents(sb))
+		return;
+
+	sbi->s_awu_min = max(sb->s_blocksize,
+			      bdev_atomic_write_unit_min_bytes(bdev));
+	sbi->s_awu_max = min(sb->s_blocksize,
+			      bdev_atomic_write_unit_max_bytes(bdev));
+	if (sbi->s_awu_min && sbi->s_awu_max &&
+	    sbi->s_awu_min <= sbi->s_awu_max) {
+		ext4_msg(sb, KERN_NOTICE, "Supports (experimental) DIO atomic writes awu_min: %u, awu_max: %u",
+			 sbi->s_awu_min, sbi->s_awu_max);
+	} else {
+		sbi->s_awu_min = 0;
+		sbi->s_awu_max = 0;
+	}
+}
+
 static void ext4_fast_commit_init(struct super_block *sb)
 {
 	struct ext4_sb_info *sbi = EXT4_SB(sb);
@@ -5336,6 +5366,7 @@ static int __ext4_fill_super(struct fs_context *fc, struct super_block *sb)
 
 	spin_lock_init(&sbi->s_bdev_wb_lock);
 
+	ext4_atomic_write_init(sb);
 	ext4_fast_commit_init(sb);
 
 	sb->s_root = NULL;

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;
 		}

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" }, \

fs/read_write.c

@@ -1830,18 +1830,22 @@ int generic_file_rw_checks(struct file *file_in, struct file *file_out)
 	return 0;
 }
 
-bool generic_atomic_write_valid(struct iov_iter *iter, loff_t pos)
+int generic_atomic_write_valid(struct kiocb *iocb, struct iov_iter *iter)
 {
 	size_t len = iov_iter_count(iter);
 
 	if (!iter_is_ubuf(iter))
-		return false;
+		return -EINVAL;
 
 	if (!is_power_of_2(len))
-		return false;
+		return -EINVAL;
+
+	if (!IS_ALIGNED(iocb->ki_pos, len))
+		return -EINVAL;
 
-	if (!IS_ALIGNED(pos, len))
-		return false;
+	if (!(iocb->ki_flags & IOCB_DIRECT))
+		return -EOPNOTSUPP;
 
-	return true;
+	return 0;
 }
+EXPORT_SYMBOL_GPL(generic_atomic_write_valid);