linux-kernel - [PATCH v1 12/12] mm/rmap: keep mapcount untouched for device-exclusive entries

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Message-ID: <20250129115411.2077152-13-david@redhat.com>
Date: Wed, 29 Jan 2025 12:54:10 +0100
From: David Hildenbrand <david@...hat.com>
To: linux-kernel@...r.kernel.org
Cc: linux-doc@...r.kernel.org,
	dri-devel@...ts.freedesktop.org,
	linux-mm@...ck.org,
	nouveau@...ts.freedesktop.org,
	David Hildenbrand <david@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jérôme Glisse <jglisse@...hat.com>,
	Jonathan Corbet <corbet@....net>,
	Alex Shi <alexs@...nel.org>,
	Yanteng Si <si.yanteng@...ux.dev>,
	Karol Herbst <kherbst@...hat.com>,
	Lyude Paul <lyude@...hat.com>,
	Danilo Krummrich <dakr@...nel.org>,
	David Airlie <airlied@...il.com>,
	Simona Vetter <simona@...ll.ch>,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Jann Horn <jannh@...gle.com>,
	Pasha Tatashin <pasha.tatashin@...een.com>,
	Peter Xu <peterx@...hat.com>,
	Alistair Popple <apopple@...dia.com>,
	Jason Gunthorpe <jgg@...dia.com>
Subject: [PATCH v1 12/12] mm/rmap: keep mapcount untouched for device-exclusive entries

Now that conversion to device-exclusive does no longer perform an
rmap walk and the main page_vma_mapped_walk() users were taught to
properly handle nonswap entries, let's treat device-exclusive entries just
as if they would be present, similar to how we handle device-private
entries already.

This fixes swapout/migration of folios with device-exclusive entries.

Likely there are still some page_vma_mapped_walk() callers that are not
fully prepared for these entries, and where we simply want to refuse
!pte_present() entries. They have to be fixed independently; the ones in
mm/rmap.c are prepared.

Fixes: b756a3b5e7ea ("mm: device exclusive memory access")
Signed-off-by: David Hildenbrand <david@...hat.com>
---
 mm/memory.c | 17 +----------------
 mm/rmap.c   |  7 -------
 2 files changed, 1 insertion(+), 23 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index db38d6ae4e74..cd689cd8a7c8 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -743,20 +743,6 @@ static void restore_exclusive_pte(struct vm_area_struct *vma,
 
 	VM_BUG_ON_FOLIO(pte_write(pte) && (!folio_test_anon(folio) &&
 					   PageAnonExclusive(page)), folio);
-
-	/*
-	 * No need to take a page reference as one was already
-	 * created when the swap entry was made.
-	 */
-	if (folio_test_anon(folio))
-		folio_add_anon_rmap_pte(folio, page, vma, address, RMAP_NONE);
-	else
-		/*
-		 * Currently device exclusive access only supports anonymous
-		 * memory so the entry shouldn't point to a filebacked page.
-		 */
-		WARN_ON_ONCE(1);
-
 	set_pte_at(vma->vm_mm, address, ptep, pte);
 
 	/*
@@ -1628,8 +1614,7 @@ static inline int zap_nonpresent_ptes(struct mmu_gather *tlb,
 		 */
 		WARN_ON_ONCE(!vma_is_anonymous(vma));
 		rss[mm_counter(folio)]--;
-		if (is_device_private_entry(entry))
-			folio_remove_rmap_pte(folio, page, vma);
+		folio_remove_rmap_pte(folio, page, vma);
 		folio_put(folio);
 	} else if (!non_swap_entry(entry)) {
 		/* Genuine swap entries, hence a private anon pages */
diff --git a/mm/rmap.c b/mm/rmap.c
index 9e2002d97d6f..4acc9f6d743a 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -2495,13 +2495,6 @@ struct page *make_device_exclusive(struct mm_struct *mm, unsigned long addr,
 	/* The pte is writable, uffd-wp does not apply. */
 	set_pte_at(mm, addr, fw.ptep, swp_pte);
 
-	/*
-	 * TODO: The device-exclusive non-swap PTE holds a folio reference but
-	 * does not count as a mapping (mapcount), which is wrong and must be
-	 * fixed, otherwise RMAP walks don't behave as expected.
-	 */
-	folio_remove_rmap_pte(folio, page, vma);
-
 	folio_walk_end(&fw, vma);
 	*foliop = folio;
 	return page;
-- 
2.48.1