linux-kernel - [RFC PATCH 1/6] mm/hmm: Add flag to track device private PFNs

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Message-Id: <20251128044146.80050-2-jniethe@nvidia.com>
Date: Fri, 28 Nov 2025 15:41:41 +1100
From: Jordan Niethe <jniethe@...dia.com>
To: linux-mm@...ck.org
Cc: balbirs@...dia.com,
	matthew.brost@...el.com,
	akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org,
	dri-devel@...ts.freedesktop.org,
	david@...hat.com,
	ziy@...dia.com,
	apopple@...dia.com,
	lorenzo.stoakes@...cle.com,
	lyude@...hat.com,
	dakr@...nel.org,
	airlied@...il.com,
	simona@...ll.ch,
	rcampbell@...dia.com,
	mpenttil@...hat.com,
	jgg@...dia.com,
	willy@...radead.org
Subject: [RFC PATCH 1/6] mm/hmm: Add flag to track device private PFNs

A future change will remove device private pages from the physical
address space. This will mean that device private pages no longer have
normal PFN and must be handled separately.

Prepare for this by adding a HMM_PFN_DEVICE_PRIVATE flag to indicate
that a hmm_pfn contains a PFN for a device private page.

Signed-off-by: Jordan Niethe <jniethe@...dia.com>
Signed-off-by: Alistair Popple <apopple@...dia.com>
---
 include/linux/hmm.h | 2 ++
 mm/hmm.c            | 2 +-
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/include/linux/hmm.h b/include/linux/hmm.h
index db75ffc949a7..df571fa75a44 100644
--- a/include/linux/hmm.h
+++ b/include/linux/hmm.h
@@ -23,6 +23,7 @@ struct mmu_interval_notifier;
  * HMM_PFN_WRITE - if the page memory can be written to (requires HMM_PFN_VALID)
  * HMM_PFN_ERROR - accessing the pfn is impossible and the device should
  *                 fail. ie poisoned memory, special pages, no vma, etc
+ * HMM_PFN_DEVICE_PRIVATE - the pfn field contains a DEVICE_PRIVATE pfn.
  * HMM_PFN_P2PDMA - P2P page
  * HMM_PFN_P2PDMA_BUS - Bus mapped P2P transfer
  * HMM_PFN_DMA_MAPPED - Flag preserved on input-to-output transformation
@@ -40,6 +41,7 @@ enum hmm_pfn_flags {
 	HMM_PFN_VALID = 1UL << (BITS_PER_LONG - 1),
 	HMM_PFN_WRITE = 1UL << (BITS_PER_LONG - 2),
 	HMM_PFN_ERROR = 1UL << (BITS_PER_LONG - 3),
+	HMM_PFN_DEVICE_PRIVATE = 1UL << (BITS_PER_LONG - 7),
 	/*
 	 * Sticky flags, carried from input to output,
 	 * don't forget to update HMM_PFN_INOUT_FLAGS
diff --git a/mm/hmm.c b/mm/hmm.c
index 87562914670a..1cff68ade1d4 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -262,7 +262,7 @@ static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
 		if (is_device_private_entry(entry) &&
 		    page_pgmap(pfn_swap_entry_to_page(entry))->owner ==
 		    range->dev_private_owner) {
-			cpu_flags = HMM_PFN_VALID;
+			cpu_flags = HMM_PFN_VALID | HMM_PFN_DEVICE_PRIVATE;
 			if (is_writable_device_private_entry(entry))
 				cpu_flags |= HMM_PFN_WRITE;
 			new_pfn_flags = swp_offset_pfn(entry) | cpu_flags;
-- 
2.34.1