[RFC 4/8] ext4: Add statx and other atomic write helper routines

[Ritesh Harjani (IBM)]

This patch adds the statx (STATX_WRITE_ATOMIC) support in ext4_getattr()
to query for atomic_write_unit_min(awu_min), awu_max and other
attributes for atomic writes.
This adds a new runtime mount flag (EXT4_MF_ATOMIC_WRITE_FSAWU),
for querying whether ext4 supports atomic write using fsawu
(filesystem atomic write unit).

Co-developed-by: Ojaswin Mujoo <ojaswin@xxxxxxxxxxxxx>
Signed-off-by: Ojaswin Mujoo <ojaswin@xxxxxxxxxxxxx>
Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@xxxxxxxxx>
---
 fs/ext4/ext4.h  | 53 ++++++++++++++++++++++++++++++++++++++++++++++++-
 fs/ext4/inode.c | 16 +++++++++++++++
 2 files changed, 68 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 023571f8dd1b..1d2bce26e616 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -1817,7 +1817,8 @@ static inline int ext4_valid_inum(struct super_block *sb, unsigned long ino)
  */
 enum {
 	EXT4_MF_MNTDIR_SAMPLED,
-	EXT4_MF_FC_INELIGIBLE	/* Fast commit ineligible */
+	EXT4_MF_FC_INELIGIBLE,		/* Fast commit ineligible */
+	EXT4_MF_ATOMIC_WRITE_FSAWU	/* Atomic write via FSAWU */
 };
 
 static inline void ext4_set_mount_flag(struct super_block *sb, int bit)
@@ -3839,6 +3840,56 @@ static inline int ext4_buffer_uptodate(struct buffer_head *bh)
 	return buffer_uptodate(bh);
 }
 
+#define ext4_can_atomic_write_fsawu(sb)				\
+	ext4_test_mount_flag(sb, EXT4_MF_ATOMIC_WRITE_FSAWU)
+
+/**
+ * ext4_atomic_write_fsawu	Returns EXT4 filesystem atomic write unit.
+ *  @sb				super_block
+ *  This returns the filesystem min|max atomic write units.
+ *  For !bigalloc it is filesystem blocksize (fsawu_min)
+ *  For bigalloc it should be either blocksize or multiple of blocksize
+ *  (fsawu_min)
+ */
+static inline void ext4_atomic_write_fsawu(struct super_block *sb,
+					   unsigned int *fsawu_min,
+					   unsigned int *fsawu_max)
+{
+	u8 blkbits = sb->s_blocksize_bits;
+	unsigned int blocksize = 1U << blkbits;
+	unsigned int clustersize = blocksize;
+	struct block_device *bdev = sb->s_bdev;
+	unsigned int awu_min =
+			queue_atomic_write_unit_min_bytes(bdev->bd_queue);
+	unsigned int awu_max =
+			queue_atomic_write_unit_max_bytes(bdev->bd_queue);
+
+	if (ext4_has_feature_bigalloc(sb))
+		clustersize = 1U << (EXT4_SB(sb)->s_cluster_bits + blkbits);
+
+	/* fs min|max should respect awu_[min|max] units */
+	if (unlikely(awu_min > clustersize || awu_max < blocksize))
+		goto not_supported;
+
+	/* in case of !bigalloc fsawu_[min|max] should be same as blocksize */
+	if (!ext4_has_feature_bigalloc(sb)) {
+		*fsawu_min = blocksize;
+		*fsawu_max = blocksize;
+		return;
+	}
+
+	/* bigalloc can support write in blocksize units. So advertize it */
+	*fsawu_min = max(blocksize, awu_min);
+	*fsawu_max = min(clustersize, awu_max);
+
+	/* This should never happen, but let's keep a WARN_ON_ONCE */
+	WARN_ON_ONCE(!IS_ALIGNED(clustersize, *fsawu_min));
+	return;
+not_supported:
+	*fsawu_min = 0;
+	*fsawu_max = 0;
+}
+
 #endif	/* __KERNEL__ */
 
 #define EFSBADCRC	EBADMSG		/* Bad CRC detected */
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 2ccf3b5e3a7c..ea009ca9085d 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -5536,6 +5536,22 @@ int ext4_getattr(struct mnt_idmap *idmap, const struct path *path,
 		}
 	}
 
+	if (request_mask & STATX_WRITE_ATOMIC) {
+		unsigned int fsawu_min = 0, fsawu_max = 0;
+
+		/*
+		 * Get fsawu_[min|max] value which we can advertise to userspace
+		 * in statx call, if we support atomic writes using
+		 * EXT4_MF_ATOMIC_WRITE_FSAWU.
+		 */
+		if (ext4_can_atomic_write_fsawu(inode->i_sb)) {
+			ext4_atomic_write_fsawu(inode->i_sb, &fsawu_min,
+						&fsawu_max);
+		}
+
+		generic_fill_statx_atomic_writes(stat, fsawu_min, fsawu_max);
+	}
+
 	flags = ei->i_flags & EXT4_FL_USER_VISIBLE;
 	if (flags & EXT4_APPEND_FL)
 		stat->attributes |= STATX_ATTR_APPEND;
-- 
2.43.0