linux-kernel - [PATCH v1 1/2] mm/huge_memory: don't mark refcounted pages special in vmf_insert_folio

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Message-ID: <20250603211634.2925015-2-david@redhat.com>
Date: Tue,  3 Jun 2025 23:16:33 +0200
From: David Hildenbrand <david@...hat.com>
To: linux-kernel@...r.kernel.org
Cc: linux-mm@...ck.org,
	nvdimm@...ts.linux.dev,
	linux-cxl@...r.kernel.org,
	David Hildenbrand <david@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alistair Popple <apopple@...dia.com>,
	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>,
	Dan Williams <dan.j.williams@...el.com>
Subject: [PATCH v1 1/2] mm/huge_memory: don't mark refcounted pages special in vmf_insert_folio_pmd()

Marking PMDs that map a "normal" refcounted folios as special is
against our rules documented for vm_normal_page().

Fortunately, there are not that many pmd_special() check that can be
mislead, and most vm_normal_page_pmd()/vm_normal_folio_pmd() users that
would get this wrong right now are rather harmless: e.g., none so far
bases decisions whether to grab a folio reference on that decision.

Well, and GUP-fast will fallback to GUP-slow. All in all, so far no big
implications as it seems.

Getting this right will get more important as we use
folio_normal_page_pmd() in more places.

Fix it by just inlining the relevant code, making the whole
pmd_none() handling cleaner. We can now use folio_mk_pmd().

While at it, make sure that a pmd that is not-none is actually present
before comparing PFNs.

Fixes: 6c88f72691f8 ("mm/huge_memory: add vmf_insert_folio_pmd()")
Signed-off-by: David Hildenbrand <david@...hat.com>
---
 mm/huge_memory.c | 39 ++++++++++++++++++++++++++++++++-------
 1 file changed, 32 insertions(+), 7 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index d3e66136e41a3..f9e23dfea76f8 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1474,9 +1474,10 @@ vm_fault_t vmf_insert_folio_pmd(struct vm_fault *vmf, struct folio *folio,
 	struct vm_area_struct *vma = vmf->vma;
 	unsigned long addr = vmf->address & PMD_MASK;
 	struct mm_struct *mm = vma->vm_mm;
+	pmd_t *pmd = vmf->pmd;
 	spinlock_t *ptl;
 	pgtable_t pgtable = NULL;
-	int error;
+	pmd_t entry;
 
 	if (addr < vma->vm_start || addr >= vma->vm_end)
 		return VM_FAULT_SIGBUS;
@@ -1490,17 +1491,41 @@ vm_fault_t vmf_insert_folio_pmd(struct vm_fault *vmf, struct folio *folio,
 			return VM_FAULT_OOM;
 	}
 
-	ptl = pmd_lock(mm, vmf->pmd);
-	if (pmd_none(*vmf->pmd)) {
+	ptl = pmd_lock(mm, pmd);
+	if (pmd_none(*pmd)) {
 		folio_get(folio);
 		folio_add_file_rmap_pmd(folio, &folio->page, vma);
 		add_mm_counter(mm, mm_counter_file(folio), HPAGE_PMD_NR);
+
+		entry = folio_mk_pmd(folio, vma->vm_page_prot);
+		if (write) {
+			entry = pmd_mkyoung(pmd_mkdirty(entry));
+			entry = maybe_pmd_mkwrite(entry, vma);
+		}
+		set_pmd_at(mm, addr, pmd, entry);
+		update_mmu_cache_pmd(vma, addr, pmd);
+
+		if (pgtable) {
+			pgtable_trans_huge_deposit(mm, pmd, pgtable);
+			mm_inc_nr_ptes(mm);
+			pgtable = NULL;
+		}
+	} else if (pmd_present(*pmd) && write) {
+		/*
+		 * We only allow for upgrading write permissions if the
+		 * same folio is already mapped.
+		 */
+		if (pmd_pfn(*pmd) == folio_pfn(folio)) {
+			entry = pmd_mkyoung(*pmd);
+			entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
+			if (pmdp_set_access_flags(vma, addr, pmd, entry, 1))
+				update_mmu_cache_pmd(vma, addr, pmd);
+		} else {
+			WARN_ON_ONCE(!is_huge_zero_pmd(*pmd));
+		}
 	}
-	error = insert_pfn_pmd(vma, addr, vmf->pmd,
-			pfn_to_pfn_t(folio_pfn(folio)), vma->vm_page_prot,
-			write, pgtable);
 	spin_unlock(ptl);
-	if (error && pgtable)
+	if (pgtable)
 		pte_free(mm, pgtable);
 
 	return VM_FAULT_NOPAGE;
-- 
2.49.0