linux-kernel - [PATCH] mm/hmm: Do not fault in device private pages owned by the caller

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Message-ID: <20250722193445.1588348-1-francois.dugast@intel.com>
Date: Tue, 22 Jul 2025 21:34:45 +0200
From: Francois Dugast <francois.dugast@...el.com>
To: balbirs@...dia.com
Cc: airlied@...il.com,
	akpm@...ux-foundation.org,
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Subject: [PATCH] mm/hmm: Do not fault in device private pages owned by the caller

When the PMD swap entry is device private and owned by the caller,
skip the range faulting and instead just set the correct HMM PFNs.
This is similar to the logic for PTEs in hmm_vma_handle_pte().

For now, each hmm_pfns[i] entry is populated as it is currently done
in hmm_vma_handle_pmd() but this might not be necessary. A follow-up
optimization could be to make use of the order and skip populating
subsequent PFNs.

Signed-off-by: Francois Dugast <francois.dugast@...el.com>
---
 mm/hmm.c | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/mm/hmm.c b/mm/hmm.c
index f2415b4b2cdd..63ec1b18a656 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -355,6 +355,31 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
 	}
 
 	if (!pmd_present(pmd)) {
+		swp_entry_t entry = pmd_to_swp_entry(pmd);
+
+		/*
+		 * Don't fault in device private pages owned by the caller,
+		 * just report the PFNs.
+		 */
+		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;
+
+			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;
+		}
+
 		if (hmm_range_need_fault(hmm_vma_walk, hmm_pfns, npages, 0))
 			return -EFAULT;
 		return hmm_pfns_fill(start, end, range, HMM_PFN_ERROR);
-- 
2.43.0