mm: delete unsafe BUG from page_cache_add_speculative()

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.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 020853b ("mm: Add folio_try_get_rcu()")
Signed-off-by: Hugh Dickins <[email protected]>
Acked-by: Kirill A. Shutemov <[email protected]>
Reviewed-by: Matthew Wilcox (Oracle) <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: William Kucharski <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Mike Rapoport <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: Hugh Dickins

include/linux/pagemap.h

@@ -285,7 +285,6 @@ static inline struct inode *folio_inode(struct folio *folio)
 
 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);
 }