[PATCH v3 05/11] mm/huge_memory: mark PMD mappings of the huge zero folio special

[David Hildenbrand <david@redhat.com>]

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.

Reviewed-by: Oscar Salvador <osalvador@suse.de>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 mm/huge_memory.c |  8 ++++++--
 mm/memory.c      | 15 ++++++++++-----
 2 files changed, 16 insertions(+), 7 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index ec89e0607424e..58bac83e7fa31 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1309,6 +1309,7 @@ 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);
@@ -1418,7 +1419,9 @@ 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);
@@ -1643,7 +1646,8 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
 	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);
diff --git a/mm/memory.c b/mm/memory.c
index 0ba4f6b718471..626caedce35e0 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -555,7 +555,14 @@ 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: 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_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
@@ -667,7 +673,6 @@ 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