[PATCH 0/6] btrfs: delay compression to bbio submission time

[PATCH 0/6] btrfs: delay compression to bbio submission time

[Qu Wenruo]

[CHANGELOG]
PoC->v1:
- Fix the ordered extent leak caused by incorrect ref count of child OEs
- Fix the reserved space leakage in ranges without a real OE
- Fix the hang caused by incorrect extent lock/unlock pair
  All exposed by fsstress runs

- Fix the OE range check in btrfs_wait_ordered_extents() that affects
  snapshot creation
  All exposed by fstests runs

[BACKGROUND]
Btrfs currently goes with async submission for compressed write, I'll go
the following example to explain the async submission:

The page and fs block sizes are all 4K, no large folio involved.
The dirty range is [0, 4K), [8K, 128K).

    0  4K  8K                                        128K
    |//|   |/////////////////////////////////////////|

- Write back folio 0
  * Delalloc
    writepage_delalloc() will find the delalloc range [0, 4K), and since
    it can not be inlined and too small for compression, it will be go
    through COW path, thus a new data extent is allocated, with
    corresponding EM/OE created.

  * Submission
    That folio 0 will be added into a bbio, and since we reached the OE
    end, the bbio will be submitted immediately.

- Write back folio 8K
  * Delalloc
    writepage_delalloc() find the delalloc range [8K, 128K) and go
    compression.
    Instead of allocating an extent immediately, it queues the work into
    delalloc_workers.

    Please note that the range [8K, 128K) is completely locked during
    compression.

  * Skip submission
    As the whole folio 8K went through async submission, we skip bbio
    submission.

- Write back folio 12K
  We wait for the folio to be unlocked (after compression is done and
  compressed bio is submitted).
  When the folio is unlocked, the folio will have writeback flag set and
  its dirty flag cleared. Thus we either wait for the writeback or skip
  the folio completely.

  This step repeats for the range [8K, 128K).

AFAIK the async submission is required as we can not submit two
different bbios for a single compressed range.
Which is different from the uncompressed write path, where we can have
several different bbios for a single ordered extent.

[PROBLEMS]
The async submission has the following problems:

- Non-sequential writeback
  Especially when large folios are involved, we can have some blocks
  submitted immediately (uncompressed), and some submitted later
  (compressed).

  That breaks the assumption of iomap and DONTCACHE writes, which
  requires all blocks inside a folio to be submitted in one go.

- Not really async
  As the example given above, we keep the whole range locked during
  compression.
  This means if we want to read a cached folio in that range, we still
  need to wait for the compression.

[DELAYED COMPRESSION]
The new idea is to delay the compression at bbio submission time.
Now the workflow will be:

- Write back folio 0
  The same, submitting it immediately

- Write back folio 8K
  * Delalloc
    writepage_delalloc() find the delalloc range [8K, 128K) and go
    compression, but this time we allocated delayed EM and OE for the
    range [8K, 128K).

  * Submission
    That folio 8K will be added into a bbio, with its dirty flag removed
    and writeback flag set.

- Writeback folio 12K ~ 124K
  * Delalloc
    No new delalloc range.
 
  * Submission
    Those folios will be added to the same bbio above.
    And after the last folio 124K is queued, we reached the OE end, and
    will submit the delayed bbio.

- Delayed bbio submission
  As the bbio has a special @is_delayed flag set, it will not be
  submitted directly, but queued into a workqueue for compression.

  * Compression in the workqueue
  * Real delalloc
    Now an on-disk extent is reserved. The real EM will replace the
    delayed one.
    And the real OE will be added as a child of the original delayed
    one.
  * Compressed data submission
  * Delayed bbio finish
    When all child compressed/uncompressed writes finished, the delayed
    bbio will finish.

    The full delayed OE is also finished, which will insert all of its
    child OEs into the subvolume tree.

This solves both the problems mentioned above, but is definitely way
more complex than the current async submission:

- Layered OEs
  And we need to manage the child/parent OEs properly
  But still it brings the minimal amount of changes to the existing OE
  users, and keep the scheme that every block going through
  extent_writepage_io() has a corresponding OE.

- Possible extra split
  Since the delayed OE is allocated first, we can still submit two
  different delayed bbio for the same OE.

  This means we can have two smaller compressed extents compared to one,
  which may reduce the compression ratio.

- More complex error handling
  We need to handle cases where some part of the delayed OE has no child
  one. In that case we need to manually release the reserved data/meta
  space.

Qu Wenruo (6):
  btrfs: add skeleton for delayed btrfs bio
  btrfs: add delayed ordered extent support
  btrfs: introduce the skeleton of delayed bbio endio function
  btrfs: introduce compression for delayed bbio
  btrfs: implement uncompressed fallback for delayed bbio
  btrfs: enable experimental delayed compression support

 fs/btrfs/bio.c          |   1 +
 fs/btrfs/bio.h          |   3 +
 fs/btrfs/btrfs_inode.h  |   3 +
 fs/btrfs/extent_io.c    |  29 ++-
 fs/btrfs/extent_map.h   |   9 +-
 fs/btrfs/inode.c        | 492 +++++++++++++++++++++++++++++++++++++++-
 fs/btrfs/ordered-data.c | 181 +++++++++++----
 fs/btrfs/ordered-data.h |  14 ++
 8 files changed, 678 insertions(+), 54 deletions(-)

-- 
2.53.0

[PATCH 1/6] btrfs: add skeleton for delayed btrfs bio

[Qu Wenruo]

The objective of such new delayed btrfs bio infrastructure is to allow
compressed write to go the regular extent_writepage_io() path, without
going through the async submission path.

This will make it easier to align our write path to iomap.

The core ideas of delayed btrfs bio are:

- A place holder ordered extent created at delalloc time
  No space is reserved at that time, and is not implemented in this
  patch.

- A delayed extent map created at delalloc time
  It will have a special disk_bytenr (-4) to indicate the range is
  delayed.
  And a new EXTENT_FLAG_DELAYED flag.

- Delayed btrfs bios will be limited to BTRFS_MAX_COMPRESSED size
  As only compression will go through delayed btrfs bio.

- Delayed btrfs bios will have @is_delayed flag set
  And such bio will have 0 as bi_sector, but will never be submitted
  directly through btrfs_submit_bio().

  Currently the submission of a delayed btrfs bio is not here yet, and
  will be implemented by later patches.

- Btrfs bio assembly mostly follows the regular path
  There are several small exceptions:
  * btrfs_bio_is_contig() needs to handle delayed disk_bytenr/bbio
  * New bbio needs to have its is_delayed flag set if disk_bytenr
    is EXTENT_MAP_DELAYED

- Real ordered extents will be created at bbio submission time
  This part is not implemented in this patch.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/bio.c         |  1 +
 fs/btrfs/bio.h         |  3 +++
 fs/btrfs/btrfs_inode.h |  3 +++
 fs/btrfs/extent_io.c   | 29 +++++++++++++++++++++++++----
 fs/btrfs/extent_map.h  |  9 ++++++++-
 fs/btrfs/inode.c       | 41 +++++++++++++++++++++++++++++++++++++++++
 6 files changed, 81 insertions(+), 5 deletions(-)

diff --git a/fs/btrfs/bio.c b/fs/btrfs/bio.c
index 2a2a21aec817..513cf2eeff4d 100644
--- a/fs/btrfs/bio.c
+++ b/fs/btrfs/bio.c
@@ -900,6 +900,7 @@ void btrfs_submit_bbio(struct btrfs_bio *bbio, int mirror_num)
 {
 	/* If bbio->inode is not populated, its file_offset must be 0. */
 	ASSERT(bbio->inode || bbio->file_offset == 0);
+	ASSERT(!bbio->is_delayed);
 
 	assert_bbio_alignment(bbio);
 
diff --git a/fs/btrfs/bio.h b/fs/btrfs/bio.h
index 303ed6c7103d..49ebdc7ce6e6 100644
--- a/fs/btrfs/bio.h
+++ b/fs/btrfs/bio.h
@@ -99,6 +99,9 @@ struct btrfs_bio {
 	/* Whether the bio is written using zone append. */
 	bool can_use_append:1;
 
+	/* If the bio is delayed (aka, no backing OE). */
+	bool is_delayed:1;
+
 	/*
 	 * This member must come last, bio_alloc_bioset will allocate enough
 	 * bytes for entire btrfs_bio but relies on bio being last.
diff --git a/fs/btrfs/btrfs_inode.h b/fs/btrfs/btrfs_inode.h
index 55c272fe5d92..080ede55b1d6 100644
--- a/fs/btrfs/btrfs_inode.h
+++ b/fs/btrfs/btrfs_inode.h
@@ -669,5 +669,8 @@ u64 btrfs_get_extent_allocation_hint(struct btrfs_inode *inode, u64 start,
 struct extent_map *btrfs_create_io_em(struct btrfs_inode *inode, u64 start,
 				      const struct btrfs_file_extent *file_extent,
 				      int type);
+struct extent_map *btrfs_create_delayed_em(struct btrfs_inode *inode,
+					   u64 start, u32 length);
+void btrfs_submit_delayed_write(struct btrfs_bio *bbio);
 
 #endif
diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index 33b1afbee0a6..5fdc78915046 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -180,12 +180,16 @@ static void submit_one_bio(struct btrfs_bio_ctrl *bio_ctrl)
 
 	/* Caller should ensure the bio has at least some range added */
 	ASSERT(bbio->bio.bi_iter.bi_size);
-
+	/* Delayed bbio is only for write. */
+	if (bbio->is_delayed)
+		ASSERT(btrfs_op(&bbio->bio) == BTRFS_MAP_WRITE);
 	bio_set_csum_search_commit_root(bio_ctrl);
 
 	if (btrfs_op(&bbio->bio) == BTRFS_MAP_READ &&
 	    bio_ctrl->compress_type != BTRFS_COMPRESS_NONE)
 		btrfs_submit_compressed_read(bbio);
+	else if (bbio->is_delayed)
+		btrfs_submit_delayed_write(bbio);
 	else
 		btrfs_submit_bbio(bbio, 0);
 
@@ -723,6 +727,14 @@ static bool btrfs_bio_is_contig(struct btrfs_bio_ctrl *bio_ctrl,
 	struct bio *bio = &bio_ctrl->bbio->bio;
 	const sector_t sector = disk_bytenr >> SECTOR_SHIFT;
 
+	/* One is delayed bbio and one is not, definitely not contig. */
+	if (bio_ctrl->bbio->is_delayed != (disk_bytenr == EXTENT_MAP_DELAYED))
+		return false;
+
+	/* For delayed bbio, only need to check if the file range is contig. */
+	if (bio_ctrl->bbio->is_delayed)
+		return bio_ctrl->next_file_offset == file_offset;
+
 	if (bio_ctrl->compress_type != BTRFS_COMPRESS_NONE) {
 		/*
 		 * For compression, all IO should have its logical bytenr set
@@ -748,7 +760,13 @@ static void alloc_new_bio(struct btrfs_inode *inode,
 
 	bbio = btrfs_bio_alloc(BIO_MAX_VECS, bio_ctrl->opf, inode,
 			       file_offset, bio_ctrl->end_io_func, NULL);
-	bbio->bio.bi_iter.bi_sector = disk_bytenr >> SECTOR_SHIFT;
+	if (disk_bytenr == EXTENT_MAP_DELAYED) {
+		bbio->is_delayed = true;
+		bbio->bio.bi_iter.bi_sector = 0;
+	} else {
+		bbio->is_delayed = false;
+		bbio->bio.bi_iter.bi_sector = disk_bytenr >> SECTOR_SHIFT;
+	}
 	bbio->bio.bi_write_hint = inode->vfs_inode.i_write_hint;
 	bio_ctrl->bbio = bbio;
 	bio_ctrl->len_to_oe_boundary = U32_MAX;
@@ -762,7 +780,7 @@ static void alloc_new_bio(struct btrfs_inode *inode,
 		if (ordered) {
 			bio_ctrl->len_to_oe_boundary = min_t(u32, U32_MAX,
 					ordered->file_offset +
-					ordered->disk_num_bytes - file_offset);
+					ordered->num_bytes - file_offset);
 			bbio->ordered = ordered;
 		}
 
@@ -1688,7 +1706,10 @@ static int submit_one_sector(struct btrfs_inode *inode,
 	ASSERT(IS_ALIGNED(em->len, sectorsize));
 
 	block_start = btrfs_extent_map_block_start(em);
-	disk_bytenr = btrfs_extent_map_block_start(em) + extent_offset;
+	if (block_start == EXTENT_MAP_DELAYED)
+		disk_bytenr = block_start;
+	else
+		disk_bytenr = block_start + extent_offset;
 
 	ASSERT(!btrfs_extent_map_is_compressed(em));
 	ASSERT(block_start != EXTENT_MAP_HOLE);
diff --git a/fs/btrfs/extent_map.h b/fs/btrfs/extent_map.h
index 6f685f3c9327..e45e9f96443a 100644
--- a/fs/btrfs/extent_map.h
+++ b/fs/btrfs/extent_map.h
@@ -13,7 +13,8 @@
 struct btrfs_inode;
 struct btrfs_fs_info;
 
-#define EXTENT_MAP_LAST_BYTE ((u64)-4)
+#define EXTENT_MAP_LAST_BYTE ((u64)-5)
+#define EXTENT_MAP_DELAYED ((u64)-4)
 #define EXTENT_MAP_HOLE ((u64)-3)
 #define EXTENT_MAP_INLINE ((u64)-2)
 
@@ -30,6 +31,12 @@ enum {
 	ENUM_BIT(EXTENT_FLAG_LOGGING),
 	/* This em is merged from two or more physically adjacent ems */
 	ENUM_BIT(EXTENT_FLAG_MERGED),
+	/*
+	 * This real on-disk extent allocation is delayed until bio submission.
+	 * For now it's only a place holder with EXTENT_MAP_DELAYED as
+	 * its disk_bytenr.
+	 */
+	ENUM_BIT(EXTENT_FLAG_DELAYED),
 };
 
 /*
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index acfef903ac8b..0551b8e755ed 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -7552,6 +7552,47 @@ struct extent_map *btrfs_create_io_em(struct btrfs_inode *inode, u64 start,
 	return em;
 }
 
+struct extent_map *btrfs_create_delayed_em(struct btrfs_inode *inode,
+					   u64 start, u32 length)
+{
+	struct extent_map *em;
+	int ret;
+
+	em = btrfs_alloc_extent_map();
+	if (!em)
+		return ERR_PTR(-ENOMEM);
+
+	em->start = start;
+	em->len = length;
+	em->disk_bytenr = EXTENT_MAP_DELAYED;
+	em->disk_num_bytes = 0;
+	em->ram_bytes = 0;
+	em->generation = -1;
+	em->offset = 0;
+	em->flags = EXTENT_FLAG_DELAYED | EXTENT_FLAG_PINNED;
+
+	ret = btrfs_replace_extent_map_range(inode, em, true);
+	if (ret) {
+		btrfs_free_extent_map(em);
+		return ERR_PTR(ret);
+	}
+
+	/* em got 2 refs now, callers needs to do btrfs_free_extent_map once. */
+	return em;
+}
+
+void btrfs_submit_delayed_write(struct btrfs_bio *bbio)
+{
+	ASSERT(bbio->is_delayed);
+
+	/*
+	 * Not yet implemented, and should not hit this path as we have no
+	 * caller to create delayed extent map.
+	 */
+	ASSERT(0);
+	bio_put(&bbio->bio);
+}
+
 /*
  * For release_folio() and invalidate_folio() we have a race window where
  * folio_end_writeback() is called but the subpage spinlock is not yet released.
-- 
2.53.0

[PATCH 2/6] btrfs: add delayed ordered extent support

[Qu Wenruo]

A delayed ordered extent has the following features:

- A new BTRFS_ORDERED_DELAYED flag
  And this new flag must be set along BTRFS_ORDERED_REGULAR flag.

- No allocation of any on-disk space
  As a delayed ordered extent doesn't take any on-disk space yet, it
  won't release any reserved data/meta space either.

- Zero or more real OEs can be added to the parent
  If a real OE is allocated, it must be inside the parent OE.
  And such real OE will go through the regular data/meta space
  reservation path.

- Children OEs will not be added to the per-inode OE rb-tree nor
  per-root list
  Only the parent OE is added to the per-inode rb-tree and per-root
  list.
  So anything waiting for ordered extents should only work on the parent
  one.

  There is a special corner case to btrfs_wait_ordered_extents(), as
  delayed parent OEs have 0 disk_bytenr and disk_num_bytes, they will
  be considered out of the [0, U64_MAX] range.
  Thus we have to always wait for any delayed OEs of a root, no matter
  if a block group range is given or not.

- When the parent OE finishes, all children OEs will also be finished
  And reserved space handling is all handled by the children OEs.

- Any range not covered by child OE will be manually cleaned up

Above features allows us to use the existing ordered extent interfaces
to allocate new real OEs, and wait for them properly.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/inode.c        |  76 +++++++++++++++++
 fs/btrfs/ordered-data.c | 181 ++++++++++++++++++++++++++++++----------
 fs/btrfs/ordered-data.h |  14 ++++
 3 files changed, 225 insertions(+), 46 deletions(-)

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 0551b8e755ed..799e3d5c80f3 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -3162,6 +3162,79 @@ static int insert_ordered_extent_file_extent(struct btrfs_trans_handle *trans,
 					   update_inode_bytes, oe->qgroup_rsv);
 }
 
+static int finish_delayed_ordered(struct btrfs_ordered_extent *oe)
+{
+	struct btrfs_inode *inode = oe->inode;
+	struct btrfs_fs_info *fs_info = inode->root->fs_info;
+	struct btrfs_ordered_extent *child;
+	struct btrfs_ordered_extent *tmp;
+	struct extent_state *cached = NULL;
+	const u32 nr_bits = oe->num_bytes >> fs_info->sectorsize_bits;
+	bool io_error = test_bit(BTRFS_ORDERED_IOERR, &oe->flags);
+	u64 cur = oe->file_offset;
+	int ret = 0;
+	int saved_ret = 0;
+
+	/* Finish each child OE. */
+	list_for_each_entry_safe(child, tmp, &oe->child_list, child_list) {
+		list_del_init(&child->child_list);
+		refcount_inc(&child->refs);
+
+		/* The range should have been marked in the bitmap. */
+		ASSERT(bitmap_test_range_all_set(oe->child_bitmap,
+			(child->file_offset - oe->file_offset) >> fs_info->sectorsize_bits,
+			child->num_bytes >> fs_info->sectorsize_bits));
+
+		if (io_error)
+			set_bit(BTRFS_ORDERED_IOERR, &child->flags);
+
+		ret = btrfs_finish_one_ordered(child);
+		if (ret && !saved_ret)
+			saved_ret = ret;
+	}
+
+	/* For ranges that doesn't have a child OE, manually clean them up. */
+	while (cur < oe->file_offset + oe->num_bytes) {
+		const u32 cur_bit = (cur - oe->file_offset) >> fs_info->sectorsize_bits;
+		u32 first_zero;
+		u32 next_set;
+		u64 range_start;
+		u64 range_end;
+		u32 range_len;
+
+		first_zero = find_next_zero_bit(oe->child_bitmap, nr_bits, cur_bit);
+		if (first_zero >= nr_bits)
+			break;
+		next_set = find_next_bit(oe->child_bitmap, nr_bits, first_zero);
+		ASSERT(next_set > first_zero);
+
+		range_start = oe->file_offset + (first_zero << fs_info->sectorsize_bits);
+		range_len = (next_set - first_zero) << fs_info->sectorsize_bits;
+		range_end = range_start + range_len - 1;
+
+		btrfs_lock_extent(&inode->io_tree, range_start, range_end, &cached);
+		/*
+		 * The range has reserved data/metadata but no real OE, thus we have
+		 * to manually release them.
+		 */
+		btrfs_delalloc_release_space(inode, NULL, range_start, range_len, true);
+		/*
+		 * Also need to remove/drop the pinned extent map range.
+		 * Here we do not want the extent map to stay, as they do not represent
+		 * any real extent map.
+		 */
+		btrfs_drop_extent_map_range(inode, range_start, range_end, false);
+		btrfs_clear_extent_bit(&inode->io_tree, range_start, range_end,
+				EXTENT_LOCKED | EXTENT_DELALLOC_NEW | EXTENT_DEFRAG |
+				EXTENT_DO_ACCOUNTING, &cached);
+		cur = range_end + 1;
+	}
+	btrfs_remove_ordered_extent(oe);
+	btrfs_put_ordered_extent(oe);
+	btrfs_put_ordered_extent(oe);
+	return saved_ret;
+}
+
 /*
  * As ordered data IO finishes, this gets called so we can finish
  * an ordered extent if the range of bytes in the file it covers are
@@ -3184,6 +3257,9 @@ int btrfs_finish_one_ordered(struct btrfs_ordered_extent *ordered_extent)
 	bool clear_reserved_extent = true;
 	unsigned int clear_bits = EXTENT_DEFRAG;
 
+	if (test_bit(BTRFS_ORDERED_DELAYED, &ordered_extent->flags))
+		return finish_delayed_ordered(ordered_extent);
+
 	start = ordered_extent->file_offset;
 	end = start + ordered_extent->num_bytes - 1;
 
diff --git a/fs/btrfs/ordered-data.c b/fs/btrfs/ordered-data.c
index bc88b904d024..c013609b6192 100644
--- a/fs/btrfs/ordered-data.c
+++ b/fs/btrfs/ordered-data.c
@@ -155,6 +155,7 @@ static struct btrfs_ordered_extent *alloc_ordered_extent(
 	u64 qgroup_rsv = 0;
 	const bool is_nocow = (flags &
 	       ((1U << BTRFS_ORDERED_NOCOW) | (1U << BTRFS_ORDERED_PREALLOC)));
+	const bool is_delayed = test_bit(BTRFS_ORDERED_DELAYED, &flags);
 
 	/* Only one type flag can be set. */
 	ASSERT(hweight_long(flags & BTRFS_ORDERED_EXCLUSIVE_FLAGS) == 1);
@@ -169,6 +170,17 @@ static struct btrfs_ordered_extent *alloc_ordered_extent(
 	if (test_bit(BTRFS_ORDERED_ENCODED, &flags))
 		ASSERT(test_bit(BTRFS_ORDERED_COMPRESSED, &flags));
 
+	/*
+	 * DELAYED can only be set with REGULAR, no DIRECT/ENCODED, and should
+	 * not exceed BTRFS_MAX_COMPRESSED size.
+	 */
+	if (test_bit(BTRFS_ORDERED_DELAYED, &flags)) {
+		ASSERT(test_bit(BTRFS_ORDERED_REGULAR, &flags));
+		ASSERT(!test_bit(BTRFS_ORDERED_DIRECT, &flags));
+		ASSERT(!test_bit(BTRFS_ORDERED_ENCODED, &flags));
+		ASSERT(num_bytes <= BTRFS_MAX_COMPRESSED);
+	}
+
 	/*
 	 * For a NOCOW write we can free the qgroup reserve right now. For a COW
 	 * one we transfer the reserved space from the inode's iotree into the
@@ -177,13 +189,14 @@ static struct btrfs_ordered_extent *alloc_ordered_extent(
 	 * completing the ordered extent, when running the data delayed ref it
 	 * creates, we free the reserved data with btrfs_qgroup_free_refroot().
 	 */
-	if (is_nocow)
-		ret = btrfs_qgroup_free_data(inode, NULL, file_offset, num_bytes, &qgroup_rsv);
-	else
-		ret = btrfs_qgroup_release_data(inode, file_offset, num_bytes, &qgroup_rsv);
-
-	if (ret < 0)
-		return ERR_PTR(ret);
+	if (!is_delayed) {
+		if (is_nocow)
+			ret = btrfs_qgroup_free_data(inode, NULL, file_offset, num_bytes, &qgroup_rsv);
+		else
+			ret = btrfs_qgroup_release_data(inode, file_offset, num_bytes, &qgroup_rsv);
+		if (ret < 0)
+			return ERR_PTR(ret);
+	}
 
 	entry = kmem_cache_zalloc(btrfs_ordered_extent_cache, GFP_NOFS);
 	if (!entry) {
@@ -215,19 +228,23 @@ static struct btrfs_ordered_extent *alloc_ordered_extent(
 	INIT_LIST_HEAD(&entry->root_extent_list);
 	INIT_LIST_HEAD(&entry->work_list);
 	INIT_LIST_HEAD(&entry->bioc_list);
+	INIT_LIST_HEAD(&entry->child_list);
 	init_completion(&entry->completion);
+	RB_CLEAR_NODE(&entry->rb_node);
 
 	/*
 	 * We don't need the count_max_extents here, we can assume that all of
 	 * that work has been done at higher layers, so this is truly the
 	 * smallest the extent is going to get.
 	 */
-	spin_lock(&inode->lock);
-	btrfs_mod_outstanding_extents(inode, 1);
-	spin_unlock(&inode->lock);
+	if (!is_delayed) {
+		spin_lock(&inode->lock);
+		btrfs_mod_outstanding_extents(inode, 1);
+		spin_unlock(&inode->lock);
+	}
 
 out:
-	if (IS_ERR(entry) && !is_nocow)
+	if (IS_ERR(entry) && !is_nocow && !is_delayed)
 		btrfs_qgroup_free_refroot(inode->root->fs_info,
 					  btrfs_root_id(inode->root),
 					  qgroup_rsv, BTRFS_QGROUP_RSV_DATA);
@@ -235,12 +252,43 @@ static struct btrfs_ordered_extent *alloc_ordered_extent(
 	return entry;
 }
 
+static void add_child_oe(struct btrfs_ordered_extent *parent,
+			 struct btrfs_ordered_extent *child)
+{
+	struct btrfs_inode *inode = parent->inode;
+	struct btrfs_fs_info *fs_info = inode->root->fs_info;
+	const u32 start_bit = (child->file_offset - parent->file_offset) >>
+			      fs_info->sectorsize_bits;
+	const u32 nr_bits = child->num_bytes >> fs_info->sectorsize_bits;
+
+	lockdep_assert_held(&inode->ordered_tree_lock);
+	/* Basic flags check for parent and child. */
+	ASSERT(test_bit(BTRFS_ORDERED_DELAYED, &parent->flags));
+	ASSERT(!test_bit(BTRFS_ORDERED_DELAYED, &child->flags));
+
+	/* Child should not belong to any parent yet. */
+	ASSERT(list_empty(&child->child_list));
+
+	/* Child should be fully inside parent's range. */
+	ASSERT(child->file_offset >= parent->file_offset);
+	ASSERT(child->file_offset + child->num_bytes <=
+	       parent->file_offset + parent->num_bytes);
+
+	/* There should be no existing child in the range. */
+	ASSERT(bitmap_test_range_all_zero(parent->child_bitmap, start_bit, nr_bits));
+
+	list_add_tail(&child->child_list, &parent->child_list);
+
+	bitmap_set(parent->child_bitmap, start_bit, nr_bits);
+}
+
 static void insert_ordered_extent(struct btrfs_ordered_extent *entry)
 {
 	struct btrfs_inode *inode = entry->inode;
 	struct btrfs_root *root = inode->root;
 	struct btrfs_fs_info *fs_info = root->fs_info;
 	struct rb_node *node;
+	bool is_child = false;
 
 	trace_btrfs_ordered_extent_add(inode, entry);
 
@@ -253,17 +301,25 @@ static void insert_ordered_extent(struct btrfs_ordered_extent *entry)
 	spin_lock(&inode->ordered_tree_lock);
 	node = tree_insert(&inode->ordered_tree, entry->file_offset,
 			   &entry->rb_node);
-	if (unlikely(node)) {
+	if (node) {
 		struct btrfs_ordered_extent *exist =
 			rb_entry(node, struct btrfs_ordered_extent, rb_node);
 
-		btrfs_panic(fs_info, -EEXIST,
+		if (test_bit(BTRFS_ORDERED_DELAYED, &exist->flags)) {
+			add_child_oe(exist, entry);
+			is_child = true;
+		} else {
+			btrfs_panic(fs_info, -EEXIST,
 "existing oe file_offset=%llu num_bytes=%llu flags=0x%lx new oe file_offset=%llu num_bytes=%llu flags=0x%lx",
-			    exist->file_offset, exist->num_bytes, exist->flags,
-			    entry->file_offset, entry->num_bytes, entry->flags);
+				    exist->file_offset, exist->num_bytes, exist->flags,
+				    entry->file_offset, entry->num_bytes, entry->flags);
+		}
 	}
 	spin_unlock(&inode->ordered_tree_lock);
 
+	/* Child OE shouldn't be added to per-root oe list. */
+	if (is_child)
+		return;
 	spin_lock(&root->ordered_extent_lock);
 	list_add_tail(&entry->root_extent_list,
 		      &root->ordered_extents);
@@ -336,6 +392,20 @@ struct btrfs_ordered_extent *btrfs_alloc_ordered_extent(
 	return entry;
 }
 
+struct btrfs_ordered_extent *btrfs_alloc_delayed_ordered_extent(
+			struct btrfs_inode *inode, u64 file_offset, u32 length)
+{
+	struct btrfs_ordered_extent *entry;
+
+	entry = alloc_ordered_extent(inode, file_offset, length, length, 0, 0, 0,
+				     (1UL << BTRFS_ORDERED_REGULAR) |
+				     (1UL << BTRFS_ORDERED_DELAYED),
+				     BTRFS_COMPRESS_NONE);
+	if (!IS_ERR(entry))
+		insert_ordered_extent(entry);
+	return entry;
+}
+
 /*
  * Add a struct btrfs_ordered_sum into the list of checksums to be inserted
  * when an ordered extent is finished.  If the list covers more than one
@@ -643,8 +713,9 @@ void btrfs_remove_ordered_extent(struct btrfs_ordered_extent *entry)
 	struct btrfs_root *root = btrfs_inode->root;
 	struct btrfs_fs_info *fs_info = root->fs_info;
 	struct rb_node *node;
-	bool pending;
+	bool pending = false;
 	bool freespace_inode;
+	const bool is_delayed = test_bit(BTRFS_ORDERED_DELAYED, &entry->flags);
 
 	/*
 	 * If this is a free space inode the thread has not acquired the ordered
@@ -653,33 +724,37 @@ void btrfs_remove_ordered_extent(struct btrfs_ordered_extent *entry)
 	freespace_inode = btrfs_is_free_space_inode(btrfs_inode);
 
 	btrfs_lockdep_acquire(fs_info, btrfs_trans_pending_ordered);
-	/* This is paired with alloc_ordered_extent(). */
-	spin_lock(&btrfs_inode->lock);
-	btrfs_mod_outstanding_extents(btrfs_inode, -1);
-	spin_unlock(&btrfs_inode->lock);
-	if (root != fs_info->tree_root) {
-		u64 release;
+	if (!is_delayed) {
+		/* This is paired with alloc_ordered_extent(). */
+		spin_lock(&btrfs_inode->lock);
+		btrfs_mod_outstanding_extents(btrfs_inode, -1);
+		spin_unlock(&btrfs_inode->lock);
 
-		if (test_bit(BTRFS_ORDERED_ENCODED, &entry->flags))
-			release = entry->disk_num_bytes;
-		else
-			release = entry->num_bytes;
-		btrfs_delalloc_release_metadata(btrfs_inode, release,
+		if (root != fs_info->tree_root) {
+			u64 release;
+
+			if (test_bit(BTRFS_ORDERED_ENCODED, &entry->flags))
+				release = entry->disk_num_bytes;
+			else
+				release = entry->num_bytes;
+			btrfs_delalloc_release_metadata(btrfs_inode, release,
 						test_bit(BTRFS_ORDERED_IOERR,
 							 &entry->flags));
+		}
 	}
-
 	percpu_counter_add_batch(&fs_info->ordered_bytes, -entry->num_bytes,
 				 fs_info->delalloc_batch);
 
 	spin_lock(&btrfs_inode->ordered_tree_lock);
-	node = &entry->rb_node;
-	rb_erase(node, &btrfs_inode->ordered_tree);
-	RB_CLEAR_NODE(node);
-	if (btrfs_inode->ordered_tree_last == node)
-		btrfs_inode->ordered_tree_last = NULL;
-	set_bit(BTRFS_ORDERED_COMPLETE, &entry->flags);
-	pending = test_and_clear_bit(BTRFS_ORDERED_PENDING, &entry->flags);
+	if (!RB_EMPTY_NODE(&entry->rb_node)) {
+		node = &entry->rb_node;
+		rb_erase(node, &btrfs_inode->ordered_tree);
+		RB_CLEAR_NODE(node);
+		if (btrfs_inode->ordered_tree_last == node)
+			btrfs_inode->ordered_tree_last = NULL;
+		set_bit(BTRFS_ORDERED_COMPLETE, &entry->flags);
+		pending = test_and_clear_bit(BTRFS_ORDERED_PENDING, &entry->flags);
+	}
 	spin_unlock(&btrfs_inode->ordered_tree_lock);
 
 	/*
@@ -711,17 +786,21 @@ void btrfs_remove_ordered_extent(struct btrfs_ordered_extent *entry)
 
 	btrfs_lockdep_release(fs_info, btrfs_trans_pending_ordered);
 
-	spin_lock(&root->ordered_extent_lock);
-	list_del_init(&entry->root_extent_list);
-	root->nr_ordered_extents--;
-
 	trace_btrfs_ordered_extent_remove(btrfs_inode, entry);
 
-	if (!root->nr_ordered_extents) {
-		spin_lock(&fs_info->ordered_root_lock);
-		BUG_ON(list_empty(&root->ordered_root));
-		list_del_init(&root->ordered_root);
-		spin_unlock(&fs_info->ordered_root_lock);
+	spin_lock(&root->ordered_extent_lock);
+	/* For child OEs, they are not added to per-root OEs. */
+	if (!list_empty(&entry->root_extent_list)) {
+		list_del_init(&entry->root_extent_list);
+		root->nr_ordered_extents--;
+
+
+		if (!root->nr_ordered_extents) {
+			spin_lock(&fs_info->ordered_root_lock);
+			BUG_ON(list_empty(&root->ordered_root));
+			list_del_init(&root->ordered_root);
+			spin_unlock(&fs_info->ordered_root_lock);
+		}
 	}
 	spin_unlock(&root->ordered_extent_lock);
 	wake_up(&entry->wait);
@@ -770,8 +849,18 @@ u64 btrfs_wait_ordered_extents(struct btrfs_root *root, u64 nr,
 		ordered = list_first_entry(&splice, struct btrfs_ordered_extent,
 					   root_extent_list);
 
-		if (range_end <= ordered->disk_bytenr ||
-		    ordered->disk_bytenr + ordered->disk_num_bytes <= range_start) {
+		/*
+		 * Delayed OEs have 0 disk_bytenr and 0 disk_num_bytes, thus
+		 * they will be considered out of the [0, U64_MAX) range.
+		 * And we do not know where they will really land until the
+		 * writeback finished.
+		 *
+		 * So here we must exclude delayed OEs from the bg range check,
+		 * and always wait for them.
+		 */
+		if (!test_bit(BTRFS_ORDERED_DELAYED, &ordered->flags) &&
+		    (range_end <= ordered->disk_bytenr ||
+		     ordered->disk_bytenr + ordered->disk_num_bytes <= range_start)) {
 			list_move_tail(&ordered->root_extent_list, &skipped);
 			cond_resched_lock(&root->ordered_extent_lock);
 			continue;
diff --git a/fs/btrfs/ordered-data.h b/fs/btrfs/ordered-data.h
index 2664ea455229..8a1800f109e8 100644
--- a/fs/btrfs/ordered-data.h
+++ b/fs/btrfs/ordered-data.h
@@ -13,6 +13,7 @@
 #include <linux/rbtree.h>
 #include <linux/wait.h>
 #include "async-thread.h"
+#include "compression.h"
 
 struct inode;
 struct page;
@@ -87,6 +88,12 @@ enum {
 	 */
 	BTRFS_ORDERED_DIRECT,
 
+	/*
+	 * Extra bit for delayed OE, can only be set for REGULAR.
+	 * Can not be set with COMPRESSED/ENCODED/DIRECT.
+	 */
+	BTRFS_ORDERED_DELAYED,
+
 	BTRFS_ORDERED_NR_FLAGS,
 };
 static_assert(BTRFS_ORDERED_NR_FLAGS <= BITS_PER_LONG);
@@ -155,6 +162,11 @@ struct btrfs_ordered_extent {
 	/* a per root list of all the pending ordered extents */
 	struct list_head root_extent_list;
 
+	/* Child ordered extent list for delayed OE. */
+	struct list_head child_list;
+
+	unsigned long child_bitmap[BITS_TO_LONGS(BTRFS_MAX_COMPRESSED / BTRFS_MIN_BLOCKSIZE)];
+
 	struct btrfs_work work;
 
 	struct completion completion;
@@ -192,6 +204,8 @@ struct btrfs_file_extent {
 struct btrfs_ordered_extent *btrfs_alloc_ordered_extent(
 			struct btrfs_inode *inode, u64 file_offset,
 			const struct btrfs_file_extent *file_extent, unsigned long flags);
+struct btrfs_ordered_extent *btrfs_alloc_delayed_ordered_extent(
+			struct btrfs_inode *inode, u64 file_offset, u32 length);
 void btrfs_add_ordered_sum(struct btrfs_ordered_extent *entry,
 			   struct btrfs_ordered_sum *sum);
 struct btrfs_ordered_extent *btrfs_lookup_ordered_extent(struct btrfs_inode *inode,
-- 
2.53.0

[PATCH 3/6] btrfs: introduce the skeleton of delayed bbio endio function

[Qu Wenruo]

A delayed bbio will not be directly submitted, but queued into a
workqueue, doing the heavy lifting compression there.

The compression and uncompressed fallback are not implemented in this
patch.

Only the main endio function and helper to queue workload into a
workqueue is implemented.

The endio function is mostly the same as end_bbio_data_write(), except
the extra memory allocation/freeing for the bbio->private.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/inode.c | 68 +++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 64 insertions(+), 4 deletions(-)

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 799e3d5c80f3..03018cc3bd25 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -97,6 +97,12 @@ struct data_reloc_warn {
 	int mirror_num;
 };
 
+struct delayed_bio_private {
+	struct work_struct work;
+	struct btrfs_bio *delayed_bbio;
+	atomic_t pending_ios;
+};
+
 /*
  * For the file_extent_tree, we want to hold the inode lock when we lookup and
  * update the disk_i_size, but lockdep will complain because our io_tree we hold
@@ -7657,18 +7663,72 @@ struct extent_map *btrfs_create_delayed_em(struct btrfs_inode *inode,
 	return em;
 }
 
-void btrfs_submit_delayed_write(struct btrfs_bio *bbio)
+static void run_delayed_bbio(struct work_struct *work)
 {
-	ASSERT(bbio->is_delayed);
+	struct delayed_bio_private *dbp = container_of(work, struct delayed_bio_private, work);
+	struct btrfs_bio *parent = dbp->delayed_bbio;
 
 	/*
-	 * Not yet implemented, and should not hit this path as we have no
-	 * caller to create delayed extent map.
+	 * Increase the pending_ios so that parent bbio won't end
+	 * until all child ones are submitted.
 	 */
+	atomic_inc(&dbp->pending_ios);
+	/* Compressed and uncompressed fallback is not yet implemented. */
 	ASSERT(0);
+	if (atomic_dec_and_test(&dbp->pending_ios))
+		btrfs_bio_end_io(parent, parent->status);
+}
+
+static void end_bbio_delayed(struct btrfs_bio *bbio)
+{
+	struct delayed_bio_private *dbp = bbio->private;
+	struct btrfs_inode *inode = bbio->inode;
+	struct btrfs_fs_info *fs_info = inode->root->fs_info;
+	struct folio_iter fi;
+	const u32 bio_size = bio_get_size(&bbio->bio);
+	const bool uptodate = bbio->status == BLK_STS_OK;
+
+	ASSERT(bbio->is_delayed);
+
+	bio_for_each_folio_all(fi, &bbio->bio) {
+		u64 start = folio_pos(fi.folio) + fi.offset;
+		u32 len = fi.length;
+
+		btrfs_folio_clear_ordered(fs_info, fi.folio, start, len);
+		btrfs_folio_clear_writeback(fs_info, fi.folio, start, len);
+	}
+	btrfs_mark_ordered_io_finished(inode, bbio->file_offset, bio_size, uptodate);
+	kfree(dbp);
 	bio_put(&bbio->bio);
 }
 
+void btrfs_submit_delayed_write(struct btrfs_bio *bbio)
+{
+	struct delayed_bio_private *dbp;
+
+	ASSERT(bbio->is_delayed);
+
+	bbio->end_io = end_bbio_delayed;
+	dbp = kzalloc(sizeof(struct delayed_bio_private), GFP_NOFS);
+	if (!dbp) {
+		btrfs_bio_end_io(bbio, errno_to_blk_status(-ENOMEM));
+		return;
+	}
+	atomic_set(&dbp->pending_ios, 0);
+	dbp->delayed_bbio = bbio;
+	bbio->private = dbp;
+	/*
+	 * TODO: find a way to properly allow sequential extent allocation.
+	 *
+	 * The existing btrfs async workqueue will execute the sequential workload
+	 * twice, the second one to free the structure.
+	 * But our current submission path can only be called once, after that
+	 * the bbio will be gone thus can not afford to use btrfs async workqueue.
+	 */
+	INIT_WORK(&dbp->work, run_delayed_bbio);
+	schedule_work(&dbp->work);
+}
+
 /*
  * For release_folio() and invalidate_folio() we have a race window where
  * folio_end_writeback() is called but the subpage spinlock is not yet released.
-- 
2.53.0

[PATCH 4/6] btrfs: introduce compression for delayed bbio

[Qu Wenruo]

The compressed write path inside a delayed bbio is mostly the same as
regular compression, but with some differences:

- The error handling should not touch folio flags
  It will be handled by the parent delayed bbio.
  And those folios already have WRITEBACK flag set, not the LOCKED flag
  of the async submission path.

- A successful compression will lead to a child compressed bio
  That compressed bio will be properly submitted, and if there is no
  more pending ios of the delayed bbio, end the delayed bbio.

  There is minor note, since we're going through the regular
  extent_writepage_io() path, we can have multiple bbios for the same
  delayed ordered extent.

  This means we may have a slightly lower compression ratio if by
  whatever reason the write back path choose to skip the bio.

- No sequential execution of data extent reservation
  The existing async thread has one quirk related to the ordered
  function execution, which is not suitable for this call site.

  After the compressed bio is submitted, we can no longer touch the
  child compressed bio (it can finished immediately and also finish the
  parent delayed bbio).
  Meanwhile the async ordered function needs different entries to handle
  the workload and free involved structures.

  This will be the major changes compared to the existing compressed
  write.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/inode.c | 112 ++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 111 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 03018cc3bd25..ef1f5efb68ca 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -7663,6 +7663,111 @@ struct extent_map *btrfs_create_delayed_em(struct btrfs_inode *inode,
 	return em;
 }
 
+static void end_bbio_delayed_compressed(struct btrfs_bio *bbio)
+{
+	struct delayed_bio_private *dbp = bbio->private;
+	struct btrfs_bio *parent = dbp->delayed_bbio;
+	struct folio_iter fi;
+
+	bio_for_each_folio_all(fi, &bbio->bio)
+		btrfs_free_compr_folio(fi.folio);
+	bio_put(&bbio->bio);
+
+	cmpxchg(&parent->status, BLK_STS_OK, bbio->status);
+	if (atomic_dec_and_test(&dbp->pending_ios))
+		btrfs_bio_end_io(parent, parent->status);
+}
+
+static bool try_submit_compressed(struct btrfs_bio *parent)
+{
+	struct delayed_bio_private *dbp = parent->private;
+	struct btrfs_bio *bbio = dbp->delayed_bbio;
+	struct btrfs_inode *inode = bbio->inode;
+	struct btrfs_fs_info *fs_info = inode->root->fs_info;
+	struct btrfs_key ins;
+	struct compressed_bio *cb;
+	struct extent_state *cached = NULL;
+	struct extent_map *em;
+	struct btrfs_ordered_extent *ordered;
+	struct btrfs_file_extent file_extent;
+	u64 alloc_hint;
+	const u32 len = bio_get_size(&bbio->bio);
+	const u64 fileoff = bbio->file_offset;
+	const u64 end = fileoff + len - 1;
+	u32 compressed_size;
+	int compress_type = fs_info->compress_type;
+	int compress_level = fs_info->compress_level;
+	int ret;
+
+	if (!btrfs_inode_can_compress(inode) ||
+	    !inode_need_compress(inode, fileoff, end, false))
+		return false;
+
+	if (inode->defrag_compress > 0 &&
+	    inode->defrag_compress < BTRFS_NR_COMPRESS_TYPES) {
+		compress_type = inode->defrag_compress;
+		compress_level = inode->defrag_compress_level;
+	} else if (inode->prop_compress) {
+		compress_type = inode->prop_compress;
+	}
+	cb = btrfs_compress_bio(inode, fileoff, len, compress_type,
+				compress_level, 0);
+	if (IS_ERR(cb))
+		return false;
+
+	round_up_last_block(cb, fs_info->sectorsize);
+	compressed_size = cb->bbio.bio.bi_iter.bi_size;
+
+	alloc_hint = btrfs_get_extent_allocation_hint(inode, fileoff, len);
+	ret = btrfs_reserve_extent(inode->root, len,
+				   compressed_size, compressed_size,
+				   0, alloc_hint, &ins, true, true);
+	if (ret < 0) {
+		cleanup_compressed_bio(cb);
+		return false;
+	}
+	btrfs_lock_extent(&inode->io_tree, fileoff, end, &cached);
+	file_extent.disk_bytenr = ins.objectid;
+	file_extent.disk_num_bytes = ins.offset;
+	file_extent.ram_bytes = len;
+	file_extent.num_bytes = len;
+	file_extent.offset = 0;
+	file_extent.compression = cb->compress_type;
+
+	cb->bbio.bio.bi_iter.bi_sector = ins.objectid >> SECTOR_SHIFT;
+	em = btrfs_create_io_em(inode, fileoff, &file_extent, BTRFS_ORDERED_COMPRESSED);
+	if (IS_ERR(em)) {
+		ret = PTR_ERR(em);
+		goto out_free_reserve;
+	}
+	btrfs_free_extent_map(em);
+
+	ordered = btrfs_alloc_ordered_extent(inode, fileoff, &file_extent,
+					     1U << BTRFS_ORDERED_COMPRESSED);
+	if (IS_ERR(ordered)) {
+		btrfs_drop_extent_map_range(inode, fileoff, end, false);
+		ret = PTR_ERR(ordered);
+		goto out_free_reserve;
+	}
+	cb->bbio.ordered = ordered;
+	btrfs_dec_block_group_reservations(fs_info, ins.objectid);
+	btrfs_unlock_extent(&inode->io_tree, fileoff, end, &cached);
+
+	cb->bbio.end_io = end_bbio_delayed_compressed;
+	cb->bbio.private = dbp;
+	atomic_inc(&dbp->pending_ios);
+	btrfs_submit_bbio(&cb->bbio, 0);
+	return true;
+
+out_free_reserve:
+	btrfs_dec_block_group_reservations(fs_info, ins.objectid);
+	btrfs_free_reserved_extent(fs_info, ins.objectid, ins.offset, true);
+	mapping_set_error(inode->vfs_inode.i_mapping, -EIO);
+	btrfs_unlock_extent(&inode->io_tree, fileoff, end, &cached);
+	cleanup_compressed_bio(cb);
+	return false;
+}
+
 static void run_delayed_bbio(struct work_struct *work)
 {
 	struct delayed_bio_private *dbp = container_of(work, struct delayed_bio_private, work);
@@ -7673,8 +7778,13 @@ static void run_delayed_bbio(struct work_struct *work)
 	 * until all child ones are submitted.
 	 */
 	atomic_inc(&dbp->pending_ios);
-	/* Compressed and uncompressed fallback is not yet implemented. */
+	if (try_submit_compressed(parent))
+		goto finish;
+
+	/* Uncompressed fallback is not yet implemented. */
 	ASSERT(0);
+
+finish:
 	if (atomic_dec_and_test(&dbp->pending_ios))
 		btrfs_bio_end_io(parent, parent->status);
 }
-- 
2.53.0

[PATCH 5/6] btrfs: implement uncompressed fallback for delayed bbio

[Qu Wenruo]

When the compression failed (either bad ratio, fragmented free space, or
writeback path choose to submit the bio early), we have to fall back to
uncompressed writes.

The uncompressed fallback is mostly the same as cow_file_range() but
with some changes:

- Endio function is slightly different from the compressed path
  Only in the folio freeing handling.

- Uncompressed fallback error handling
  Since at the stage, the folios already have WRITEBACK flag set, we do
  not need to do the usual page unlock/end writeback, but just free the
  reserved space and call it a day.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/inode.c | 149 ++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 147 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index ef1f5efb68ca..183fb3c83c10 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -7768,10 +7768,144 @@ static bool try_submit_compressed(struct btrfs_bio *parent)
 	return false;
 }
 
+static void end_bbio_delayed_uncompressed(struct btrfs_bio *bbio)
+{
+	struct delayed_bio_private *dbp = bbio->private;
+	struct btrfs_bio *parent = dbp->delayed_bbio;
+	struct folio_iter fi;
+
+	bio_for_each_folio_all(fi, &bbio->bio)
+		folio_put(fi.folio);
+	bio_put(&bbio->bio);
+
+	cmpxchg(&parent->status, BLK_STS_OK, bbio->status);
+	if (atomic_dec_and_test(&dbp->pending_ios))
+		btrfs_bio_end_io(parent, parent->status);
+}
+
+static struct btrfs_bio *child_bbio_from_page_cache(struct btrfs_bio *parent,
+						    u64 fileoff, u32 len)
+{
+	struct btrfs_inode *inode = parent->inode;
+	struct address_space *mapping = inode->vfs_inode.i_mapping;
+	struct btrfs_bio *bbio;
+	struct folio_iter fi;
+	u64 cur = fileoff;
+	int ret;
+
+	bbio = btrfs_bio_alloc(round_up(len, PAGE_SIZE) >> PAGE_SHIFT, REQ_OP_WRITE,
+			       inode, fileoff, end_bbio_delayed_uncompressed,
+			       parent->private);
+
+	while (cur < fileoff + len) {
+		struct folio *folio;
+		u32 cur_len;
+
+		folio = filemap_get_folio(mapping, cur >> PAGE_SHIFT);
+		if (IS_ERR(folio)) {
+			ret = PTR_ERR(folio);
+			goto error;
+		}
+		cur_len = min_t(u64, folio_next_pos(folio), fileoff + len) - cur;
+		ret = bio_add_folio(&bbio->bio, folio, cur_len,
+				    offset_in_folio(folio, cur));
+		ASSERT(ret);
+		cur += cur_len;
+	}
+
+	return bbio;
+error:
+	bio_for_each_folio_all(fi, &bbio->bio)
+		folio_put(fi.folio);
+	bio_put(&bbio->bio);
+	return ERR_PTR(ret);
+}
+
+static int submit_one_uncompressed_range(struct btrfs_bio *parent, struct btrfs_key *ins,
+					 struct extent_state **cached, u64 file_offset,
+					 u32 num_bytes, u64 alloc_hint, u32 *ret_alloc_size)
+{
+	struct btrfs_inode *inode = parent->inode;
+	struct delayed_bio_private *dbp = parent->private;
+	struct btrfs_root *root = inode->root;
+	struct btrfs_fs_info *fs_info = root->fs_info;
+	struct btrfs_ordered_extent *ordered;
+	struct btrfs_file_extent file_extent;
+	struct btrfs_bio *child;
+	struct extent_map *em;
+	u64 cur_end;
+	u32 cur_len = 0;
+	int ret;
+
+	ret = btrfs_reserve_extent(root, num_bytes, num_bytes, fs_info->sectorsize,
+				   0, alloc_hint, ins, true, true);
+	if (ret < 0)
+		return ret;
+
+	cur_len = ins->offset;
+	cur_end = file_offset + cur_len - 1;
+
+	file_extent.disk_bytenr = ins->objectid;
+	file_extent.disk_num_bytes = ins->offset;
+	file_extent.num_bytes = ins->offset;
+	file_extent.ram_bytes = ins->offset;
+	file_extent.offset = 0;
+	file_extent.compression = BTRFS_COMPRESS_NONE;
+
+	btrfs_lock_extent(&inode->io_tree, file_offset, cur_end, cached);
+	em = btrfs_create_io_em(inode, file_offset, &file_extent, BTRFS_ORDERED_REGULAR);
+	if (IS_ERR(em)) {
+		ret = PTR_ERR(em);
+		btrfs_unlock_extent(&inode->io_tree, file_offset, cur_end, cached);
+		goto free_reserved;
+	}
+	btrfs_free_extent_map(em);
+	ordered = btrfs_alloc_ordered_extent(inode, file_offset, &file_extent,
+					     1U << BTRFS_ORDERED_REGULAR);
+	if (IS_ERR(ordered)) {
+		btrfs_drop_extent_map_range(inode, file_offset, cur_end, false);
+		btrfs_unlock_extent(&inode->io_tree, file_offset, cur_end, cached);
+		ret = PTR_ERR(ordered);
+		goto free_reserved;
+	}
+	btrfs_dec_block_group_reservations(fs_info, ins->objectid);
+	btrfs_unlock_extent(&inode->io_tree, file_offset, cur_end, cached);
+	child = child_bbio_from_page_cache(parent, file_offset, cur_len);
+	if (IS_ERR(child)) {
+		btrfs_put_ordered_extent(ordered);
+		btrfs_drop_extent_map_range(inode, file_offset, cur_end, false);
+		ret = PTR_ERR(ordered);
+		goto free_reserved;
+	}
+	child->ordered = ordered;
+	child->private = parent->private;
+	child->end_io = end_bbio_delayed_uncompressed;
+	child->bio.bi_iter.bi_sector = ins->objectid >> SECTOR_SHIFT;
+	atomic_inc(&dbp->pending_ios);
+	btrfs_submit_bbio(child, 0);
+	*ret_alloc_size = cur_len;
+	return 0;
+
+free_reserved:
+	btrfs_qgroup_free_data(inode, NULL, file_offset, cur_len, NULL);
+	btrfs_dec_block_group_reservations(fs_info, ins->objectid);
+	btrfs_free_reserved_extent(fs_info, ins->objectid, ins->offset, true);
+	ASSERT(ret != -EAGAIN);
+	return ret;
+}
+
 static void run_delayed_bbio(struct work_struct *work)
 {
 	struct delayed_bio_private *dbp = container_of(work, struct delayed_bio_private, work);
 	struct btrfs_bio *parent = dbp->delayed_bbio;
+	struct btrfs_key ins;
+	struct extent_state *cached = NULL;
+	const u32 uncompressed_size = bio_get_size(&parent->bio);
+	const u64 start = parent->file_offset;
+	const u64 end = start + uncompressed_size - 1;
+	u64 cur = start;
+	u64 alloc_hint;
+	int ret = 0;
 
 	/*
 	 * Increase the pending_ios so that parent bbio won't end
@@ -7781,8 +7915,19 @@ static void run_delayed_bbio(struct work_struct *work)
 	if (try_submit_compressed(parent))
 		goto finish;
 
-	/* Uncompressed fallback is not yet implemented. */
-	ASSERT(0);
+	alloc_hint = btrfs_get_extent_allocation_hint(parent->inode, start,
+						      uncompressed_size);
+	while (cur < end) {
+		u32 cur_len;
+
+		ret = submit_one_uncompressed_range(parent, &ins, &cached,
+						    cur, end + 1 - cur,
+						    alloc_hint, &cur_len);
+		if (ret < 0)
+			goto finish;
+		cur += cur_len;
+		alloc_hint += cur_len;
+	}
 
 finish:
 	if (atomic_dec_and_test(&dbp->pending_ios))
-- 
2.53.0

[PATCH 6/6] btrfs: enable experimental delayed compression support

[Qu Wenruo]

Instead of the existing async submission path, the new delayed bbio will
handle compressed write by:

- Allocating delayed em/oe at run_delalloc_*() time
  Thus no data extent is reserved at that time.

- Delayed bbio will be assembled at extent_writepage_io() time

- Delayed bbio will be intercepted just before submission
  Which will run compression (or fallback to uncompressed writes) in
  workqueue.
  Data extents will only be reserved at that time, and the delayed em
  will be replaced by real ones.

  Meanwhile the real OE will be added as a child of the parent delayed
  OE, and when the parent OE finishes, the child OE will be finished
  with their file extents inserted.

This has some benefits:

- Higher concurrency
  Previously async submission will hold the folio and io tree range
  locked, this means we can not even read the uptodate folio.

  Furthermore although the compressed write is queued into a workqueue
  for submission and extent_writepage_io() will skip the compressed
  range, when we need to write the next folio of the compressed range,
  we will need to wait for the folio to be unlocked.

  This makes async submission less async.

- Future DONTCACHE writes support
  We do not support DONTCACHE because that feature requires writeback path
  to clear the folio dirty and submit them sequentially.

  Meanwhile async submission makes the writeback async, breaking the
  sequential submission requirement.

  This is also why we need a complex per-block tracking for writeback
  flags, meanwhile iomap only requires a counter tracking.

  With the new delayed compression, the lifespan of a folio aligns with
  DONTCACHE and iomap.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/inode.c | 60 +++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 57 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 183fb3c83c10..c95207009210 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -1673,6 +1673,57 @@ static bool run_delalloc_compressed(struct btrfs_inode *inode,
 	return true;
 }
 
+static int run_delalloc_delayed(struct btrfs_inode *inode, struct folio *locked_folio,
+				u64 start, u64 end)
+{
+	struct btrfs_root *root = inode->root;
+	struct btrfs_fs_info *fs_info = root->fs_info;
+	struct extent_state *cached = NULL;
+	u64 cur = start;
+	int ret;
+
+	if (btrfs_is_shutdown(fs_info)) {
+		ret = -EIO;
+		goto error;
+	}
+	while (cur < end) {
+		struct extent_map *em;
+		struct btrfs_ordered_extent *oe;
+		u32 cur_len = min_t(u64, end + 1 - cur, BTRFS_MAX_COMPRESSED);
+
+		btrfs_lock_extent(&inode->io_tree, cur, cur + cur_len - 1, &cached);
+		em = btrfs_create_delayed_em(inode, cur, cur_len);
+		if (IS_ERR(em)) {
+			ret = PTR_ERR(em);
+			goto error;
+		}
+		btrfs_free_extent_map(em);
+		oe = btrfs_alloc_delayed_ordered_extent(inode, cur, cur_len);
+		if (IS_ERR(oe)) {
+			btrfs_drop_extent_map_range(inode, cur, cur + cur_len - 1, false);
+			ret = PTR_ERR(em);
+			goto error;
+		}
+		btrfs_put_ordered_extent(oe);
+
+		cur += cur_len;
+	}
+	extent_clear_unlock_delalloc(inode, start, end, locked_folio, &cached,
+				     EXTENT_LOCKED | EXTENT_DELALLOC,
+				     PAGE_UNLOCK | PAGE_SET_ORDERED);
+	return 0;
+error:
+	/* We have to drop any created delayed extent maps. */
+	if (start < cur)
+		btrfs_drop_extent_map_range(inode, start, cur - 1, false);
+	/* No range has any extent reserved, just clear them all. */
+	extent_clear_unlock_delalloc(inode, start, end, locked_folio, &cached,
+			EXTENT_LOCKED | EXTENT_DELALLOC | EXTENT_DELALLOC_NEW |
+			EXTENT_DEFRAG | EXTENT_DO_ACCOUNTING,
+			PAGE_UNLOCK | PAGE_START_WRITEBACK | PAGE_END_WRITEBACK);
+	return ret;
+}
+
 /*
  * Run the delalloc range from start to end, and write back any dirty pages
  * covered by the range.
@@ -2436,9 +2487,12 @@ int btrfs_run_delalloc_range(struct btrfs_inode *inode, struct folio *locked_fol
 		return run_delalloc_nocow(inode, locked_folio, start, end);
 
 	if (btrfs_inode_can_compress(inode) &&
-	    inode_need_compress(inode, start, end, false) &&
-	    run_delalloc_compressed(inode, locked_folio, start, end, wbc))
-		return 1;
+	    inode_need_compress(inode, start, end, false)) {
+		if (IS_ENABLED(CONFIG_BTRFS_EXPERIMENTAL))
+			return run_delalloc_delayed(inode, locked_folio, start, end);
+		else if (run_delalloc_compressed(inode, locked_folio, start, end, wbc))
+			return 1;
+	}
 
 	if (zoned)
 		return run_delalloc_cow(inode, locked_folio, start, end, wbc, true);
-- 
2.53.0