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>

1 files changed, 6 insertions, 6 deletions

diff --git a/fs/btrfs/tests/extent-io-tests.c b/fs/btrfs/tests/extent-io-tests.c
index 73e96d505f4f..c2e72e7a8ff0 100644
--- a/fs/btrfs/tests/extent-io-tests.c
+++ b/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