Subject: [PATCH v1 4/4] powernv/memtrace: don't abuse memory hot(un)plug infrastructure for memory allocations

From: David Hildenbrand <[email-masked]>
Date: Thu, 29 Oct 2020 17:27:18 +0100

Let's use alloc_contig_pages() for allocating memory and remove the
linear mapping manually via arch_remove_linear_mapping(). Mark all pages
PG_offline, such that they will definitely not get touched - e.g.,
when hibernating. When freeing memory, try to revert what we did.

The original idea was discussed in:
 https://lkml.kernel.org/r/[email-masked]

This is similar to CONFIG_DEBUG_PAGEALLOC handling on other
architectures, whereby only single pages are unmapped from the linear
mapping. Let's mimic what memory hot(un)plug would do with the linear
mapping.

We now need MEMORY_HOTPLUG and CONTIG_ALLOC as dependencies.

Simple test under QEMU TCG (10GB RAM, single NUMA node):

sh-5.0# mount -t debugfs none /sys/kernel/debug/
sh-5.0# cat /sys/devices/system/memory/block_size_bytes
40000000
sh-5.0# echo 0x40000000 > /sys/kernel/debug/powerpc/memtrace/enable
[   71.052836][  T356] memtrace: Allocated trace memory on node 0 at 0x0000000080000000
sh-5.0# echo 0x80000000 > /sys/kernel/debug/powerpc/memtrace/enable
[   75.424302][  T356] radix-mmu: Mapped 0x0000000080000000-0x00000000c0000000 with 64.0 KiB pages
[   75.430549][  T356] memtrace: Freed trace memory back on node 0
[   75.604520][  T356] memtrace: Allocated trace memory on node 0 at 0x0000000080000000
sh-5.0# echo 0x100000000 > /sys/kernel/debug/powerpc/memtrace/enable
[   80.418835][  T356] radix-mmu: Mapped 0x0000000080000000-0x0000000100000000 with 64.0 KiB pages
[   80.430493][  T356] memtrace: Freed trace memory back on node 0
[   80.433882][  T356] memtrace: Failed to allocate trace memory on node 0
sh-5.0# echo 0x40000000 > /sys/kernel/debug/powerpc/memtrace/enable
[   91.920158][  T356] memtrace: Allocated trace memory on node 0 at 0x0000000080000000

Note 1: We currently won't be allocating from ZONE_MOVABLE - because our
	pages are not movable. However, as we don't run with any memory
	hot(un)plug mechanism around, we could make an exception to
	increase the chance of allocations succeeding.

Note 2: PG_reserved isn't sufficient. E.g., kernel_page_present() used
	along PG_reserved in hibernation code will always return "true"
	on powerpc, resulting in the pages getting touched. It's too
	generic - e.g., indicates boot allocations.

Note 3: For now, we keep using memory_block_size_bytes() as minimum
	granularity. I'm not able to come up with a better guess (most
	probably, doing it on a section basis could be possible).

Suggested-by: Michal Hocko <[email-masked]>
Cc: Michael Ellerman <[email-masked]>
Cc: Benjamin Herrenschmidt <[email-masked]>
Cc: Paul Mackerras <[email-masked]>
Cc: Rashmica Gupta <[email-masked]>
Cc: Andrew Morton <[email-masked]>
Cc: Mike Rapoport <[email-masked]>
Cc: Michal Hocko <[email-masked]>
Cc: Oscar Salvador <[email-masked]>
Cc: Wei Yang <[email-masked]>
Signed-off-by: David Hildenbrand <[email-masked]>
---
 arch/powerpc/platforms/powernv/Kconfig    |   8 +-
 arch/powerpc/platforms/powernv/memtrace.c | 134 ++++++++--------------
 2 files changed, 49 insertions(+), 93 deletions(-)

diff --git a/arch/powerpc/platforms/powernv/Kconfig b/arch/powerpc/platforms/powernv/Kconfig
index 938803eab0ad..619b093a0657 100644
--- a/arch/powerpc/platforms/powernv/Kconfig
+++ b/arch/powerpc/platforms/powernv/Kconfig
@@ -27,11 +27,11 @@ config OPAL_PRD
 	  recovery diagnostics on OpenPower machines
 
 config PPC_MEMTRACE
-	bool "Enable removal of RAM from kernel mappings for tracing"
-	depends on PPC_POWERNV && MEMORY_HOTREMOVE
+	bool "Enable runtime allocation of RAM for tracing"
+	depends on PPC_POWERNV && MEMORY_HOTPLUG && CONTIG_ALLOC
 	help
-	  Enabling this option allows for the removal of memory (RAM)
-	  from the kernel mappings to be used for hardware tracing.
+	  Enabling this option allows for runtime allocation of memory (RAM)
+	  for hardware tracing.
 
 config PPC_VAS
 	bool "IBM Virtual Accelerator Switchboard (VAS)"
diff --git a/arch/powerpc/platforms/powernv/memtrace.c b/arch/powerpc/platforms/powernv/memtrace.c
index 6828108486f8..8f47797a78c2 100644
--- a/arch/powerpc/platforms/powernv/memtrace.c
+++ b/arch/powerpc/platforms/powernv/memtrace.c
@@ -50,83 +50,50 @@ static const struct file_operations memtrace_fops = {
 	.open	= simple_open,
 };
 
-static int check_memblock_online(struct memory_block *mem, void *arg)
-{
-	if (mem->state != MEM_ONLINE)
-		return -1;
-
-	return 0;
-}
-
-static int change_memblock_state(struct memory_block *mem, void *arg)
-{
-	unsigned long state = (unsigned long)arg;
-
-	mem->state = state;
-
-	return 0;
-}
-
-/* called with device_hotplug_lock held */
-static bool memtrace_offline_pages(u32 nid, u64 start_pfn, u64 nr_pages)
+static u64 memtrace_alloc_node(u32 nid, u64 size)
 {
-	const unsigned long start = PFN_PHYS(start_pfn);
-	const unsigned long size = PFN_PHYS(nr_pages);
+	const unsigned long nr_pages = PHYS_PFN(size);
+	unsigned long pfn, start_pfn;
+	struct page *page;
 
-	if (walk_memory_blocks(start, size, NULL, check_memblock_online))
-		return false;
-
-	walk_memory_blocks(start, size, (void *)MEM_GOING_OFFLINE,
-			   change_memblock_state);
-
-	if (offline_pages(start_pfn, nr_pages)) {
-		walk_memory_blocks(start, size, (void *)MEM_ONLINE,
-				   change_memblock_state);
-		return false;
-	}
+	/*
+	 * Trace memory needs to be aligned to the size, which is guaranteed
+	 * by alloc_contig_pages().
+	 */
+	page = alloc_contig_pages(nr_pages, __GFP_THISNODE | __GFP_NOWARN,
+				  nid, NULL);
+	if (!page)
+		return 0;
 
-	walk_memory_blocks(start, size, (void *)MEM_OFFLINE,
-			   change_memblock_state);
+	/*
+	 * Set pages PageOffline(), to indicate that nobody (e.g., hibernation,
+	 * dumping, ...) should be touching these pages.
+	 */
+	start_pfn = page_to_pfn(page);
+	for (pfn = start_pfn; pfn < start_pfn + nr_pages; pfn++)
+		__SetPageOffline(pfn_to_page(pfn));
 
+	arch_remove_linear_mapping(PFN_PHYS(start_pfn), size);
 
-	return true;
+	return PFN_PHYS(start_pfn);
 }
 
-static u64 memtrace_alloc_node(u32 nid, u64 size)
+static int memtrace_free(int nid, u64 start, u64 size)
 {
-	u64 start_pfn, end_pfn, nr_pages, pfn;
-	u64 base_pfn;
-	u64 bytes = memory_block_size_bytes();
+	struct mhp_params params = { .pgprot = PAGE_KERNEL };
+	const unsigned long nr_pages = PHYS_PFN(size);
+	const unsigned long start_pfn = PHYS_PFN(start);
+	unsigned long pfn;
+	int ret;
 
-	if (!node_spanned_pages(nid))
-		return 0;
+	ret = arch_create_linear_mapping(nid, start, size, &params);
+	if (ret)
+		return ret;
 
-	start_pfn = node_start_pfn(nid);
-	end_pfn = node_end_pfn(nid);
-	nr_pages = size >> PAGE_SHIFT;
-
-	/* Trace memory needs to be aligned to the size */
-	end_pfn = round_down(end_pfn - nr_pages, nr_pages);
-
-	lock_device_hotplug();
-	for (base_pfn = end_pfn; base_pfn > start_pfn; base_pfn -= nr_pages) {
-		if (memtrace_offline_pages(nid, base_pfn, nr_pages) == true) {
-			/*
-			 * Remove memory in memory block size chunks so that
-			 * iomem resources are always split to the same size and
-			 * we never try to remove memory that spans two iomem
-			 * resources.
-			 */
-			end_pfn = base_pfn + nr_pages;
-			for (pfn = base_pfn; pfn < end_pfn; pfn += bytes>> PAGE_SHIFT) {
-				__remove_memory(nid, pfn << PAGE_SHIFT, bytes);
-			}
-			unlock_device_hotplug();
-			return base_pfn << PAGE_SHIFT;
-		}
-	}
-	unlock_device_hotplug();
+	for (pfn = start_pfn; pfn < start_pfn + nr_pages; pfn++)
+		__ClearPageOffline(pfn_to_page(pfn));
 
+	free_contig_range(start_pfn, nr_pages);
 	return 0;
 }
 
@@ -197,16 +164,11 @@ static int memtrace_init_debugfs(void)
 	return ret;
 }
 
-static int online_mem_block(struct memory_block *mem, void *arg)
-{
-	return device_online(&mem->dev);
-}
-
 /*
- * Iterate through the chunks of memory we have removed from the kernel
- * and attempt to add them back to the kernel.
+ * Iterate through the chunks of memory we allocated and attempt to expose
+ * them back to the kernel.
  */
-static int memtrace_online(void)
+static int memtrace_free_regions(void)
 {
 	int i, ret = 0;
 	struct memtrace_entry *ent;
@@ -214,7 +176,7 @@ static int memtrace_online(void)
 	for (i = memtrace_array_nr - 1; i >= 0; i--) {
 		ent = &memtrace_array[i];
 
-		/* We have onlined this chunk previously */
+		/* We have freed this chunk previously */
 		if (ent->nid == NUMA_NO_NODE)
 			continue;
 
@@ -224,30 +186,25 @@ static int memtrace_online(void)
 			ent->mem = 0;
 		}
 
-		if (add_memory(ent->nid, ent->start, ent->size, MHP_NONE)) {
-			pr_err("Failed to add trace memory to node %d\n",
+		if (memtrace_free(ent->nid, ent->start, ent->size)) {
+			pr_err("Failed to free trace memory on node %d\n",
 				ent->nid);
 			ret += 1;
 			continue;
 		}
 
-		lock_device_hotplug();
-		walk_memory_blocks(ent->start, ent->size, NULL,
-				   online_mem_block);
-		unlock_device_hotplug();
-
 		/*
-		 * Memory was added successfully so clean up references to it
-		 * so on reentry we can tell that this chunk was added.
+		 * Memory was freed successfully so clean up references to it
+		 * so on reentry we can tell that this chunk was freed.
 		 */
 		debugfs_remove_recursive(ent->dir);
-		pr_info("Added trace memory back to node %d\n", ent->nid);
+		pr_info("Freed trace memory back on node %d\n", ent->nid);
 		ent->size = ent->start = ent->nid = NUMA_NO_NODE;
 	}
 	if (ret)
 		return ret;
 
-	/* If all chunks of memory were added successfully, reset globals */
+	/* If all chunks of memory were freed successfully, reset globals */
 	kfree(memtrace_array);
 	memtrace_array = NULL;
 	memtrace_size = 0;
@@ -269,16 +226,15 @@ static int memtrace_enable_set(void *data, u64 val)
 		return -EINVAL;
 	}
 
-	/* Re-add/online previously removed/offlined memory */
+	/* Free all previously allocated memory. */
 	if (memtrace_size) {
-		if (memtrace_online())
+		if (memtrace_free_regions())
 			return -EAGAIN;
 	}
 
 	if (!val)
 		return 0;
 
-	/* Offline and remove memory */
 	if (memtrace_init_regions_runtime(val))
 		return -EINVAL;
 
-- 
2.26.2