mm: introduce pfnmap_track() and pfnmap_untrack() and use them for memremap

Let's provide variants of track_pfn_remap() and untrack_pfn() that won't
mess with VMAs, and replace the usage in mm/memremap.c.

Add some documentation.

Link: https://lkml.kernel.org/r/20250512123424.637989-4-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Acked-by: Ingo Molnar <mingo@kernel.org>	[x86 bits]
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Betkov <bp@alien8.de>
Cc: Dave Airlie <airlied@gmail.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Jann Horn <jannh@google.com>
Cc: Jonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleinxer <tglx@linutronix.de>
Cc: Tvrtko Ursulin <tursulin@ursulin.net>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

arch/x86/mm/pat/memtype.c

@@ -1068,6 +1068,20 @@ int pfnmap_setup_cachemode(unsigned long pfn, unsigned long size, pgprot_t *prot
 	return 0;
 }
 
+int pfnmap_track(unsigned long pfn, unsigned long size, pgprot_t *prot)
+{
+	const resource_size_t paddr = (resource_size_t)pfn << PAGE_SHIFT;
+
+	return reserve_pfn_range(paddr, size, prot, 0);
+}
+
+void pfnmap_untrack(unsigned long pfn, unsigned long size)
+{
+	const resource_size_t paddr = (resource_size_t)pfn << PAGE_SHIFT;
+
+	free_pfn_range(paddr, size);
+}
+
 /*
  * untrack_pfn is called while unmapping a pfnmap for a region.
  * untrack can be called for a specific region indicated by pfn and size or

include/linux/pgtable.h

@@ -1502,6 +1502,16 @@ static inline int pfnmap_setup_cachemode(unsigned long pfn, unsigned long size,
 	return 0;
 }
 
+static inline int pfnmap_track(unsigned long pfn, unsigned long size,
+		pgprot_t *prot)
+{
+	return 0;
+}
+
+static inline void pfnmap_untrack(unsigned long pfn, unsigned long size)
+{
+}
+
 /*
  * track_pfn_copy is called when a VM_PFNMAP VMA is about to get the page
  * tables copied during copy_page_range(). Will store the pfn to be
@@ -1575,6 +1585,35 @@ extern int track_pfn_remap(struct vm_area_struct *vma, pgprot_t *prot,
  */
 int pfnmap_setup_cachemode(unsigned long pfn, unsigned long size,
 		pgprot_t *prot);
+
+/**
+ * pfnmap_track - track a pfn range
+ * @pfn: the start of the pfn range
+ * @size: the size of the pfn range in bytes
+ * @prot: the pgprot to track
+ *
+ * Requested the pfn range to be 'tracked' by a hardware implementation and
+ * setup the cachemode in @prot similar to pfnmap_setup_cachemode().
+ *
+ * This allows for fine-grained control of memory cache behaviour at page
+ * level granularity. Tracking memory this way is persisted across VMA splits
+ * (VMA merging does not apply for VM_PFNMAP).
+ *
+ * Currently, there is only one implementation for this - x86 Page Attribute
+ * Table (PAT). See Documentation/arch/x86/pat.rst for more details.
+ *
+ * Returns 0 on success and -EINVAL on error.
+ */
+int pfnmap_track(unsigned long pfn, unsigned long size, pgprot_t *prot);
+
+/**
+ * pfnmap_untrack - untrack a pfn range
+ * @pfn: the start of the pfn range
+ * @size: the size of the pfn range in bytes
+ *
+ * Untrack a pfn range previously tracked through pfnmap_track().
+ */
+void pfnmap_untrack(unsigned long pfn, unsigned long size);
 extern int track_pfn_copy(struct vm_area_struct *dst_vma,
 		struct vm_area_struct *src_vma, unsigned long *pfn);
 extern void untrack_pfn_copy(struct vm_area_struct *dst_vma,

mm/memremap.c

@@ -130,7 +130,7 @@ static void pageunmap_range(struct dev_pagemap *pgmap, int range_id)
 	}
 	mem_hotplug_done();
 
-	untrack_pfn(NULL, PHYS_PFN(range->start), range_len(range), true);
+	pfnmap_untrack(PHYS_PFN(range->start), range_len(range));
 	pgmap_array_delete(range);
 }
 
@@ -211,8 +211,8 @@ static int pagemap_range(struct dev_pagemap *pgmap, struct mhp_params *params,
 	if (nid < 0)
 		nid = numa_mem_id();
 
-	error = track_pfn_remap(NULL, &params->pgprot, PHYS_PFN(range->start), 0,
-			range_len(range));
+	error = pfnmap_track(PHYS_PFN(range->start), range_len(range),
+			     &params->pgprot);
 	if (error)
 		goto err_pfn_remap;
 
@@ -277,7 +277,7 @@ err_add_memory:
 	if (!is_private)
 		kasan_remove_zero_shadow(__va(range->start), range_len(range));
 err_kasan:
-	untrack_pfn(NULL, PHYS_PFN(range->start), range_len(range), true);
+	pfnmap_untrack(PHYS_PFN(range->start), range_len(range));
 err_pfn_remap:
 	pgmap_array_delete(range);
 	return error;