3 files changed, 94 insertions, 15 deletions

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index 4220ef4ff7d0..8cb51da0e16d 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -1841,14 +1841,18 @@ static int process_one_page(struct btrfs_fs_info *fs_info,
 	if (page_ops & PAGE_END_WRITEBACK)
 		btrfs_page_clamp_clear_writeback(fs_info, page, start, len);
 	if (page_ops & PAGE_LOCK) {
-		lock_page(page);
+		int ret;
+
+		ret = btrfs_page_start_writer_lock(fs_info, page, start, len);
+		if (ret)
+			return ret;
 		if (!PageDirty(page) || page->mapping != mapping) {
-			unlock_page(page);
+			btrfs_page_end_writer_lock(fs_info, page, start, len);
 			return -EAGAIN;
 		}
 	}
 	if (page_ops & PAGE_UNLOCK)
-		unlock_page(page);
+		btrfs_page_end_writer_lock(fs_info, page, start, len);
 	return 0;
 }
 
diff --git a/fs/btrfs/subpage.c b/fs/btrfs/subpage.c
index a6cf1776f3f9..69a0dbf35362 100644
--- a/fs/btrfs/subpage.c
+++ b/fs/btrfs/subpage.c
@@ -110,10 +110,12 @@ int btrfs_alloc_subpage(const struct btrfs_fs_info *fs_info,
 	if (!*ret)
 		return -ENOMEM;
 	spin_lock_init(&(*ret)->lock);
-	if (type == BTRFS_SUBPAGE_METADATA)
+	if (type == BTRFS_SUBPAGE_METADATA) {
 		atomic_set(&(*ret)->eb_refs, 0);
-	else
+	} else {
 		atomic_set(&(*ret)->readers, 0);
+		atomic_set(&(*ret)->writers, 0);
+	}
 	return 0;
 }
 
@@ -203,6 +205,79 @@ void btrfs_subpage_end_reader(const struct btrfs_fs_info *fs_info,
 		unlock_page(page);
 }
 
+static void btrfs_subpage_clamp_range(struct page *page, u64 *start, u32 *len)
+{
+	u64 orig_start = *start;
+	u32 orig_len = *len;
+
+	*start = max_t(u64, page_offset(page), orig_start);
+	*len = min_t(u64, page_offset(page) + PAGE_SIZE,
+		     orig_start + orig_len) - *start;
+}
+
+void btrfs_subpage_start_writer(const struct btrfs_fs_info *fs_info,
+		struct page *page, u64 start, u32 len)
+{
+	struct btrfs_subpage *subpage = (struct btrfs_subpage *)page->private;
+	const int nbits = (len >> fs_info->sectorsize_bits);
+	int ret;
+
+	btrfs_subpage_assert(fs_info, page, start, len);
+
+	ASSERT(atomic_read(&subpage->readers) == 0);
+	ret = atomic_add_return(nbits, &subpage->writers);
+	ASSERT(ret == nbits);
+}
+
+bool btrfs_subpage_end_and_test_writer(const struct btrfs_fs_info *fs_info,
+		struct page *page, u64 start, u32 len)
+{
+	struct btrfs_subpage *subpage = (struct btrfs_subpage *)page->private;
+	const int nbits = (len >> fs_info->sectorsize_bits);
+
+	btrfs_subpage_assert(fs_info, page, start, len);
+
+	ASSERT(atomic_read(&subpage->writers) >= nbits);
+	return atomic_sub_and_test(nbits, &subpage->writers);
+}
+
+/*
+ * Lock a page for delalloc page writeback.
+ *
+ * Return -EAGAIN if the page is not properly initialized.
+ * Return 0 with the page locked, and writer counter updated.
+ *
+ * Even with 0 returned, the page still need extra check to make sure
+ * it's really the correct page, as the caller is using
+ * find_get_pages_contig(), which can race with page invalidating.
+ */
+int btrfs_page_start_writer_lock(const struct btrfs_fs_info *fs_info,
+		struct page *page, u64 start, u32 len)
+{
+	if (unlikely(!fs_info) || fs_info->sectorsize == PAGE_SIZE) {
+		lock_page(page);
+		return 0;
+	}
+	lock_page(page);
+	if (!PagePrivate(page) || !page->private) {
+		unlock_page(page);
+		return -EAGAIN;
+	}
+	btrfs_subpage_clamp_range(page, &start, &len);
+	btrfs_subpage_start_writer(fs_info, page, start, len);
+	return 0;
+}
+
+void btrfs_page_end_writer_lock(const struct btrfs_fs_info *fs_info,
+		struct page *page, u64 start, u32 len)
+{
+	if (unlikely(!fs_info) || fs_info->sectorsize == PAGE_SIZE)
+		return unlock_page(page);
+	btrfs_subpage_clamp_range(page, &start, &len);
+	if (btrfs_subpage_end_and_test_writer(fs_info, page, start, len))
+		unlock_page(page);
+}
+
 /*
  * Convert the [start, start + len) range into a u16 bitmap
  *
@@ -354,16 +429,6 @@ void btrfs_subpage_clear_writeback(const struct btrfs_fs_info *fs_info,
 	spin_unlock_irqrestore(&subpage->lock, flags);
 }
 
-static void btrfs_subpage_clamp_range(struct page *page, u64 *start, u32 *len)
-{
-	u64 orig_start = *start;
-	u32 orig_len = *len;
-
-	*start = max_t(u64, page_offset(page), orig_start);
-	*len = min_t(u64, page_offset(page) + PAGE_SIZE,
-		     orig_start + orig_len) - *start;
-}
-
 /*
  * Unlike set/clear which is dependent on each page status, for test all bits
  * are tested in the same way.
diff --git a/fs/btrfs/subpage.h b/fs/btrfs/subpage.h
index 291cb1932f27..9d087ab3244e 100644
--- a/fs/btrfs/subpage.h
+++ b/fs/btrfs/subpage.h
@@ -33,6 +33,7 @@ struct btrfs_subpage {
 		/* Structures only used by data */
 		struct {
 			atomic_t readers;
+			atomic_t writers;
 		};
 	};
 };
@@ -63,6 +64,15 @@ void btrfs_subpage_start_reader(const struct btrfs_fs_info *fs_info,
 void btrfs_subpage_end_reader(const struct btrfs_fs_info *fs_info,
 		struct page *page, u64 start, u32 len);
 
+void btrfs_subpage_start_writer(const struct btrfs_fs_info *fs_info,
+		struct page *page, u64 start, u32 len);
+bool btrfs_subpage_end_and_test_writer(const struct btrfs_fs_info *fs_info,
+		struct page *page, u64 start, u32 len);
+int btrfs_page_start_writer_lock(const struct btrfs_fs_info *fs_info,
+		struct page *page, u64 start, u32 len);
+void btrfs_page_end_writer_lock(const struct btrfs_fs_info *fs_info,
+		struct page *page, u64 start, u32 len);
+
 /*
  * Template for subpage related operations.
  *