btrfs: don't double put our subpage reference in alloc_extent_buffer

This fixes as case in "btrfs: refactor alloc_extent_buffer() to
allocate-then-attach method".

We have been seeing panics in the CI for the subpage stuff recently, it
happens on btrfs/187 but could potentially happen anywhere.

In the subpage case, if we race with somebody else inserting the same
extent buffer, the error case will end up calling
detach_extent_buffer_page() on the page twice.

This is done first in the bit

for (int i = 0; i < attached; i++)
	detach_extent_buffer_page(eb, eb->pages[i];

and then again in btrfs_release_extent_buffer().

This works fine for !subpage because we're the only person who ever has
ourselves on the private, and so when we do the initial
detach_extent_buffer_page() we know we've completely removed it.

However for subpage we could be using this page private elsewhere, so
this results in a double put on the subpage, which can result in an
early freeing.

The fix here is to clear eb->pages[i] for everything we detach.  Then
anything still attached to the eb is freed in
btrfs_release_extent_buffer().

Because of this change we must update
btrfs_release_extent_buffer_pages() to not use num_extent_folios,
because it assumes eb->folio[0] is set properly.  Since this is only
interested in freeing any pages we have on the extent buffer we can
simply use INLINE_EXTENT_BUFFER_PAGES.
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_io.c

@@ -3175,11 +3175,9 @@ static void detach_extent_buffer_folio(struct extent_buffer *eb, struct folio *f
 /* Release all pages attached to the extent buffer */
 static void btrfs_release_extent_buffer_pages(struct extent_buffer *eb)
 {
-	int num_folios = num_extent_folios(eb);
-
 	ASSERT(!extent_buffer_under_io(eb));
 
-	for (int i = 0; i < num_folios; i++) {
+	for (int i = 0; i < INLINE_EXTENT_BUFFER_PAGES; i++) {
 		struct folio *folio = eb->folios[i];
 
 		if (!folio)
@@ -3747,10 +3745,28 @@ struct extent_buffer *alloc_extent_buffer(struct btrfs_fs_info *fs_info,
 
 out:
 	WARN_ON(!atomic_dec_and_test(&eb->refs));
+
+	/*
+	 * Any attached folios need to be detached before we unlock them.  This
+	 * is because when we're inserting our new folios into the mapping, and
+	 * then attaching our eb to that folio.  If we fail to insert our folio
+	 * we'll lookup the folio for that index, and grab that EB.  We do not
+	 * want that to grab this eb, as we're getting ready to free it.  So we
+	 * have to detach it first and then unlock it.
+	 *
+	 * We have to drop our reference and NULL it out here because in the
+	 * subpage case detaching does a btrfs_folio_dec_eb_refs() for our eb.
+	 * Below when we call btrfs_release_extent_buffer() we will call
+	 * detach_extent_buffer_folio() on our remaining pages in the !subpage
+	 * case.  If we left eb->folios[i] populated in the subpage case we'd
+	 * double put our reference and be super sad.
+	 */
 	for (int i = 0; i < attached; i++) {
 		ASSERT(eb->folios[i]);
 		detach_extent_buffer_folio(eb, eb->folios[i]);
 		unlock_page(folio_page(eb->folios[i], 0));
+		folio_put(eb->folios[i]);
+		eb->folios[i] = NULL;
 	}
 	/*
 	 * Now all pages of that extent buffer is unmapped, set UNMAPPED flag,