+ mm-huge_memory-mark-pmd-mappings-of-the-huge-zero-folio-special.patch added to mm-new branch

+ mm-huge_memory-mark-pmd-mappings-of-the-huge-zero-folio-special.patch added to mm-new branch

[Andrew Morton]

The patch titled
     Subject: mm/huge_memory: mark PMD mappings of the huge zero folio special
has been added to the -mm mm-new branch.  Its filename is
     mm-huge_memory-mark-pmd-mappings-of-the-huge-zero-folio-special.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-huge_memory-mark-pmd-mappings-of-the-huge-zero-folio-special.patch

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

Note, mm-new is a provisional staging ground for work-in-progress
patches, and acceptance into mm-new is a notification for others take
notice and to finish up reviews.  Please do not hesitate to respond to
review feedback and post updated versions to replace or incrementally
fixup patches in mm-new.

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------------------------------------------------------
From: David Hildenbrand <david@redhat.com>
Subject: mm/huge_memory: mark PMD mappings of the huge zero folio special
Date: Mon, 11 Aug 2025 13:26:25 +0200

The huge zero folio is refcounted (+mapcounted -- is that a word?)
differently than "normal" folios, similarly (but different) to the
ordinary shared zeropage.

For this reason, we special-case these pages in
vm_normal_page*/vm_normal_folio*, and only allow selected callers to still
use them (e.g., GUP can still take a reference on them).

vm_normal_page_pmd() already filters out the huge zero folio, to indicate
it a special (return NULL).  However, so far we are not making use of
pmd_special() on architectures that support it
(CONFIG_ARCH_HAS_PTE_SPECIAL), like we would with the ordinary shared
zeropage.

Let's mark PMD mappings of the huge zero folio similarly as special, so we
can avoid the manual check for the huge zero folio with
CONFIG_ARCH_HAS_PTE_SPECIAL next, and only perform the check on
!CONFIG_ARCH_HAS_PTE_SPECIAL.

In copy_huge_pmd(), where we have a manual pmd_special() check to handle
PFNMAP, we have to manually rule out the huge zero folio.  That code needs
a serious cleanup, but that's something for another day.

While at it, update the doc regarding the shared zero folios.

No functional change intended: vm_normal_page_pmd() still returns NULL
when it encounters the huge zero folio.

Link: https://lkml.kernel.org/r/20250811112631.759341-6-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: David Vrabel <david.vrabel@citrix.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Jann Horn <jannh@google.com>
Cc: Juegren Gross <jgross@suse.com>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Mariano Pache <npache@redhat.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Wei Yang <richard.weiyang@gmail.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/huge_memory.c |    8 ++++++--
 mm/memory.c      |   15 ++++++++++-----
 2 files changed, 16 insertions(+), 7 deletions(-)

--- a/mm/huge_memory.c~mm-huge_memory-mark-pmd-mappings-of-the-huge-zero-folio-special
+++ a/mm/huge_memory.c
@@ -1309,6 +1309,7 @@ static void set_huge_zero_folio(pgtable_
 {
 	pmd_t entry;
 	entry = folio_mk_pmd(zero_folio, vma->vm_page_prot);
+	entry = pmd_mkspecial(entry);
 	pgtable_trans_huge_deposit(mm, pmd, pgtable);
 	set_pmd_at(mm, haddr, pmd, entry);
 	mm_inc_nr_ptes(mm);
@@ -1418,7 +1419,9 @@ static vm_fault_t insert_pmd(struct vm_a
 	if (fop.is_folio) {
 		entry = folio_mk_pmd(fop.folio, vma->vm_page_prot);
 
-		if (!is_huge_zero_folio(fop.folio)) {
+		if (is_huge_zero_folio(fop.folio)) {
+			entry = pmd_mkspecial(entry);
+		} else {
 			folio_get(fop.folio);
 			folio_add_file_rmap_pmd(fop.folio, &fop.folio->page, vma);
 			add_mm_counter(mm, mm_counter_file(fop.folio), HPAGE_PMD_NR);
@@ -1643,7 +1646,8 @@ int copy_huge_pmd(struct mm_struct *dst_
 	int ret = -ENOMEM;
 
 	pmd = pmdp_get_lockless(src_pmd);
-	if (unlikely(pmd_present(pmd) && pmd_special(pmd))) {
+	if (unlikely(pmd_present(pmd) && pmd_special(pmd) &&
+		     !is_huge_zero_pmd(pmd))) {
 		dst_ptl = pmd_lock(dst_mm, dst_pmd);
 		src_ptl = pmd_lockptr(src_mm, src_pmd);
 		spin_lock_nested(src_ptl, SINGLE_DEPTH_NESTING);
--- a/mm/memory.c~mm-huge_memory-mark-pmd-mappings-of-the-huge-zero-folio-special
+++ a/mm/memory.c
@@ -555,7 +555,14 @@ static void print_bad_pte(struct vm_area
  *
  * "Special" mappings do not wish to be associated with a "struct page" (either
  * it doesn't exist, or it exists but they don't want to touch it). In this
- * case, NULL is returned here. "Normal" mappings do have a struct page.
+ * case, NULL is returned here. "Normal" mappings do have a struct page and
+ * are ordinarily refcounted.
+ *
+ * Page mappings of the shared zero folios are always considered "special", as
+ * they are not ordinarily refcounted: neither the refcount nor the mapcount
+ * of these folios is adjusted when mapping them into user page tables.
+ * Selected page table walkers (such as GUP) can still identify mappings of the
+ * shared zero folios and work with the underlying "struct page".
  *
  * There are 2 broad cases. Firstly, an architecture may define a pte_special()
  * pte bit, in which case this function is trivial. Secondly, an architecture
@@ -585,9 +592,8 @@ static void print_bad_pte(struct vm_area
  *
  * VM_MIXEDMAP mappings can likewise contain memory with or without "struct
  * page" backing, however the difference is that _all_ pages with a struct
- * page (that is, those where pfn_valid is true) are refcounted and considered
- * normal pages by the VM. The only exception are zeropages, which are
- * *never* refcounted.
+ * page (that is, those where pfn_valid is true, except the shared zero
+ * folios) are refcounted and considered normal pages by the VM.
  *
  * The disadvantage is that pages are refcounted (which can be slower and
  * simply not an option for some PFNMAP users). The advantage is that we
@@ -667,7 +673,6 @@ struct page *vm_normal_page_pmd(struct v
 {
 	unsigned long pfn = pmd_pfn(pmd);
 
-	/* Currently it's only used for huge pfnmaps */
 	if (unlikely(pmd_special(pmd)))
 		return NULL;
 
_

Patches currently in -mm which might be from david@redhat.com are

mm-migrate-remove-migratepage_unmap.patch
treewide-remove-migratepage_success.patch
mm-huge_memory-move-more-common-code-into-insert_pmd.patch
mm-huge_memory-move-more-common-code-into-insert_pud.patch
mm-huge_memory-support-huge-zero-folio-in-vmf_insert_folio_pmd.patch
fs-dax-use-vmf_insert_folio_pmd-to-insert-the-huge-zero-folio.patch
mm-huge_memory-mark-pmd-mappings-of-the-huge-zero-folio-special.patch
mm-rmap-convert-enum-rmap_level-to-enum-pgtable_level.patch
mm-memory-convert-print_bad_pte-to-print_bad_page_map.patch
mm-memory-factor-out-common-code-from-vm_normal_page_.patch
mm-introduce-and-use-vm_normal_page_pud.patch
mm-rename-vm_ops-find_special_page-to-vm_ops-find_normal_page.patch