linux-kernel - [PATCH v1 5/9] mm/huge_memory: mark PMD mappings of the huge zero folio special

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Message-ID: <20250715132350.2448901-6-david@redhat.com>
Date: Tue, 15 Jul 2025 15:23:46 +0200
From: David Hildenbrand <david@...hat.com>
To: linux-kernel@...r.kernel.org
Cc: linux-mm@...ck.org,
	xen-devel@...ts.xenproject.org,
	linux-fsdevel@...r.kernel.org,
	nvdimm@...ts.linux.dev,
	David Hildenbrand <david@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Juergen Gross <jgross@...e.com>,
	Stefano Stabellini <sstabellini@...nel.org>,
	Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
	Dan Williams <dan.j.williams@...el.com>,
	Matthew Wilcox <willy@...radead.org>,
	Jan Kara <jack@...e.cz>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Mike Rapoport <rppt@...nel.org>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Michal Hocko <mhocko@...e.com>,
	Zi Yan <ziy@...dia.com>,
	Baolin Wang <baolin.wang@...ux.alibaba.com>,
	Nico Pache <npache@...hat.com>,
	Ryan Roberts <ryan.roberts@....com>,
	Dev Jain <dev.jain@....com>,
	Barry Song <baohua@...nel.org>,
	Jann Horn <jannh@...gle.com>,
	Pedro Falcato <pfalcato@...e.de>,
	Hugh Dickins <hughd@...gle.com>,
	Oscar Salvador <osalvador@...e.de>,
	Lance Yang <lance.yang@...ux.dev>
Subject: [PATCH v1 5/9] mm/huge_memory: mark PMD mappings of the huge zero folio special

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. However,
so far we are not marking it as special like we do with the ordinary
shared zeropage. Let's mark it as special, so we can further refactor
vm_normal_page_pmd() and vm_normal_page().

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

Reviewed-by: Oscar Salvador <osalvador@...e.de>
Signed-off-by: David Hildenbrand <david@...hat.com>
---
 mm/huge_memory.c |  5 ++++-
 mm/memory.c      | 14 +++++++++-----
 2 files changed, 13 insertions(+), 6 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 9ec7f48efde09..24aff14d22a1e 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1320,6 +1320,7 @@ static void set_huge_zero_folio(pgtable_t pgtable, struct mm_struct *mm,
 {
 	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);
@@ -1429,7 +1430,9 @@ static vm_fault_t insert_pmd(struct vm_area_struct *vma, unsigned long addr,
 	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);
diff --git a/mm/memory.c b/mm/memory.c
index 3dd6c57e6511e..a4f62923b961c 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -543,7 +543,13 @@ static void print_bad_pte(struct vm_area_struct *vma, unsigned long addr,
  *
  * "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. However, selected page table walkers
+ * (such as GUP) can still identify these mappings 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
@@ -573,9 +579,8 @@ static void print_bad_pte(struct vm_area_struct *vma, unsigned long addr,
  *
  * 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
@@ -655,7 +660,6 @@ struct page *vm_normal_page_pmd(struct vm_area_struct *vma, unsigned long addr,
 {
 	unsigned long pfn = pmd_pfn(pmd);
 
-	/* Currently it's only used for huge pfnmaps */
 	if (unlikely(pmd_special(pmd)))
 		return NULL;
 
-- 
2.50.1