mm: move _pincount in folio to page[2] on 32bit

Let's free up some space on 32bit in page[1] by moving the _pincount to
page[2].

For order-1 folios (never anon folios!) on 32bit, we will now also use the
GUP_PIN_COUNTING_BIAS approach.  A fully-mapped order-1 folio requires 2
references.  With GUP_PIN_COUNTING_BIAS being 1024, we'd detect such
folios as "maybe pinned" with 512 full mappings, instead of 1024 for
order-0.  As anon folios are out of the picture (which are the most
relevant users of checking for pinnings on *mapped* pages) and we are
talking about 32bit, this is not expected to cause any trouble.

In __dump_page(), copy one additional folio page if we detect a folio with
an order > 1, so we can dump the pincount on order > 1 folios reliably.

Note that THPs on 32bit are not particularly common (and we don't care too
much about performance), but we want to keep it working reliably, because
likely we want to use large folios there as well in the future,
independent of PMD leaf support.

Once we dynamically allocate "struct folio", fortunately the 32bit
specifics will likely go away again; even small folios could then have a
pincount and folio_has_pincount() would essentially always return "true".

Link: https://lkml.kernel.org/r/20250303163014.1128035-6-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Cc: Andy Lutomirks^H^Hski <luto@kernel.org>
Cc: Borislav Betkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jann Horn <jannh@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Lance Yang <ioworker0@gmail.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Matthew Wilcow (Oracle) <willy@infradead.org>
Cc: Michal Koutn <mkoutny@suse.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: tejun heo <tj@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Zefan Li <lizefan.x@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 4 insertions, 4 deletions

diff --git a/mm/gup.c b/mm/gup.c
index e5040657870e..2944fe8cf317 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -109,7 +109,7 @@ static void gup_put_folio(struct folio *folio, int refs, unsigned int flags)
 		if (is_zero_folio(folio))
 			return;
 		node_stat_mod_folio(folio, NR_FOLL_PIN_RELEASED, refs);
-		if (folio_test_large(folio))
+		if (folio_has_pincount(folio))
 			atomic_sub(refs, &folio->_pincount);
 		else
 			refs *= GUP_PIN_COUNTING_BIAS;
@@ -164,7 +164,7 @@ int __must_check try_grab_folio(struct folio *folio, int refs,
 		 * Increment the normal page refcount field at least once,
 		 * so that the page really is pinned.
 		 */
-		if (folio_test_large(folio)) {
+		if (folio_has_pincount(folio)) {
 			folio_ref_add(folio, refs);
 			atomic_add(refs, &folio->_pincount);
 		} else {
@@ -223,7 +223,7 @@ void folio_add_pin(struct folio *folio)
 	 * page refcount field at least once, so that the page really is
 	 * pinned.
 	 */
-	if (folio_test_large(folio)) {
+	if (folio_has_pincount(folio)) {
 		WARN_ON_ONCE(atomic_read(&folio->_pincount) < 1);
 		folio_ref_inc(folio);
 		atomic_inc(&folio->_pincount);
@@ -575,7 +575,7 @@ static struct folio *try_grab_folio_fast(struct page *page, int refs,
 	 * is pinned.  That's why the refcount from the earlier
 	 * try_get_folio() is left intact.
 	 */
-	if (folio_test_large(folio))
+	if (folio_has_pincount(folio))
 		atomic_add(refs, &folio->_pincount);
 	else
 		folio_ref_add(folio,