[PATCH v2 0/2] btrfs: unify the read and writer locks for btrfs_subpage

[PATCH v2 0/2] btrfs: unify the read and writer locks for btrfs_subpage

[Qu Wenruo]

[CHANGELOG]
v2:
- Rename btrfs_subpage::locked to btrfs_subpage::nr_locked

When the handling of sector size < page size is introduced, there are
two types of locking, reader and writer lock.

The main reason for the reader lock is to handle metadata to make sure
the page::private is not released when there is still a metadata being
read.

However since commit d7172f52e993 ("btrfs: use per-buffer locking for
extent_buffer reading"), metadata read no longer relies on
btrfs_subpage::readers.

Making the writer lock as the only utilized subpage locking.

This patchset converts all the existing reader lock usage and rename the
writer lock into a generic lock.

This patchset relies on this patch "btrfs: fix the delalloc range
locking if sector size < page size", as it removes the last user of
btrfs_folio_start_writer_lock().

Qu Wenruo (2):
  btrfs: unify to use writer locks for subpage locking
  btrfs: rename btrfs_folio_(set|start|end)_writer_lock()

 fs/btrfs/compression.c |   3 +-
 fs/btrfs/extent_io.c   |  20 +++-----
 fs/btrfs/subpage.c     | 108 ++++++++++-------------------------------
 fs/btrfs/subpage.h     |  33 +++++--------
 4 files changed, 45 insertions(+), 119 deletions(-)

-- 
2.46.2

[PATCH v2 1/2] btrfs: unify to use writer locks for subpage locking

[Qu Wenruo]

Since commit d7172f52e993 ("btrfs: use per-buffer locking for
extent_buffer reading"), metadata read no longer relies on the subpage
reader locking.

This means we do not need to maintain a different metadata/data split
for locking, so we can convert the existing reader lock users by:

- add_ra_bio_pages()
  Convert to btrfs_folio_set_writer_lock()

- end_folio_read()
  Convert to btrfs_folio_end_writer_lock()

- begin_folio_read()
  Convert to btrfs_folio_set_writer_lock()

- folio_range_has_eb()
  Remove the subpage->readers checks, since it is always 0.

- Remove btrfs_subpage_start_reader() and btrfs_subpage_end_reader()

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/compression.c |  3 +-
 fs/btrfs/extent_io.c   | 10 ++-----
 fs/btrfs/subpage.c     | 62 ++----------------------------------------
 fs/btrfs/subpage.h     | 13 ---------
 4 files changed, 5 insertions(+), 83 deletions(-)

diff --git a/fs/btrfs/compression.c b/fs/btrfs/compression.c
index 6e9c4a5e0d51..74799270eb78 100644
--- a/fs/btrfs/compression.c
+++ b/fs/btrfs/compression.c
@@ -545,8 +545,7 @@ static noinline int add_ra_bio_pages(struct inode *inode,
 		 * subpage::readers and to unlock the page.
 		 */
 		if (fs_info->sectorsize < PAGE_SIZE)
-			btrfs_subpage_start_reader(fs_info, folio, cur,
-						   add_size);
+			btrfs_folio_set_writer_lock(fs_info, folio, cur, add_size);
 		folio_put(folio);
 		cur += add_size;
 	}
diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index bfa745258e9b..06d2b882bf8c 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -438,7 +438,7 @@ static void end_folio_read(struct folio *folio, bool uptodate, u64 start, u32 le
 	if (!btrfs_is_subpage(fs_info, folio->mapping))
 		folio_unlock(folio);
 	else
-		btrfs_subpage_end_reader(fs_info, folio, start, len);
+		btrfs_folio_end_writer_lock(fs_info, folio, start, len);
 }
 
 /*
@@ -495,7 +495,7 @@ static void begin_folio_read(struct btrfs_fs_info *fs_info, struct folio *folio)
 		return;
 
 	ASSERT(folio_test_private(folio));
-	btrfs_subpage_start_reader(fs_info, folio, folio_pos(folio), PAGE_SIZE);
+	btrfs_folio_set_writer_lock(fs_info, folio, folio_pos(folio), PAGE_SIZE);
 }
 
 /*
@@ -2517,12 +2517,6 @@ static bool folio_range_has_eb(struct btrfs_fs_info *fs_info, struct folio *foli
 		subpage = folio_get_private(folio);
 		if (atomic_read(&subpage->eb_refs))
 			return true;
-		/*
-		 * Even there is no eb refs here, we may still have
-		 * end_folio_read() call relying on page::private.
-		 */
-		if (atomic_read(&subpage->readers))
-			return true;
 	}
 	return false;
 }
diff --git a/fs/btrfs/subpage.c b/fs/btrfs/subpage.c
index 26d4d05dd165..6c3d54a45956 100644
--- a/fs/btrfs/subpage.c
+++ b/fs/btrfs/subpage.c
@@ -140,12 +140,10 @@ struct btrfs_subpage *btrfs_alloc_subpage(const struct btrfs_fs_info *fs_info,
 		return ERR_PTR(-ENOMEM);
 
 	spin_lock_init(&ret->lock);
-	if (type == BTRFS_SUBPAGE_METADATA) {
+	if (type == BTRFS_SUBPAGE_METADATA)
 		atomic_set(&ret->eb_refs, 0);
-	} else {
-		atomic_set(&ret->readers, 0);
+	else
 		atomic_set(&ret->writers, 0);
-	}
 	return ret;
 }
 
@@ -221,62 +219,6 @@ static void btrfs_subpage_assert(const struct btrfs_fs_info *fs_info,
 	__start_bit;							\
 })
 
-void btrfs_subpage_start_reader(const struct btrfs_fs_info *fs_info,
-				struct folio *folio, u64 start, u32 len)
-{
-	struct btrfs_subpage *subpage = folio_get_private(folio);
-	const int start_bit = subpage_calc_start_bit(fs_info, folio, locked, start, len);
-	const int nbits = len >> fs_info->sectorsize_bits;
-	unsigned long flags;
-
-
-	btrfs_subpage_assert(fs_info, folio, start, len);
-
-	spin_lock_irqsave(&subpage->lock, flags);
-	/*
-	 * Even though it's just for reading the page, no one should have
-	 * locked the subpage range.
-	 */
-	ASSERT(bitmap_test_range_all_zero(subpage->bitmaps, start_bit, nbits));
-	bitmap_set(subpage->bitmaps, start_bit, nbits);
-	atomic_add(nbits, &subpage->readers);
-	spin_unlock_irqrestore(&subpage->lock, flags);
-}
-
-void btrfs_subpage_end_reader(const struct btrfs_fs_info *fs_info,
-			      struct folio *folio, u64 start, u32 len)
-{
-	struct btrfs_subpage *subpage = folio_get_private(folio);
-	const int start_bit = subpage_calc_start_bit(fs_info, folio, locked, start, len);
-	const int nbits = len >> fs_info->sectorsize_bits;
-	unsigned long flags;
-	bool is_data;
-	bool last;
-
-	btrfs_subpage_assert(fs_info, folio, start, len);
-	is_data = is_data_inode(BTRFS_I(folio->mapping->host));
-
-	spin_lock_irqsave(&subpage->lock, flags);
-
-	/* The range should have already been locked. */
-	ASSERT(bitmap_test_range_all_set(subpage->bitmaps, start_bit, nbits));
-	ASSERT(atomic_read(&subpage->readers) >= nbits);
-
-	bitmap_clear(subpage->bitmaps, start_bit, nbits);
-	last = atomic_sub_and_test(nbits, &subpage->readers);
-
-	/*
-	 * For data we need to unlock the page if the last read has finished.
-	 *
-	 * And please don't replace @last with atomic_sub_and_test() call
-	 * inside if () condition.
-	 * As we want the atomic_sub_and_test() to be always executed.
-	 */
-	if (is_data && last)
-		folio_unlock(folio);
-	spin_unlock_irqrestore(&subpage->lock, flags);
-}
-
 static void btrfs_subpage_clamp_range(struct folio *folio, u64 *start, u32 *len)
 {
 	u64 orig_start = *start;
diff --git a/fs/btrfs/subpage.h b/fs/btrfs/subpage.h
index e4076c5b06bc..c150aba9318e 100644
--- a/fs/btrfs/subpage.h
+++ b/fs/btrfs/subpage.h
@@ -45,14 +45,6 @@ enum {
 struct btrfs_subpage {
 	/* Common members for both data and metadata pages */
 	spinlock_t lock;
-	/*
-	 * Both data and metadata needs to track how many readers are for the
-	 * page.
-	 * Data relies on @readers to unlock the page when last reader finished.
-	 * While metadata doesn't need page unlock, it needs to prevent
-	 * page::private get cleared before the last end_page_read().
-	 */
-	atomic_t readers;
 	union {
 		/*
 		 * Structures only used by metadata
@@ -95,11 +87,6 @@ void btrfs_free_subpage(struct btrfs_subpage *subpage);
 void btrfs_folio_inc_eb_refs(const struct btrfs_fs_info *fs_info, struct folio *folio);
 void btrfs_folio_dec_eb_refs(const struct btrfs_fs_info *fs_info, struct folio *folio);
 
-void btrfs_subpage_start_reader(const struct btrfs_fs_info *fs_info,
-				struct folio *folio, u64 start, u32 len);
-void btrfs_subpage_end_reader(const struct btrfs_fs_info *fs_info,
-			      struct folio *folio, u64 start, u32 len);
-
 void btrfs_folio_end_writer_lock(const struct btrfs_fs_info *fs_info,
 				 struct folio *folio, u64 start, u32 len);
 void btrfs_folio_set_writer_lock(const struct btrfs_fs_info *fs_info,
-- 
2.46.2

[PATCH v2 2/2] btrfs: rename btrfs_folio_(set|start|end)_writer_lock()

[Qu Wenruo]

Since there is no user of reader locks, rename the writer locks into a
more generic name, by removing the "_writer" part from the name.

And also rename btrfs_subpage::writer into btrfs_subpage::locked.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/compression.c |  2 +-
 fs/btrfs/extent_io.c   | 14 ++++++-------
 fs/btrfs/subpage.c     | 46 +++++++++++++++++++++---------------------
 fs/btrfs/subpage.h     | 20 ++++++++++--------
 4 files changed, 43 insertions(+), 39 deletions(-)

diff --git a/fs/btrfs/compression.c b/fs/btrfs/compression.c
index 74799270eb78..dfa872cf09d5 100644
--- a/fs/btrfs/compression.c
+++ b/fs/btrfs/compression.c
@@ -545,7 +545,7 @@ static noinline int add_ra_bio_pages(struct inode *inode,
 		 * subpage::readers and to unlock the page.
 		 */
 		if (fs_info->sectorsize < PAGE_SIZE)
-			btrfs_folio_set_writer_lock(fs_info, folio, cur, add_size);
+			btrfs_folio_set_lock(fs_info, folio, cur, add_size);
 		folio_put(folio);
 		cur += add_size;
 	}
diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index 06d2b882bf8c..f94def65e83b 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -190,7 +190,7 @@ static void process_one_folio(struct btrfs_fs_info *fs_info,
 		btrfs_folio_clamp_clear_writeback(fs_info, folio, start, len);
 
 	if (folio != locked_folio && (page_ops & PAGE_UNLOCK))
-		btrfs_folio_end_writer_lock(fs_info, folio, start, len);
+		btrfs_folio_end_lock(fs_info, folio, start, len);
 }
 
 static void __process_folios_contig(struct address_space *mapping,
@@ -276,7 +276,7 @@ static noinline int lock_delalloc_folios(struct inode *inode,
 			range_start = max_t(u64, folio_pos(folio), start);
 			range_len = min_t(u64, folio_pos(folio) + folio_size(folio),
 					  end + 1) - range_start;
-			btrfs_folio_set_writer_lock(fs_info, folio, range_start, range_len);
+			btrfs_folio_set_lock(fs_info, folio, range_start, range_len);
 
 			processed_end = range_start + range_len - 1;
 		}
@@ -438,7 +438,7 @@ static void end_folio_read(struct folio *folio, bool uptodate, u64 start, u32 le
 	if (!btrfs_is_subpage(fs_info, folio->mapping))
 		folio_unlock(folio);
 	else
-		btrfs_folio_end_writer_lock(fs_info, folio, start, len);
+		btrfs_folio_end_lock(fs_info, folio, start, len);
 }
 
 /*
@@ -495,7 +495,7 @@ static void begin_folio_read(struct btrfs_fs_info *fs_info, struct folio *folio)
 		return;
 
 	ASSERT(folio_test_private(folio));
-	btrfs_folio_set_writer_lock(fs_info, folio, folio_pos(folio), PAGE_SIZE);
+	btrfs_folio_set_lock(fs_info, folio, folio_pos(folio), PAGE_SIZE);
 }
 
 /*
@@ -1184,7 +1184,7 @@ static noinline_for_stack int writepage_delalloc(struct btrfs_inode *inode,
 	for_each_set_bit(bit, &bio_ctrl->submit_bitmap, fs_info->sectors_per_page) {
 		u64 start = page_start + (bit << fs_info->sectorsize_bits);
 
-		btrfs_folio_set_writer_lock(fs_info, folio, start, fs_info->sectorsize);
+		btrfs_folio_set_lock(fs_info, folio, start, fs_info->sectorsize);
 	}
 
 	/* Lock all (subpage) delalloc ranges inside the folio first. */
@@ -1520,7 +1520,7 @@ static int extent_writepage(struct folio *folio, struct btrfs_bio_ctrl *bio_ctrl
 	 * Only unlock ranges that are submitted. As there can be some async
 	 * submitted ranges inside the folio.
 	 */
-	btrfs_folio_end_writer_lock_bitmap(fs_info, folio, bio_ctrl->submit_bitmap);
+	btrfs_folio_end_lock_bitmap(fs_info, folio, bio_ctrl->submit_bitmap);
 	ASSERT(ret <= 0);
 	return ret;
 }
@@ -2298,7 +2298,7 @@ void extent_write_locked_range(struct inode *inode, const struct folio *locked_f
 						       cur, cur_len, !ret);
 			mapping_set_error(mapping, ret);
 		}
-		btrfs_folio_end_writer_lock(fs_info, folio, cur, cur_len);
+		btrfs_folio_end_lock(fs_info, folio, cur, cur_len);
 		if (ret < 0)
 			found_error = true;
 next_page:
diff --git a/fs/btrfs/subpage.c b/fs/btrfs/subpage.c
index 6c3d54a45956..d4cab3c55742 100644
--- a/fs/btrfs/subpage.c
+++ b/fs/btrfs/subpage.c
@@ -143,7 +143,7 @@ struct btrfs_subpage *btrfs_alloc_subpage(const struct btrfs_fs_info *fs_info,
 	if (type == BTRFS_SUBPAGE_METADATA)
 		atomic_set(&ret->eb_refs, 0);
 	else
-		atomic_set(&ret->writers, 0);
+		atomic_set(&ret->nr_locked, 0);
 	return ret;
 }
 
@@ -237,8 +237,8 @@ static void btrfs_subpage_clamp_range(struct folio *folio, u64 *start, u32 *len)
 			     orig_start + orig_len) - *start;
 }
 
-static bool btrfs_subpage_end_and_test_writer(const struct btrfs_fs_info *fs_info,
-					      struct folio *folio, u64 start, u32 len)
+static bool btrfs_subpage_end_and_test_lock(const struct btrfs_fs_info *fs_info,
+					    struct folio *folio, u64 start, u32 len)
 {
 	struct btrfs_subpage *subpage = folio_get_private(folio);
 	const int start_bit = subpage_calc_start_bit(fs_info, folio, locked, start, len);
@@ -256,9 +256,9 @@ static bool btrfs_subpage_end_and_test_writer(const struct btrfs_fs_info *fs_inf
 	 * extent_clear_unlock_delalloc() for compression path.
 	 *
 	 * This @locked_page is locked by plain lock_page(), thus its
-	 * subpage::writers is 0.  Handle them in a special way.
+	 * subpage::locked is 0.  Handle them in a special way.
 	 */
-	if (atomic_read(&subpage->writers) == 0) {
+	if (atomic_read(&subpage->nr_locked) == 0) {
 		spin_unlock_irqrestore(&subpage->lock, flags);
 		return true;
 	}
@@ -267,8 +267,8 @@ static bool btrfs_subpage_end_and_test_writer(const struct btrfs_fs_info *fs_inf
 		clear_bit(bit, subpage->bitmaps);
 		cleared++;
 	}
-	ASSERT(atomic_read(&subpage->writers) >= cleared);
-	last = atomic_sub_and_test(cleared, &subpage->writers);
+	ASSERT(atomic_read(&subpage->nr_locked) >= cleared);
+	last = atomic_sub_and_test(cleared, &subpage->nr_locked);
 	spin_unlock_irqrestore(&subpage->lock, flags);
 	return last;
 }
@@ -289,8 +289,8 @@ static bool btrfs_subpage_end_and_test_writer(const struct btrfs_fs_info *fs_inf
  *   bitmap, reduce the writer lock number, and unlock the page if that's
  *   the last locked range.
  */
-void btrfs_folio_end_writer_lock(const struct btrfs_fs_info *fs_info,
-				 struct folio *folio, u64 start, u32 len)
+void btrfs_folio_end_lock(const struct btrfs_fs_info *fs_info,
+			  struct folio *folio, u64 start, u32 len)
 {
 	struct btrfs_subpage *subpage = folio_get_private(folio);
 
@@ -303,24 +303,24 @@ void btrfs_folio_end_writer_lock(const struct btrfs_fs_info *fs_info,
 
 	/*
 	 * For subpage case, there are two types of locked page.  With or
-	 * without writers number.
+	 * without locked number.
 	 *
-	 * Since we own the page lock, no one else could touch subpage::writers
+	 * Since we own the page lock, no one else could touch subpage::locked
 	 * and we are safe to do several atomic operations without spinlock.
 	 */
-	if (atomic_read(&subpage->writers) == 0) {
-		/* No writers, locked by plain lock_page(). */
+	if (atomic_read(&subpage->nr_locked) == 0) {
+		/* No subpage lock, locked by plain lock_page(). */
 		folio_unlock(folio);
 		return;
 	}
 
 	btrfs_subpage_clamp_range(folio, &start, &len);
-	if (btrfs_subpage_end_and_test_writer(fs_info, folio, start, len))
+	if (btrfs_subpage_end_and_test_lock(fs_info, folio, start, len))
 		folio_unlock(folio);
 }
 
-void btrfs_folio_end_writer_lock_bitmap(const struct btrfs_fs_info *fs_info,
-					struct folio *folio, unsigned long bitmap)
+void btrfs_folio_end_lock_bitmap(const struct btrfs_fs_info *fs_info,
+				 struct folio *folio, unsigned long bitmap)
 {
 	struct btrfs_subpage *subpage = folio_get_private(folio);
 	const int start_bit = fs_info->sectors_per_page * btrfs_bitmap_nr_locked;
@@ -334,8 +334,8 @@ void btrfs_folio_end_writer_lock_bitmap(const struct btrfs_fs_info *fs_info,
 		return;
 	}
 
-	if (atomic_read(&subpage->writers) == 0) {
-		/* No writers, locked by plain lock_page(). */
+	if (atomic_read(&subpage->nr_locked) == 0) {
+		/* No subpage lock, locked by plain lock_page(). */
 		folio_unlock(folio);
 		return;
 	}
@@ -345,8 +345,8 @@ void btrfs_folio_end_writer_lock_bitmap(const struct btrfs_fs_info *fs_info,
 		if (test_and_clear_bit(bit + start_bit, subpage->bitmaps))
 			cleared++;
 	}
-	ASSERT(atomic_read(&subpage->writers) >= cleared);
-	last = atomic_sub_and_test(cleared, &subpage->writers);
+	ASSERT(atomic_read(&subpage->nr_locked) >= cleared);
+	last = atomic_sub_and_test(cleared, &subpage->nr_locked);
 	spin_unlock_irqrestore(&subpage->lock, flags);
 	if (last)
 		folio_unlock(folio);
@@ -671,8 +671,8 @@ void btrfs_folio_assert_not_dirty(const struct btrfs_fs_info *fs_info,
  * This populates the involved subpage ranges so that subpage helpers can
  * properly unlock them.
  */
-void btrfs_folio_set_writer_lock(const struct btrfs_fs_info *fs_info,
-				 struct folio *folio, u64 start, u32 len)
+void btrfs_folio_set_lock(const struct btrfs_fs_info *fs_info,
+			  struct folio *folio, u64 start, u32 len)
 {
 	struct btrfs_subpage *subpage;
 	unsigned long flags;
@@ -691,7 +691,7 @@ void btrfs_folio_set_writer_lock(const struct btrfs_fs_info *fs_info,
 	/* Target range should not yet be locked. */
 	ASSERT(bitmap_test_range_all_zero(subpage->bitmaps, start_bit, nbits));
 	bitmap_set(subpage->bitmaps, start_bit, nbits);
-	ret = atomic_add_return(nbits, &subpage->writers);
+	ret = atomic_add_return(nbits, &subpage->nr_locked);
 	ASSERT(ret <= fs_info->sectors_per_page);
 	spin_unlock_irqrestore(&subpage->lock, flags);
 }
diff --git a/fs/btrfs/subpage.h b/fs/btrfs/subpage.h
index c150aba9318e..428fa9389fd4 100644
--- a/fs/btrfs/subpage.h
+++ b/fs/btrfs/subpage.h
@@ -54,8 +54,12 @@ struct btrfs_subpage {
 		 */
 		atomic_t eb_refs;
 
-		/* Structures only used by data */
-		atomic_t writers;
+		/*
+		 * Structures only used by data,
+		 *
+		 * How many sectors inside the page is locked.
+		 */
+		atomic_t nr_locked;
 	};
 	unsigned long bitmaps[];
 };
@@ -87,12 +91,12 @@ void btrfs_free_subpage(struct btrfs_subpage *subpage);
 void btrfs_folio_inc_eb_refs(const struct btrfs_fs_info *fs_info, struct folio *folio);
 void btrfs_folio_dec_eb_refs(const struct btrfs_fs_info *fs_info, struct folio *folio);
 
-void btrfs_folio_end_writer_lock(const struct btrfs_fs_info *fs_info,
-				 struct folio *folio, u64 start, u32 len);
-void btrfs_folio_set_writer_lock(const struct btrfs_fs_info *fs_info,
-				 struct folio *folio, u64 start, u32 len);
-void btrfs_folio_end_writer_lock_bitmap(const struct btrfs_fs_info *fs_info,
-					struct folio *folio, unsigned long bitmap);
+void btrfs_folio_end_lock(const struct btrfs_fs_info *fs_info,
+			  struct folio *folio, u64 start, u32 len);
+void btrfs_folio_set_lock(const struct btrfs_fs_info *fs_info,
+			  struct folio *folio, u64 start, u32 len);
+void btrfs_folio_end_lock_bitmap(const struct btrfs_fs_info *fs_info,
+				 struct folio *folio, unsigned long bitmap);
 /*
  * Template for subpage related operations.
  *
-- 
2.46.2

Re: [PATCH v2 0/2] btrfs: unify the read and writer locks for btrfs_subpage

[David Sterba]

On Wed, Oct 09, 2024 at 04:21:05PM +1030, Qu Wenruo wrote:
> [CHANGELOG]
> v2:
> - Rename btrfs_subpage::locked to btrfs_subpage::nr_locked
> 
> When the handling of sector size < page size is introduced, there are
> two types of locking, reader and writer lock.
> 
> The main reason for the reader lock is to handle metadata to make sure
> the page::private is not released when there is still a metadata being
> read.
> 
> However since commit d7172f52e993 ("btrfs: use per-buffer locking for
> extent_buffer reading"), metadata read no longer relies on
> btrfs_subpage::readers.
> 
> Making the writer lock as the only utilized subpage locking.
> 
> This patchset converts all the existing reader lock usage and rename the
> writer lock into a generic lock.
> 
> This patchset relies on this patch "btrfs: fix the delalloc range
> locking if sector size < page size", as it removes the last user of
> btrfs_folio_start_writer_lock().
> 
> Qu Wenruo (2):
>   btrfs: unify to use writer locks for subpage locking
>   btrfs: rename btrfs_folio_(set|start|end)_writer_lock()

Reviewed-by: David Sterba <dsterba@suse.com>