hugetlb: remove prep_compound_huge_page cleanup

Patch series "Fix prep_compound_gigantic_page ref count adjustment".

These patches address the possible race between
prep_compound_gigantic_page and __page_cache_add_speculative as described
by Jann Horn in [1].

The first patch simply removes the unnecessary/obsolete helper routine
prep_compound_huge_page to make the actual fix a little simpler.

The second patch is the actual fix and has a detailed explanation in the
commit message.

This potential issue has existed for almost 10 years and I am unaware of
anyone actually hitting the race.  I did not cc stable, but would be happy
to squash the patches and send to stable if anyone thinks that is a good
idea.

[1] https://lore.kernel.org/linux-mm/CAG48ez23q0Jy9cuVnwAe7t_fdhMk2S7N5Hdi-GLcCeq5bsfLxw@mail.gmail.com/

This patch (of 2):

I could not think of a reliable way to recreate the issue for testing.
Rather, I 'simulated errors' to exercise all the error paths.

The routine prep_compound_huge_page is a simple wrapper to call either
prep_compound_gigantic_page or prep_compound_page.  However, it is only
called from gather_bootmem_prealloc which only processes gigantic pages.
Eliminate the routine and call prep_compound_gigantic_page directly.

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Mike Kravetz <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Jan Kara <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: John Hubbard <[email protected]>
Cc: "Kirill A . Shutemov" <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Youquan Song <[email protected]>
Cc: Muchun Song <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/hugetlb.c

@@ -1320,8 +1320,6 @@ static struct page *alloc_gigantic_page(struct hstate *h, gfp_t gfp_mask,
 	return alloc_contig_pages(nr_pages, gfp_mask, nid, nodemask);
 }
 
-static void prep_new_huge_page(struct hstate *h, struct page *page, int nid);
-static void prep_compound_gigantic_page(struct page *page, unsigned int order);
 #else /* !CONFIG_CONTIG_ALLOC */
 static struct page *alloc_gigantic_page(struct hstate *h, gfp_t gfp_mask,
 					int nid, nodemask_t *nodemask)
@@ -2759,16 +2757,10 @@ int __alloc_bootmem_huge_page(struct hstate *h)
 	return 1;
 }
 
-static void __init prep_compound_huge_page(struct page *page,
-		unsigned int order)
-{
-	if (unlikely(order > (MAX_ORDER - 1)))
-		prep_compound_gigantic_page(page, order);
-	else
-		prep_compound_page(page, order);
-}
-
-/* Put bootmem huge pages into the standard lists after mem_map is up */
+/*
+ * Put bootmem huge pages into the standard lists after mem_map is up.
+ * Note: This only applies to gigantic (order > MAX_ORDER) pages.
+ */
 static void __init gather_bootmem_prealloc(void)
 {
 	struct huge_bootmem_page *m;
@@ -2777,20 +2769,19 @@ static void __init gather_bootmem_prealloc(void)
 		struct page *page = virt_to_page(m);
 		struct hstate *h = m->hstate;
 
+		VM_BUG_ON(!hstate_is_gigantic(h));
 		WARN_ON(page_count(page) != 1);
-		prep_compound_huge_page(page, huge_page_order(h));
+		prep_compound_gigantic_page(page, huge_page_order(h));
 		WARN_ON(PageReserved(page));
 		prep_new_huge_page(h, page, page_to_nid(page));
 		put_page(page); /* free it into the hugepage allocator */
 
 		/*
-		 * If we had gigantic hugepages allocated at boot time, we need
-		 * to restore the 'stolen' pages to totalram_pages in order to
-		 * fix confusing memory reports from free(1) and another
-		 * side-effects, like CommitLimit going negative.
+		 * We need to restore the 'stolen' pages to totalram_pages
+		 * in order to fix confusing memory reports from free(1) and
+		 * other side-effects, like CommitLimit going negative.
 		 */
-		if (hstate_is_gigantic(h))
-			adjust_managed_page_count(page, pages_per_huge_page(h));
+		adjust_managed_page_count(page, pages_per_huge_page(h));
 		cond_resched();
 	}
 }