+ mshv-use-hmm_range_fault_unlocked_timeout-for-region-faults.patch added to mm-unstable branch

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

The patch titled
     Subject: mshv: use hmm_range_fault_unlocked_timeout() for region faults
has been added to the -mm mm-unstable branch.  Its filename is
     mshv-use-hmm_range_fault_unlocked_timeout-for-region-faults.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mshv-use-hmm_range_fault_unlocked_timeout-for-region-faults.patch

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

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------------------------------------------------------
From: Stanislav Kinsburskii <skinsburskii@gmail.com>
Subject: mshv: use hmm_range_fault_unlocked_timeout() for region faults
Date: Wed, 22 Jul 2026 14:44:26 -0700

MSHV currently faults movable memory regions by taking mmap_read_lock()
around hmm_range_fault().  That prevents the fault path from handling VMAs
whose fault handlers need to drop mmap_lock, such as userfaultfd-backed
mappings.

Use hmm_range_fault_unlocked_timeout() instead.  Passing a timeout of 0
preserves MSHV's existing unbounded retry behavior while letting the HMM
helper own mmap_lock acquisition and refresh range->notifier_seq
internally before walking the range.  After the fault succeeds, MSHV still
takes mreg_mutex and checks mmu_interval_read_retry() before installing
the pages into the region, so the existing invalidation synchronization is
preserved.

Fold the small fault-and-lock helper into mshv_region_range_fault(), since
the remaining retry path is just the standard "fault, take the driver
lock, check the interval notifier sequence" pattern.

Link: https://lore.kernel.org/20260722-hmm-v10-v1-4-606464dd601a@gmail.com
Signed-off-by: Stanislav Kinsburskii <skinsburskii@gmail.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Cc: Danilo Krummrich <dakr@kernel.org>
Cc: Dave Airlie <airlied@gmail.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Dexuan Cui <decui@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: K. Y. Srinivasan <kys@microsoft.com>
Cc: Leon Romanovsky <leon@kernel.org>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lizhi Hou <lizhi.hou@amd.com>
Cc: Long Li <longli@microsoft.com>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Lyude <lyude@redhat.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Oded Gabbay <ogabbay@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Thomas Zimemrmann <tzimmermann@suse.de>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Wei Liu <wei.liu@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 drivers/hv/mshv_regions.c |   54 ++++++------------------------------
 1 file changed, 10 insertions(+), 44 deletions(-)

--- a/drivers/hv/mshv_regions.c~mshv-use-hmm_range_fault_unlocked_timeout-for-region-faults
+++ a/drivers/hv/mshv_regions.c
@@ -382,46 +382,6 @@ int mshv_region_get(struct mshv_mem_regi
 }
 
 /**
- * mshv_region_hmm_fault_and_lock - Handle HMM faults and lock the memory region
- * @region: Pointer to the memory region structure
- * @range: Pointer to the HMM range structure
- *
- * This function performs the following steps:
- * 1. Reads the notifier sequence for the HMM range.
- * 2. Acquires a read lock on the memory map.
- * 3. Handles HMM faults for the specified range.
- * 4. Releases the read lock on the memory map.
- * 5. If successful, locks the memory region mutex.
- * 6. Verifies if the notifier sequence has changed during the operation.
- *    If it has, releases the mutex and returns -EBUSY to match with
- *    hmm_range_fault() return code for repeating.
- *
- * Return: 0 on success, a negative error code otherwise.
- */
-static int mshv_region_hmm_fault_and_lock(struct mshv_mem_region *region,
-					  struct hmm_range *range)
-{
-	int ret;
-
-	range->notifier_seq = mmu_interval_read_begin(range->notifier);
-	mmap_read_lock(region->mreg_mni.mm);
-	ret = hmm_range_fault(range);
-	mmap_read_unlock(region->mreg_mni.mm);
-	if (ret)
-		return ret;
-
-	mutex_lock(&region->mreg_mutex);
-
-	if (mmu_interval_read_retry(range->notifier, range->notifier_seq)) {
-		mutex_unlock(&region->mreg_mutex);
-		cond_resched();
-		return -EBUSY;
-	}
-
-	return 0;
-}
-
-/**
  * mshv_region_range_fault - Handle memory range faults for a given region.
  * @region: Pointer to the memory region structure.
  * @page_offset: Offset of the page within the region.
@@ -452,13 +412,19 @@ static int mshv_region_range_fault(struc
 	range.start = region->start_uaddr + page_offset * HV_HYP_PAGE_SIZE;
 	range.end = range.start + page_count * HV_HYP_PAGE_SIZE;
 
-	do {
-		ret = mshv_region_hmm_fault_and_lock(region, &range);
-	} while (ret == -EBUSY);
-
+again:
+	ret = hmm_range_fault_unlocked_timeout(&range, 0);
 	if (ret)
 		goto out;
 
+	mutex_lock(&region->mreg_mutex);
+
+	if (mmu_interval_read_retry(range.notifier, range.notifier_seq)) {
+		mutex_unlock(&region->mreg_mutex);
+		cond_resched();
+		goto again;
+	}
+
 	for (i = 0; i < page_count; i++)
 		region->mreg_pages[page_offset + i] = hmm_pfn_to_page(pfns[i]);
 
_

Patches currently in -mm which might be from skinsburskii@gmail.com are

mm-hmm-move-page-fault-handling-out-of-walk-callbacks.patch
mm-hmm-add-hmm_range_fault_unlocked_timeout-for-mmap-lock-drop-support.patch
selftests-mm-add-hmm-test-for-mmap-lock-dropping-faults.patch
mshv-use-hmm_range_fault_unlocked_timeout-for-region-faults.patch
drm-nouveau-use-hmm_range_fault_unlocked_timeout-for-svm-faults.patch
rdma-umem-use-hmm_range_fault_unlocked_timeout-for-odp-faults.patch
accel-amdxdna-use-hmm_range_fault_unlocked_timeout-for-range-population.patch
drm-gpusvm-use-hmm_range_fault_unlocked_timeout-for-range-faults.patch