+ mm-hmm-populate-pfns-from-pmd-swap-entry.patch added to mm-new branch

[Andrew Morton <akpm@linux-foundation.org>]

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The patch titled
     Subject: mm/hmm: populate PFNs from PMD swap entry
has been added to the -mm mm-new branch.  Its filename is
     mm-hmm-populate-pfns-from-pmd-swap-entry.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-hmm-populate-pfns-from-pmd-swap-entry.patch

This patch will later appear in the mm-new branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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patches, and acceptance into mm-new is a notification for others take
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------------------------------------------------------
From: Francois Dugast <francois.dugast@intel.com>
Subject: mm/hmm: populate PFNs from PMD swap entry
Date: Tue, 2 Sep 2025 15:07:13 +0200

Once support for THP migration of zone device pages is enabled, device
private swap entries will be found during the walk not only for PTEs but
also for PMDs.

Therefore, it is necessary to extend to PMDs the special handling which is
already in place for PTEs when device private pages are owned by the
caller: instead of faulting or skipping the range, the correct behavior is
to use the swap entry to populate HMM PFNs.

This change is a prerequisite to make use of device-private THP in drivers
using drivers/gpu/drm/drm_pagemap, such as xe.

Even though subsequent PFNs can be inferred when handling large order
PFNs, the PFN list is still fully populated because this is currently
expected by HMM users.  In case this changes in the future, that is all
HMM users support a sparsely populated PFN list, the for() loop can be
made to skip remaining PFNs for the current order.  A quick test shows the
loop takes about 10 ns, roughly 20 times faster than without this
optimization.

Link: https://lkml.kernel.org/r/20250829080505.1020155-1-francois.dugast@intel.com
Link: https://lkml.kernel.org/r/20250902130713.1644661-1-francois.dugast@intel.com
Signed-off-by: Francois Dugast <francois.dugast@intel.com>
Acked-by: Balbir Singh <balbirs@nvidia.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Leon Romanovsky <leonro@nvidia.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Balbir Singh <balbirs@nvidia.com>
Cc: David Airlie <airlied@gmail.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Mika Penttilä <mpenttil@redhat.com>
Cc: Thomas Hellstrom <thomas.hellstrom@linux.intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/hmm.c |   29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

--- a/mm/hmm.c~mm-hmm-populate-pfns-from-pmd-swap-entry
+++ a/mm/hmm.c
@@ -355,6 +355,35 @@ again:
 	}
 
 	if (!pmd_present(pmd)) {
+#ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
+		swp_entry_t entry = pmd_to_swp_entry(pmd);
+
+		if (is_device_private_entry(entry) &&
+		    pfn_swap_entry_folio(entry)->pgmap->owner ==
+		    range->dev_private_owner) {
+			unsigned long cpu_flags = HMM_PFN_VALID |
+				hmm_pfn_flags_order(PMD_SHIFT - PAGE_SHIFT);
+			unsigned long pfn = swp_offset_pfn(entry);
+			unsigned long i;
+
+			if (is_writable_device_private_entry(entry))
+				cpu_flags |= HMM_PFN_WRITE;
+
+			/*
+			 * Fully populate the PFN list though subsequent
+			 * PFNs could be inferred, because drivers which
+			 * are not yet aware of large folios probably do
+			 * not support sparsely populated PFN lists.
+			 */
+			for (i = 0; addr < end; addr += PAGE_SIZE, i++, pfn++) {
+				hmm_pfns[i] &= HMM_PFN_INOUT_FLAGS;
+				hmm_pfns[i] |= pfn | cpu_flags;
+			}
+
+			return 0;
+		}
+#endif  /* CONFIG_ARCH_ENABLE_THP_MIGRATION */
+
 		if (hmm_range_need_fault(hmm_vma_walk, hmm_pfns, npages, 0))
 			return -EFAULT;
 		return hmm_pfns_fill(start, end, range, HMM_PFN_ERROR);
_

Patches currently in -mm which might be from francois.dugast@intel.com are

mm-hmm-populate-pfns-from-pmd-swap-entry.patch