[PATCH] mm: delete unsafe BUG from page_cache_add_speculative()

[Hugh Dickins]

It is not easily reproducible, but on 5.16-rc I have several times hit
the VM_BUG_ON_PAGE(PageTail(page), page) in page_cache_add_speculative():
usually from filemap_get_read_batch() for an ext4 read, yesterday from
next_uptodate_page() from filemap_map_pages() for a shmem fault.

That BUG used to be placed where page_ref_add_unless() had succeeded,
but now it is placed before folio_ref_add_unless() is attempted: that
is not safe, since it is only the acquired reference which makes the
page safe from racing THP collapse or split.

We could keep the BUG, checking PageTail only when folio_ref_try_add_rcu()
has succeeded; but I don't think it adds much value - just delete it.

Fixes: 020853b6f5ea ("mm: Add folio_try_get_rcu()")
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
---

 include/linux/pagemap.h |    1 -
 1 file changed, 1 deletion(-)

--- 5.16-rc4/include/linux/pagemap.h
+++ linux/include/linux/pagemap.h
@@ -285,7 +285,6 @@ static inline struct inode *folio_inode(
 
 static inline bool page_cache_add_speculative(struct page *page, int count)
 {
-	VM_BUG_ON_PAGE(PageTail(page), page);
 	return folio_ref_try_add_rcu((struct folio *)page, count);
 }