Re: [PATCH v2 1/4] ext4: Add statx support for atomic writes

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On 27/10/2024 18:17, Ritesh Harjani (IBM) wrote:
This patch adds base support for atomic writes via statx getattr.
On bs < ps systems, we can create FS with say bs of 16k. That means
both atomic write min and max unit can be set to 16k for supporting
atomic writes.

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  |  9 +++++++++
  fs/ext4/inode.c | 14 ++++++++++++++
  fs/ext4/super.c | 31 +++++++++++++++++++++++++++++++
  3 files changed, 54 insertions(+)

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 44b0d418143c..6ee49aaacd2b 100644
--- a/fs/ext4/ext4.h
+++ b/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 +3859,11 @@ static inline int ext4_buffer_uptodate(struct buffer_head *bh)
  	return buffer_uptodate(bh);
  }
+static inline bool ext4_can_atomic_write(struct super_block *sb)
+{
+	return EXT4_SB(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);
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 54bdd4884fe6..fcdee27b9aa2 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -5578,6 +5578,20 @@ int ext4_getattr(struct mnt_idmap *idmap, const struct path *path,
  		}
  	}
+ if (S_ISREG(inode->i_mode) && (request_mask & STATX_WRITE_ATOMIC)) {
+		struct ext4_sb_info *sbi = EXT4_SB(inode->i_sb);
+		unsigned int awu_min, awu_max;
+
+		if (ext4_can_atomic_write(inode->i_sb)) {
+			awu_min = sbi->s_awu_min;
+			awu_max = sbi->s_awu_max;
+		} else {
+			awu_min = awu_max = 0;
+		}
+
+		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;
diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 16a4ce704460..d6e3201a48be 100644
--- a/fs/ext4/super.c
+++ b/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))

this check is duplicated, since bdev_atomic_write_unit_{min, max}_bytes() has this check

+		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) {

This looks a bit complicated. I would just follow the XFS example and ensure bdev_atomic_write_unit_min_bytes() <= sb->s_blocksize <= bdev_atomic_write_unit_max_bytes() [which you are doing, but in a complicated way]

+		ext4_msg(sb, KERN_NOTICE, "Supports 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;





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