btrfs: subpage: avoid potential deadlock with compression and delalloc

[BUG]
With experimental subpage compression enabled, a simple fsstress can
lead to self deadlock on page 720896:

        mkfs.btrfs -f -s 4k $dev > /dev/null
        mount $dev -o compress $mnt
        $fsstress -p 1 -n 100 -w -d $mnt -v -s 1625511156

[CAUSE]
If we have a file layout looks like below:

	0	32K	64K	96K	128K
	|//|		|///////////////|
	   4K

Then we run delalloc range for the inode, it will:

- Call find_lock_delalloc_range() with @delalloc_start = 0
  Then we got a delalloc range [0, 4K).

  This range will be COWed.

- Call find_lock_delalloc_range() again with @delalloc_start = 4K
  Since find_lock_delalloc_range() never cares whether the range
  is still inside page range [0, 64K), it will return range [64K, 128K).

  This range meets the condition for subpage compression, will go
  through async COW path.

  And async COW path will return @page_started.

  But that @page_started is now for range [64K, 128K), not for range
  [0, 64K).

- writepage_dellloc() returned 1 for page [0, 64K)
  Thus page [0, 64K) will not be unlocked, nor its page dirty status
  will be cleared.

Next time when we try to lock page [0, 64K) we will deadlock, as there
is no one to release page [0, 64K).

This problem will never happen for regular page size as one page only
contains one sector.  After the first find_lock_delalloc_range() call,
the @delalloc_end will go beyond @page_end no matter if we found a
delalloc range or not

Thus this bug only happens for subpage, as now we need multiple runs to
exhaust the delalloc range of a page.

[FIX]
Fix the problem by ensuring the delalloc range we ran at least started
inside @locked_page.

So that we will never get incorrect @page_started.

And to prevent such problem from happening again:

- Make find_lock_delalloc_range() return false if the found range is
  beyond @end value passed in.

  Since @end will be utilized now, add an ASSERT() to ensure we pass
  correct @end into find_lock_delalloc_range().

  This also means, for selftests we needs to populate @end before calling
  find_lock_delalloc_range().

- New ASSERT() in find_lock_delalloc_range()
  Now we will make sure the @start/@end passed in at least covers part
  of the page.

- New ASSERT() in run_delalloc_range()
  To make sure the range at least starts inside @locked page.

- Use @delalloc_start as proper cursor, while @delalloc_end is always
  reset to @page_end.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_io.c

@@ -1975,10 +1975,18 @@ static noinline int lock_delalloc_pages(struct inode *inode,
 
 /*
  * Find and lock a contiguous range of bytes in the file marked as delalloc, no
- * more than @max_bytes.  @Start and @end are used to return the range,
+ * more than @max_bytes.
  *
- * Return: true if we find something
- *         false if nothing was in the tree
+ * @start:	The original start bytenr to search.
+ *		Will store the extent range start bytenr.
+ * @end:	The original end bytenr of the search range
+ *		Will store the extent range end bytenr.
+ *
+ * Return true if we find a delalloc range which starts inside the original
+ * range, and @start/@end will store the delalloc range start/end.
+ *
+ * Return false if we can't find any delalloc range which starts inside the
+ * original range, and @start/@end will be the non-delalloc range start/end.
  */
 EXPORT_FOR_TESTS
 noinline_for_stack bool find_lock_delalloc_range(struct inode *inode,
@@ -1986,6 +1994,8 @@ noinline_for_stack bool find_lock_delalloc_range(struct inode *inode,
 				    u64 *end)
 {
 	struct extent_io_tree *tree = &BTRFS_I(inode)->io_tree;
+	const u64 orig_start = *start;
+	const u64 orig_end = *end;
 	u64 max_bytes = BTRFS_MAX_EXTENT_SIZE;
 	u64 delalloc_start;
 	u64 delalloc_end;
@@ -1994,15 +2004,23 @@ noinline_for_stack bool find_lock_delalloc_range(struct inode *inode,
 	int ret;
 	int loops = 0;
 
+	/* Caller should pass a valid @end to indicate the search range end */
+	ASSERT(orig_end > orig_start);
+
+	/* The range should at least cover part of the page */
+	ASSERT(!(orig_start >= page_offset(locked_page) + PAGE_SIZE ||
+		 orig_end <= page_offset(locked_page)));
 again:
 	/* step one, find a bunch of delalloc bytes starting at start */
 	delalloc_start = *start;
 	delalloc_end = 0;
 	found = btrfs_find_delalloc_range(tree, &delalloc_start, &delalloc_end,
 					  max_bytes, &cached_state);
-	if (!found || delalloc_end <= *start) {
+	if (!found || delalloc_end <= *start || delalloc_start > orig_end) {
 		*start = delalloc_start;
-		*end = delalloc_end;
+
+		/* @delalloc_end can be -1, never go beyond @orig_end */
+		*end = min(delalloc_end, orig_end);
 		free_extent_state(cached_state);
 		return false;
 	}
@@ -3771,16 +3789,16 @@ static noinline_for_stack int writepage_delalloc(struct btrfs_inode *inode,
 		struct page *page, struct writeback_control *wbc,
 		unsigned long *nr_written)
 {
-	u64 page_end = page_offset(page) + PAGE_SIZE - 1;
-	bool found;
+	const u64 page_end = page_offset(page) + PAGE_SIZE - 1;
 	u64 delalloc_start = page_offset(page);
 	u64 delalloc_to_write = 0;
-	u64 delalloc_end = 0;
 	int ret;
 	int page_started = 0;
 
+	while (delalloc_start < page_end) {
+		u64 delalloc_end = page_end;
+		bool found;
 
-	while (delalloc_end < page_end) {
 		found = find_lock_delalloc_range(&inode->vfs_inode, page,
 					       &delalloc_start,
 					       &delalloc_end);

fs/btrfs/inode.c

@@ -1955,6 +1955,13 @@ int btrfs_run_delalloc_range(struct btrfs_inode *inode, struct page *locked_page
 	int ret;
 	const bool zoned = btrfs_is_zoned(inode->root->fs_info);
 
+	/*
+	 * The range must cover part of the @locked_page, or the returned
+	 * @page_started can confuse the caller.
+	 */
+	ASSERT(!(end <= page_offset(locked_page) ||
+		 start >= page_offset(locked_page) + PAGE_SIZE));
+
 	if (should_nocow(inode, start, end)) {
 		/*
 		 * Normally on a zoned device we're only doing COW writes, but

fs/btrfs/tests/extent-io-tests.c

@@ -112,7 +112,7 @@ static int test_find_delalloc(u32 sectorsize)
 	 */
 	set_extent_delalloc(tmp, 0, sectorsize - 1, 0, NULL);
 	start = 0;
-	end = 0;
+	end = start + PAGE_SIZE - 1;
 	found = find_lock_delalloc_range(inode, locked_page, &start,
 					 &end);
 	if (!found) {
@@ -143,7 +143,7 @@ static int test_find_delalloc(u32 sectorsize)
 	}
 	set_extent_delalloc(tmp, sectorsize, max_bytes - 1, 0, NULL);
 	start = test_start;
-	end = 0;
+	end = start + PAGE_SIZE - 1;
 	found = find_lock_delalloc_range(inode, locked_page, &start,
 					 &end);
 	if (!found) {
@@ -177,14 +177,14 @@ static int test_find_delalloc(u32 sectorsize)
 		goto out_bits;
 	}
 	start = test_start;
-	end = 0;
+	end = start + PAGE_SIZE - 1;
 	found = find_lock_delalloc_range(inode, locked_page, &start,
 					 &end);
 	if (found) {
 		test_err("found range when we shouldn't have");
 		goto out_bits;
 	}
-	if (end != (u64)-1) {
+	if (end != test_start + PAGE_SIZE - 1) {
 		test_err("did not return the proper end offset");
 		goto out_bits;
 	}
@@ -198,7 +198,7 @@ static int test_find_delalloc(u32 sectorsize)
 	 */
 	set_extent_delalloc(tmp, max_bytes, total_dirty - 1, 0, NULL);
 	start = test_start;
-	end = 0;
+	end = start + PAGE_SIZE - 1;
 	found = find_lock_delalloc_range(inode, locked_page, &start,
 					 &end);
 	if (!found) {
@@ -233,7 +233,7 @@ static int test_find_delalloc(u32 sectorsize)
 	/* We unlocked it in the previous test */
 	lock_page(locked_page);
 	start = test_start;
-	end = 0;
+	end = start + PAGE_SIZE - 1;
 	/*
 	 * Currently if we fail to find dirty pages in the delalloc range we
 	 * will adjust max_bytes down to PAGE_SIZE and then re-search.  If