+ mm-allow-set-clear-page_type-again.patch added to mm-hotfixes-unstable branch

+ mm-allow-set-clear-page_type-again.patch added to mm-hotfixes-unstable branch

[Andrew Morton]

The patch titled
     Subject: mm: allow set/clear page_type again
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     mm-allow-set-clear-page_type-again.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-allow-set-clear-page_type-again.patch

This patch will later appear in the mm-hotfixes-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Yu Zhao <yuzhao@google.com>
Subject: mm: allow set/clear page_type again
Date: Sat, 19 Oct 2024 22:22:12 -0600

Some page flags (page->flags) were converted to page types
(page->page_types).  A recent example is PG_hugetlb.

From the exclusive writer's perspective, e.g., a thread doing
__folio_set_hugetlb(), there is a difference between the page flag and
type APIs: the former allows the same non-atomic operation to be repeated
whereas the latter does not.  For example, calling __folio_set_hugetlb()
twice triggers VM_BUG_ON_FOLIO(), since the second call expects the type
(PG_hugetlb) not to be set previously.

Using add_hugetlb_folio() as an example, it calls __folio_set_hugetlb() in
the following error-handling path.  And when that happens, it triggers the
aforementioned VM_BUG_ON_FOLIO().

  if (folio_test_hugetlb(folio)) {
    rc = hugetlb_vmemmap_restore_folio(h, folio);
    if (rc) {
      spin_lock_irq(&hugetlb_lock);
      add_hugetlb_folio(h, folio, false);
      ...

It is possible to make hugeTLB comply with the new requirements from the
page type API.  However, a straightforward fix would be to just allow the
same page type to be set or cleared again inside the API, to avoid any
changes to its callers.

Link: https://lkml.kernel.org/r/20241020042212.296781-1-yuzhao@google.com
Fixes: d99e3140a4d3 ("mm: turn folio_test_hugetlb into a PageType")
Signed-off-by: Yu Zhao <yuzhao@google.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 include/linux/page-flags.h |    8 ++++++++
 1 file changed, 8 insertions(+)

--- a/include/linux/page-flags.h~mm-allow-set-clear-page_type-again
+++ a/include/linux/page-flags.h
@@ -975,12 +975,16 @@ static __always_inline bool folio_test_#
 }									\
 static __always_inline void __folio_set_##fname(struct folio *folio)	\
 {									\
+	if (folio_test_##fname(folio))					\
+		return;							\
 	VM_BUG_ON_FOLIO(data_race(folio->page.page_type) != UINT_MAX,	\
 			folio);						\
 	folio->page.page_type = (unsigned int)PGTY_##lname << 24;	\
 }									\
 static __always_inline void __folio_clear_##fname(struct folio *folio)	\
 {									\
+	if (folio->page.page_type == UINT_MAX)				\
+		return;							\
 	VM_BUG_ON_FOLIO(!folio_test_##fname(folio), folio);		\
 	folio->page.page_type = UINT_MAX;				\
 }
@@ -993,11 +997,15 @@ static __always_inline int Page##uname(c
 }									\
 static __always_inline void __SetPage##uname(struct page *page)		\
 {									\
+	if (Page##uname(page))						\
+		return;							\
 	VM_BUG_ON_PAGE(data_race(page->page_type) != UINT_MAX, page);	\
 	page->page_type = (unsigned int)PGTY_##lname << 24;		\
 }									\
 static __always_inline void __ClearPage##uname(struct page *page)	\
 {									\
+	if (page->page_type == UINT_MAX)				\
+		return;							\
 	VM_BUG_ON_PAGE(!Page##uname(page), page);			\
 	page->page_type = UINT_MAX;					\
 }
_

Patches currently in -mm which might be from yuzhao@google.com are

mm-allow-set-clear-page_type-again.patch