mm: prep_compound_tail() clear page->private

Although page allocation always clears page->private in the first page or
head page of an allocation, it has never made a point of clearing
page->private in the tails (though 0 is often what is already there).

But now commit 71e2d666 ("mm/huge_memory: do not clobber swp_entry_t
during THP split") issues a warning when page_tail->private is found to be
non-0 (unless it's swapcache).

Change that warning to dump page_tail (which also dumps head), instead of
just the head: so far we have seen dead000000000122, dead000000000003,
dead000000000001 or 0000000000000002 in the raw output for tail private.

We could just delete the warning, but today's consensus appears to want
page->private to be 0, unless there's a good reason for it to be set: so
now clear it in prep_compound_tail() (more general than just for THP; but
not for high order allocation, which makes no pass down the tails).

Link: https://lkml.kernel.org/r/1c4233bb-4e4d-5969-fbd4-96604268a285@google.com
Fixes: 71e2d666 ("mm/huge_memory: do not clobber swp_entry_t during THP split")
Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/huge_memory.c

@@ -2462,7 +2462,7 @@ static void __split_huge_page_tail(struct page *head, int tail,
 	 * Fix up and warn once if private is unexpectedly set.
 	 */
 	if (!folio_test_swapcache(page_folio(head))) {
-		VM_WARN_ON_ONCE_PAGE(page_tail->private != 0, head);
+		VM_WARN_ON_ONCE_PAGE(page_tail->private != 0, page_tail);
 		page_tail->private = 0;
 	}
 

mm/page_alloc.c

@@ -807,6 +807,7 @@ static void prep_compound_tail(struct page *head, int tail_idx)
 
 	p->mapping = TAIL_MAPPING;
 	set_compound_head(p, head);
+	set_page_private(p, 0);
 }
 
 void prep_compound_page(struct page *page, unsigned int order)