[PATCH v9 0/1] mm: report per-page metadata information

[PATCH v9 0/1] mm: report per-page metadata information

[Sourav Panda]

Changelog:
v9: 	- Quick Fix:
		- In fs/proc/meminfo.c, replaced tabs with spaces for
		  consistent userspace parsing.
	- Patch is ready to be taken in.
v8:
	- Addressed Powerpc (Power8 LE) boot failure.
	  	- In __populate_section_memmap instead of calling
		  mod_node_page_state (unavaialable at boot for
		  powerpc), we call mod_node_early_perpage_metadata.
		  This was a helper function that was introduced
		  for arm, to combat this exact problem.
		- Since __populate_section_memmap is tagged with
		  __meminit, we also had to modify the tag of
		  mod_node_early_perpage_metadata from __init to
		  __meminit.
	- Naming Changes:
	  	- In /proc/meminfo PageMetadata --> Memmap
		- In /proc/vmstat  nr_page_metadata --> nr_memmap
		- In /proc/vmstat  nr_page_metadata_boot -->
		  nr_memmap_boot
	- Addressed clarifications requested by Andrew Morton.
	  	- Updated the commit log to include utility or
		  potential usage for userspace.
	- Declined changing  placement of metrics after attempting:
		- No changes in /proc/meminfo since it cannot be moved
		  to the end anyway. This is because we have also
		  hugetlb_report_meminfo() and arch_report_meminfo().
	- Rebased to version 6, patchlevel 8.
v7:
	- Addressed comments from David Rientjes
                - For exporting PageMetadata to /proc/meminfo,
                  utilize global_node_page_state_pages for item
                  NR_PAGE_METADATA. This is instead of iterating
                  over nodes and accumulating the output of
                  node_page_state.
v6:
	- Interface changes
	  	- Added per-node nr_page_metadata and
		  nr_page_metadata_boot fields that are exported
		  in /sys/devices/system/node/nodeN/vmstat
		- nr_page_metadata exclusively tracks buddy
		  allocations while nr_page_metadata_boot
		  exclusively tracks memblock allocations.
		- Modified PageMetadata in /proc/meminfo to only
		  include buddy allocations so that it is
		  comparable against MemTotal in /proc/meminfo
		- Removed the PageMetadata field added in
		  /sys/devices/system/node/nodeN/meminfo
	- Addressed bugs reported by the kernel test bot.
	  	- All occurences of __mod_node_page_state have
		  been replaced by mod_node_page_state.
	- Addressed comments from Muchun Song.
	  	- Removed page meta data accouting from
		  mm/hugetlb.c. When CONFIG_SPARSEMEM_VMEMMAP
		  is disabled struct pages should not be returned
		  to buddy.
	- Modified the cover letter with the results and analysis
		- From when memory_hotplug.memmap_on_memory is
		  alternated between 0 and 1.
		- To account for the new interface changes.

v5:
	- Addressed comments from Muchun Song.
		- Fixed errors introduced in v4 when
		  CONFIG_SPARSEMEM_VMEMMAP is disabled by testing
		  against FLATMEM and SPARSEMEM memory models.
		- Handled the condition wherein the allocation of
		  walk.reuse_page fails, by moving NR_PAGE_METADATA
		  update into the clause if( walk.reuse_page ).
		- Removed the usage of DIV_ROUND_UP from
		  alloc_vmemmap_page_list since "end - start" is
		  always a multiple of PAGE_SIZE.
		- Modified alloc_vmemmap_page_list to update
		  NR_PAGE_METADATA once instead of every loop.
v4:
	- Addressed comment from Matthew Wilcox.
		- Used __node_stat_sub_folio and __node_stat_add_folio
		  instead of __mod_node_page_state in mm/hugetlb.c.
		- Used page_pgdat wherever possible in the entire patch.
		- Used DIV_ROUND_UP() wherever possible in the entire
		  patch.
v3:
	- Addressed one comment from Matthew Wilcox.
	  	- In free_page_ext, page_pgdat() is now extracted
		  prior to freeing the memory.
v2:
	- Fixed the three bugs reported by kernel test robot.
	- Enhanced the commit message as recommended by David Hildenbrand.
	- Addressed comments from Matthew Wilcox:
	  	- Simplified alloc_vmemmap_page_list() and
		  free_page_ext() as recommended.
		- Used the appropriate comment style in mm/vmstat.c.
		- Replaced writeout_early_perpage_metadata() with
		  store_early_perpage_metadata() to reduce ambiguity
		  with what swap does.
	- Addressed comments from Mike Rapoport:
	  	- Simplified the loop in alloc_vmemmap_page_list().
		- Could NOT address a comment to move
		  store_early_perpage_metadata() near where nodes
		  and page allocator are initialized.
		- Included the vmalloc()ed page_ext in accounting
		  within free_page_ext().
		- Made early_perpage_metadata[MAX_NUMNODES] static.


Previous approaches and discussions
-----------------------------------
V8:
https://lore.kernel.org/all/20240214225741.403783-1-souravpanda@google.com
V7:
https://lore.kernel.org/all/20240129224204.1812062-1-souravpanda@google.com
V6:
https://lore.kernel.org/all/20231205223118.3575485-1-souravpanda@google.com
v5:
https://lore.kernel.org/all/20231101230816.1459373-1-souravpanda@google.com
v4:
https://lore.kernel.org/all/20231031223846.827173-1-souravpanda@google.com
v3:
https://lore.kernel.org/all/20231031174459.459480-1-souravpanda@google.com
v2:
https://lore.kernel.org/all/20231018005548.3505662-1-souravpanda@google.com
v1:
https://lore.kernel.org/r/20230913173000.4016218-2-souravpanda@google.com

Hi!

This patch adds two new per-node fields, namely nr_memmap and
nr_memmap_boot to /sys/devices/system/node/nodeN/vmstat and a
global Memmap field to /proc/meminfo. This information can be
used by users to see how much memory is being used by per-page
metadata, which can vary depending on build configuration, machine
architecture, and system use.

Per-page metadata is the amount of memory that Linux needs in order to
manage memory at the page granularity. The majority of such memory is
used by "struct page" and "page_ext" data structures.


Background
----------

Kernel overhead observability is missing some of the largest
allocations during runtime, including vmemmap (struct pages) and
page_ext. This patch aims to address this problem by exporting a
new metric Memmap.

On the contrary, the kernel does provide observibility for boot memory
allocations. For example, the metric reserved_pages depicts the pages
allocated by the bootmem allocator. This can be simply calculated as
present_pages - managed_pages, which are both exported in /proc/zoneinfo.
The metric reserved_pages is primarily composed of struct pages and
page_ext.

What about the struct pages (allocated by bootmem allocator) that are
free'd during hugetlbfs allocations and then allocated by buddy-allocator
once hugtlbfs pages are free'd?

/proc/meminfo MemTotal changes: MemTotal does not include memblock
allocations but includes buddy allocations. However, during runtime
memblock allocations can be shifted into buddy allocations, and therefore
become part of MemTotal.

Once the struct pages get allocated by buddy allocator, we lose track of
these struct page allocations overhead accounting. Therefore, we must
export new metrics. nr_memmap and nr_memmap_boot exclusively track the
struct page and page ext allocations made by the buddy allocator and
memblock allocator, respectively for each node. Memmap in /proc/meminfo
would report the struct page and page ext allocations made by the buddy
allocator.

Results and analysis
--------------------

Memory model: Sparsemem-vmemmap
$ echo 1 > /proc/sys/vm/hugetlb_optimize_vmemmap

$ cat /proc/meminfo | grep MemTotal
	MemTotal:       32919124 kB
$ cat /proc/meminfo | grep Memmap
	Memmap:		   65536 kB
$ cat /sys/devices/system/node/node0/vmstat | grep "nr_memmap"
	nr_memmap 8192
	nr_memmap_boot 65536
$ cat /sys/devices/system/node/node1/vmstat | grep "nr_memmap"
        nr_memmap 8192
	nr_memmap_boot 65536

AFTER HUGTLBFS RESERVATION
$ echo 512 > /proc/sys/vm/nr_hugepages

$ cat /proc/meminfo | grep MemTotal
	MemTotal:       32935508 kB
$ cat /proc/meminfo | grep Memmap
	Memmap:      67584 kB
$ cat /sys/devices/system/node/node0/vmstat | grep "nr_memmap"
	nr_memmap 8448
	nr_memmap_boot  63488
$ cat /sys/devices/system/node/node0/vmstat | grep "nr_memmap"
	nr_memmap 8448
	nr_memmap_boot  63488


AFTER FREEING HUGTLBFS RESERVATION
$ echo 0 > /proc/sys/vm/nr_hugepages

$ cat /proc/meminfo | grep MemTotal
        MemTotal:       32935508 kB
$ cat /proc/meminfo | grep Memmap
        Memmap:      81920 kB
$ cat /sys/devices/system/node/node0/vmstat | grep "nr_memmap"
        nr_memmap 10240
	nr_memmap_boot  63488
$ cat /sys/devices/system/node/node0/vmstat | grep "nr_memmap"
        nr_memmap 10240
	nr_memmap_boot  63488

------------------------

Memory Hotplug with memory_hotplug.memmap_on_memory=1

BEFORE HOTADD
$ cat /proc/meminfo | grep MemTotal
	MemTotal:       32919124 kB
$ cat /proc/meminfo | grep Memmap
	Memmap:      65536 kB
$ cat /sys/devices/system/node/node0/vmstat | grep "nr_memmap"
	nr_memmap 8192
	nr_memmap_boot  65536
$ cat /sys/devices/system/node/node0/vmstat | grep "nr_memmap"
	nr_memmap 8192
	nr_memmap_boot  65536

AFTER HOTADDING 1GB
$ cat /proc/meminfo | grep MemTotal
	MemTotal:       33951316 kB
$ cat /proc/meminfo | grep Memmap
	Memmap:      83968 kB
$ cat /sys/devices/system/node/node0/vmstat | grep "nr_memmap"
	nr_memmap 12800
	nr_memmap_boot 65536
$ cat /sys/devices/system/node/node0/vmstat | grep "nr_memmap"
	nr_memmap 8192
	nr_memmap_boot 65536

--------------------------

Memory Hotplug with memory_hotplug.memmap_on_memory=0

BEFORE HOTADD
$ cat /proc/meminfo | grep MemTotal
	MemTotal:       32919124 kB
$ cat /proc/meminfo | grep Memmap
	Memmap:      65536 kB
$ cat /sys/devices/system/node/node0/vmstat | grep "nr_memmap"
	nr_memmap 8192
	nr_memmap_boot 65536
$ cat /sys/devices/system/node/node0/vmstat | grep "nr_memmap"
	nr_memmap 8192
	nr_memmap_boot 65536

AFTER HOTADDING 1GB
$ cat /proc/meminfo | grep MemTotal
	MemTotal:       33967700 kB
$ cat /proc/meminfo | grep Memmap
	Memmap:      83968 kB
$ cat /sys/devices/system/node/node0/vmstat | grep "nr_memmap"
	nr_memmap 12800
	nr_memmap_boot 65536
$ cat /sys/devices/system/node/node0/vmstat | grep "nr_memmap"
	nr_memmap 8192
	nr_memmap_boot 65536

Sourav Panda (1):
  mm: report per-page metadata information

 Documentation/filesystems/proc.rst |  3 +++
 fs/proc/meminfo.c                  |  4 ++++
 include/linux/mmzone.h             |  4 ++++
 include/linux/vmstat.h             |  4 ++++
 mm/hugetlb_vmemmap.c               | 17 ++++++++++++----
 mm/mm_init.c                       |  3 +++
 mm/page_alloc.c                    |  1 +
 mm/page_ext.c                      | 32 +++++++++++++++++++++---------
 mm/sparse-vmemmap.c                |  8 ++++++++
 mm/sparse.c                        |  7 ++++++-
 mm/vmstat.c                        | 26 +++++++++++++++++++++++-
 11 files changed, 94 insertions(+), 15 deletions(-)

-- 
2.44.0.rc0.258.g7320e95886-goog

[PATCH v9 1/1] mm: report per-page metadata information

[Sourav Panda]

Adds two new per-node fields, namely nr_memmap and nr_memmap_boot,
to /sys/devices/system/node/nodeN/vmstat and a global Memmap field
to /proc/meminfo. This information can be used by users to see how
much memory is being used by per-page metadata, which can vary
depending on build configuration, machine architecture, and system
use.

Per-page metadata is the amount of memory that Linux needs in order to
manage memory at the page granularity. The majority of such memory is
used by "struct page" and "page_ext" data structures. In contrast to
most other memory consumption statistics, per-page metadata might not
be included in MemTotal. For example, MemTotal does not include memblock
allocations but includes buddy allocations. In this patch, exported
field nr_memmap in /sys/devices/system/node/nodeN/vmstat would
exclusively track buddy allocations while nr_memmap_boot would
exclusively track memblock allocations. Furthermore, Memmap in
/proc/meminfo would exclusively track buddy allocations allowing it to
be compared against MemTotal.

This memory depends on build configurations, machine architectures, and
the way system is used:

Build configuration may include extra fields into "struct page",
and enable / disable "page_ext"
Machine architecture defines base page sizes. For example 4K x86,
8K SPARC, 64K ARM64 (optionally), etc. The per-page metadata
overhead is smaller on machines with larger page sizes.
System use can change per-page overhead by using vmemmap
optimizations with hugetlb pages, and emulated pmem devdax pages.
Also, boot parameters can determine whether page_ext is needed
to be allocated. This memory can be part of MemTotal or be outside
MemTotal depending on whether the memory was hot-plugged, booted with,
or hugetlb memory was returned back to the system.

Utility for userspace:

Application Optimization: Depending on the kernel version and command
line options, the kernel would relinquish a different number of pages
(that contain struct pages) when a hugetlb page is reserved (e.g., 0, 6
or 7 for a 2MB hugepage). The userspace application would want to know
the exact savings achieved through page metadata deallocation without
dealing with the intricacies of the kernel.

Observability: Struct page overhead can only be calculated on-paper at
boot time (e.g., 1.5% machine capacity). Beyond boot once hugepages are
reserved or memory is hotplugged, the computation becomes complex.
Per-page metrics will help explain part of the system memory overhead,
which shall help guide memory optimizations and memory cgroup sizing.

Debugging: Tracking the changes or absolute value in struct pages can
help detect anomalies as they can be correlated with other metrics in
the machine (e.g., memtotal, number of huge pages, etc).

page_ext overheads: Some kernel features such as page_owner
page_table_check that use page_ext can be optionally enabled via kernel
parameters. Having the total per-page metadata information helps users
precisely measure impact.

Suggested-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Signed-off-by: Sourav Panda <souravpanda@google.com>
---
 Documentation/filesystems/proc.rst |  3 +++
 fs/proc/meminfo.c                  |  4 ++++
 include/linux/mmzone.h             |  4 ++++
 include/linux/vmstat.h             |  4 ++++
 mm/hugetlb_vmemmap.c               | 17 ++++++++++++----
 mm/mm_init.c                       |  3 +++
 mm/page_alloc.c                    |  1 +
 mm/page_ext.c                      | 32 +++++++++++++++++++++---------
 mm/sparse-vmemmap.c                |  8 ++++++++
 mm/sparse.c                        |  7 ++++++-
 mm/vmstat.c                        | 26 +++++++++++++++++++++++-
 11 files changed, 94 insertions(+), 15 deletions(-)

diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
index 104c6d047d9b..c9b4de65f162 100644
--- a/Documentation/filesystems/proc.rst
+++ b/Documentation/filesystems/proc.rst
@@ -993,6 +993,7 @@ Example output. You may not have all of these fields.
     AnonPages:       4654780 kB
     Mapped:           266244 kB
     Shmem:              9976 kB
+    Memmap:           513419 kB
     KReclaimable:     517708 kB
     Slab:             660044 kB
     SReclaimable:     517708 kB
@@ -1095,6 +1096,8 @@ Mapped
               files which have been mmapped, such as libraries
 Shmem
               Total memory used by shared memory (shmem) and tmpfs
+Memmap
+              Memory used for per-page metadata
 KReclaimable
               Kernel allocations that the kernel will attempt to reclaim
               under memory pressure. Includes SReclaimable (below), and other
diff --git a/fs/proc/meminfo.c b/fs/proc/meminfo.c
index 45af9a989d40..3d3db55cfeab 100644
--- a/fs/proc/meminfo.c
+++ b/fs/proc/meminfo.c
@@ -39,6 +39,7 @@ static int meminfo_proc_show(struct seq_file *m, void *v)
 	long available;
 	unsigned long pages[NR_LRU_LISTS];
 	unsigned long sreclaimable, sunreclaim;
+	unsigned long nr_memmap;
 	int lru;
 
 	si_meminfo(&i);
@@ -57,6 +58,8 @@ static int meminfo_proc_show(struct seq_file *m, void *v)
 	sreclaimable = global_node_page_state_pages(NR_SLAB_RECLAIMABLE_B);
 	sunreclaim = global_node_page_state_pages(NR_SLAB_UNRECLAIMABLE_B);
 
+	nr_memmap = global_node_page_state_pages(NR_MEMMAP);
+
 	show_val_kb(m, "MemTotal:       ", i.totalram);
 	show_val_kb(m, "MemFree:        ", i.freeram);
 	show_val_kb(m, "MemAvailable:   ", available);
@@ -104,6 +107,7 @@ static int meminfo_proc_show(struct seq_file *m, void *v)
 	show_val_kb(m, "Mapped:         ",
 		    global_node_page_state(NR_FILE_MAPPED));
 	show_val_kb(m, "Shmem:          ", i.sharedram);
+	show_val_kb(m, "Memmap:         ", nr_memmap);
 	show_val_kb(m, "KReclaimable:   ", sreclaimable +
 		    global_node_page_state(NR_KERNEL_MISC_RECLAIMABLE));
 	show_val_kb(m, "Slab:           ", sreclaimable + sunreclaim);
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index a497f189d988..59b244092325 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -214,6 +214,10 @@ enum node_stat_item {
 	PGDEMOTE_KSWAPD,
 	PGDEMOTE_DIRECT,
 	PGDEMOTE_KHUGEPAGED,
+	NR_MEMMAP,		/* Page metadata size (struct page and page_ext)
+				 * in pages
+				 */
+	NR_MEMMAP_BOOT,		/* NR_MEMMAP for bootmem */
 	NR_VM_NODE_STAT_ITEMS
 };
 
diff --git a/include/linux/vmstat.h b/include/linux/vmstat.h
index 343906a98d6e..c3785fdd3668 100644
--- a/include/linux/vmstat.h
+++ b/include/linux/vmstat.h
@@ -632,4 +632,8 @@ static inline void lruvec_stat_sub_folio(struct folio *folio,
 {
 	lruvec_stat_mod_folio(folio, idx, -folio_nr_pages(folio));
 }
+
+void __meminit mod_node_early_perpage_metadata(int nid, long delta);
+void __meminit store_early_perpage_metadata(void);
+
 #endif /* _LINUX_VMSTAT_H */
diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index da177e49d956..2da8689aeb93 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -184,10 +184,13 @@ static int vmemmap_remap_range(unsigned long start, unsigned long end,
  */
 static inline void free_vmemmap_page(struct page *page)
 {
-	if (PageReserved(page))
+	if (PageReserved(page)) {
 		free_bootmem_page(page);
-	else
+		mod_node_page_state(page_pgdat(page), NR_MEMMAP_BOOT, -1);
+	} else {
 		__free_page(page);
+		mod_node_page_state(page_pgdat(page), NR_MEMMAP, -1);
+	}
 }
 
 /* Free a list of the vmemmap pages */
@@ -338,6 +341,7 @@ static int vmemmap_remap_free(unsigned long start, unsigned long end,
 		copy_page(page_to_virt(walk.reuse_page),
 			  (void *)walk.reuse_addr);
 		list_add(&walk.reuse_page->lru, vmemmap_pages);
+		mod_node_page_state(NODE_DATA(nid), NR_MEMMAP, 1);
 	}
 
 	/*
@@ -384,14 +388,19 @@ static int alloc_vmemmap_page_list(unsigned long start, unsigned long end,
 	unsigned long nr_pages = (end - start) >> PAGE_SHIFT;
 	int nid = page_to_nid((struct page *)start);
 	struct page *page, *next;
+	int i;
 
-	while (nr_pages--) {
+	for (i = 0; i < nr_pages; i++) {
 		page = alloc_pages_node(nid, gfp_mask, 0);
-		if (!page)
+		if (!page) {
+			mod_node_page_state(NODE_DATA(nid), NR_MEMMAP, i);
 			goto out;
+		}
 		list_add(&page->lru, list);
 	}
 
+	mod_node_page_state(NODE_DATA(nid), NR_MEMMAP, nr_pages);
+
 	return 0;
 out:
 	list_for_each_entry_safe(page, next, list, lru)
diff --git a/mm/mm_init.c b/mm/mm_init.c
index 2c19f5515e36..b61372431b7d 100644
--- a/mm/mm_init.c
+++ b/mm/mm_init.c
@@ -27,6 +27,7 @@
 #include <linux/swap.h>
 #include <linux/cma.h>
 #include <linux/crash_dump.h>
+#include <linux/vmstat.h>
 #include "internal.h"
 #include "slab.h"
 #include "shuffle.h"
@@ -1656,6 +1657,8 @@ static void __init alloc_node_mem_map(struct pglist_data *pgdat)
 		panic("Failed to allocate %ld bytes for node %d memory map\n",
 		      size, pgdat->node_id);
 	pgdat->node_mem_map = map + offset;
+	mod_node_early_perpage_metadata(pgdat->node_id,
+					DIV_ROUND_UP(size, PAGE_SIZE));
 	pr_debug("%s: node %d, pgdat %08lx, node_mem_map %08lx\n",
 		 __func__, pgdat->node_id, (unsigned long)pgdat,
 		 (unsigned long)pgdat->node_mem_map);
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 150d4f23b010..236cfdf5a8fa 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5635,6 +5635,7 @@ void __init setup_per_cpu_pageset(void)
 	for_each_online_pgdat(pgdat)
 		pgdat->per_cpu_nodestats =
 			alloc_percpu(struct per_cpu_nodestat);
+	store_early_perpage_metadata();
 }
 
 __meminit void zone_pcp_init(struct zone *zone)
diff --git a/mm/page_ext.c b/mm/page_ext.c
index 4548fcc66d74..c1e324a1427e 100644
--- a/mm/page_ext.c
+++ b/mm/page_ext.c
@@ -201,6 +201,8 @@ static int __init alloc_node_page_ext(int nid)
 		return -ENOMEM;
 	NODE_DATA(nid)->node_page_ext = base;
 	total_usage += table_size;
+	mod_node_page_state(NODE_DATA(nid), NR_MEMMAP_BOOT,
+			    DIV_ROUND_UP(table_size, PAGE_SIZE));
 	return 0;
 }
 
@@ -255,12 +257,15 @@ static void *__meminit alloc_page_ext(size_t size, int nid)
 	void *addr = NULL;
 
 	addr = alloc_pages_exact_nid(nid, size, flags);
-	if (addr) {
+	if (addr)
 		kmemleak_alloc(addr, size, 1, flags);
-		return addr;
-	}
+	else
+		addr = vzalloc_node(size, nid);
 
-	addr = vzalloc_node(size, nid);
+	if (addr) {
+		mod_node_page_state(NODE_DATA(nid), NR_MEMMAP,
+				    DIV_ROUND_UP(size, PAGE_SIZE));
+	}
 
 	return addr;
 }
@@ -303,18 +308,27 @@ static int __meminit init_section_page_ext(unsigned long pfn, int nid)
 
 static void free_page_ext(void *addr)
 {
+	size_t table_size;
+	struct page *page;
+	struct pglist_data *pgdat;
+
+	table_size = page_ext_size * PAGES_PER_SECTION;
+
 	if (is_vmalloc_addr(addr)) {
+		page = vmalloc_to_page(addr);
+		pgdat = page_pgdat(page);
 		vfree(addr);
 	} else {
-		struct page *page = virt_to_page(addr);
-		size_t table_size;
-
-		table_size = page_ext_size * PAGES_PER_SECTION;
-
+		page = virt_to_page(addr);
+		pgdat = page_pgdat(page);
 		BUG_ON(PageReserved(page));
 		kmemleak_free(addr);
 		free_pages_exact(addr, table_size);
 	}
+
+	mod_node_page_state(pgdat, NR_MEMMAP,
+			    -1L * (DIV_ROUND_UP(table_size, PAGE_SIZE)));
+
 }
 
 static void __free_page_ext(unsigned long pfn)
diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
index a2cbe44c48e1..1dda6c53370b 100644
--- a/mm/sparse-vmemmap.c
+++ b/mm/sparse-vmemmap.c
@@ -469,5 +469,13 @@ struct page * __meminit __populate_section_memmap(unsigned long pfn,
 	if (r < 0)
 		return NULL;
 
+	if (system_state == SYSTEM_BOOTING) {
+		mod_node_early_perpage_metadata(nid, DIV_ROUND_UP(end - start,
+								  PAGE_SIZE));
+	} else {
+		mod_node_page_state(NODE_DATA(nid), NR_MEMMAP,
+				    DIV_ROUND_UP(end - start, PAGE_SIZE));
+	}
+
 	return pfn_to_page(pfn);
 }
diff --git a/mm/sparse.c b/mm/sparse.c
index 338cf946dee8..eb2aeb4e226b 100644
--- a/mm/sparse.c
+++ b/mm/sparse.c
@@ -14,7 +14,7 @@
 #include <linux/swap.h>
 #include <linux/swapops.h>
 #include <linux/bootmem_info.h>
-
+#include <linux/vmstat.h>
 #include "internal.h"
 #include <asm/dma.h>
 
@@ -465,6 +465,9 @@ static void __init sparse_buffer_init(unsigned long size, int nid)
 	 */
 	sparsemap_buf = memmap_alloc(size, section_map_size(), addr, nid, true);
 	sparsemap_buf_end = sparsemap_buf + size;
+#ifndef CONFIG_SPARSEMEM_VMEMMAP
+	mod_node_early_perpage_metadata(nid, DIV_ROUND_UP(size, PAGE_SIZE));
+#endif
 }
 
 static void __init sparse_buffer_fini(void)
@@ -641,6 +644,8 @@ static void depopulate_section_memmap(unsigned long pfn, unsigned long nr_pages,
 	unsigned long start = (unsigned long) pfn_to_page(pfn);
 	unsigned long end = start + nr_pages * sizeof(struct page);
 
+	mod_node_page_state(page_pgdat(pfn_to_page(pfn)), NR_MEMMAP,
+			    -1L * (DIV_ROUND_UP(end - start, PAGE_SIZE)));
 	vmemmap_free(start, end, altmap);
 }
 static void free_map_bootmem(struct page *memmap)
diff --git a/mm/vmstat.c b/mm/vmstat.c
index db79935e4a54..79466450040e 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1252,7 +1252,8 @@ const char * const vmstat_text[] = {
 	"pgdemote_kswapd",
 	"pgdemote_direct",
 	"pgdemote_khugepaged",
-
+	"nr_memmap",
+	"nr_memmap_boot",
 	/* enum writeback_stat_item counters */
 	"nr_dirty_threshold",
 	"nr_dirty_background_threshold",
@@ -2279,4 +2280,27 @@ static int __init extfrag_debug_init(void)
 }
 
 module_init(extfrag_debug_init);
+
 #endif
+
+/*
+ * Page metadata size (struct page and page_ext) in pages
+ */
+static unsigned long early_perpage_metadata[MAX_NUMNODES] __meminitdata;
+
+void __meminit mod_node_early_perpage_metadata(int nid, long delta)
+{
+	early_perpage_metadata[nid] += delta;
+}
+
+void __meminit store_early_perpage_metadata(void)
+{
+	int nid;
+	struct pglist_data *pgdat;
+
+	for_each_online_pgdat(pgdat) {
+		nid = pgdat->node_id;
+		mod_node_page_state(NODE_DATA(nid), NR_MEMMAP_BOOT,
+				    early_perpage_metadata[nid]);
+	}
+}
-- 
2.44.0.rc0.258.g7320e95886-goog

Re: [PATCH v9 1/1] mm: report per-page metadata information

[Wei Xu]

On Tue, Feb 20, 2024 at 1:46 PM Sourav Panda <souravpanda@google.com> wrote:
>
> Adds two new per-node fields, namely nr_memmap and nr_memmap_boot,
> to /sys/devices/system/node/nodeN/vmstat and a global Memmap field
> to /proc/meminfo. This information can be used by users to see how
> much memory is being used by per-page metadata, which can vary
> depending on build configuration, machine architecture, and system
> use.

/proc/vmstat also has the system-wide nr_memmap and nr_memmap_boot.
Given that nr_memmap in /proc/vmstat provides the same info (in
different units) as Memmap in /proc/meminfo, it would be better to
remove Memmap from /proc/meminfo to avoid duplication and confusion.

> Per-page metadata is the amount of memory that Linux needs in order to
> manage memory at the page granularity. The majority of such memory is
> used by "struct page" and "page_ext" data structures. In contrast to
> most other memory consumption statistics, per-page metadata might not
> be included in MemTotal. For example, MemTotal does not include memblock
> allocations but includes buddy allocations. In this patch, exported
> field nr_memmap in /sys/devices/system/node/nodeN/vmstat would
> exclusively track buddy allocations while nr_memmap_boot would
> exclusively track memblock allocations. Furthermore, Memmap in
> /proc/meminfo would exclusively track buddy allocations allowing it to
> be compared against MemTotal.
>
> This memory depends on build configurations, machine architectures, and
> the way system is used:
>
> Build configuration may include extra fields into "struct page",
> and enable / disable "page_ext"
> Machine architecture defines base page sizes. For example 4K x86,
> 8K SPARC, 64K ARM64 (optionally), etc. The per-page metadata
> overhead is smaller on machines with larger page sizes.
> System use can change per-page overhead by using vmemmap
> optimizations with hugetlb pages, and emulated pmem devdax pages.
> Also, boot parameters can determine whether page_ext is needed
> to be allocated. This memory can be part of MemTotal or be outside
> MemTotal depending on whether the memory was hot-plugged, booted with,
> or hugetlb memory was returned back to the system.
>
> Utility for userspace:
>
> Application Optimization: Depending on the kernel version and command
> line options, the kernel would relinquish a different number of pages
> (that contain struct pages) when a hugetlb page is reserved (e.g., 0, 6
> or 7 for a 2MB hugepage). The userspace application would want to know
> the exact savings achieved through page metadata deallocation without
> dealing with the intricacies of the kernel.
>
> Observability: Struct page overhead can only be calculated on-paper at
> boot time (e.g., 1.5% machine capacity). Beyond boot once hugepages are
> reserved or memory is hotplugged, the computation becomes complex.
> Per-page metrics will help explain part of the system memory overhead,
> which shall help guide memory optimizations and memory cgroup sizing.
>
> Debugging: Tracking the changes or absolute value in struct pages can
> help detect anomalies as they can be correlated with other metrics in
> the machine (e.g., memtotal, number of huge pages, etc).
>
> page_ext overheads: Some kernel features such as page_owner
> page_table_check that use page_ext can be optionally enabled via kernel
> parameters. Having the total per-page metadata information helps users
> precisely measure impact.
>
> Suggested-by: Pasha Tatashin <pasha.tatashin@soleen.com>
> Signed-off-by: Sourav Panda <souravpanda@google.com>
> ---
>  Documentation/filesystems/proc.rst |  3 +++
>  fs/proc/meminfo.c                  |  4 ++++
>  include/linux/mmzone.h             |  4 ++++
>  include/linux/vmstat.h             |  4 ++++
>  mm/hugetlb_vmemmap.c               | 17 ++++++++++++----
>  mm/mm_init.c                       |  3 +++
>  mm/page_alloc.c                    |  1 +
>  mm/page_ext.c                      | 32 +++++++++++++++++++++---------
>  mm/sparse-vmemmap.c                |  8 ++++++++
>  mm/sparse.c                        |  7 ++++++-
>  mm/vmstat.c                        | 26 +++++++++++++++++++++++-
>  11 files changed, 94 insertions(+), 15 deletions(-)
>
> diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
> index 104c6d047d9b..c9b4de65f162 100644
> --- a/Documentation/filesystems/proc.rst
> +++ b/Documentation/filesystems/proc.rst
> @@ -993,6 +993,7 @@ Example output. You may not have all of these fields.
>      AnonPages:       4654780 kB
>      Mapped:           266244 kB
>      Shmem:              9976 kB
> +    Memmap:           513419 kB
>      KReclaimable:     517708 kB
>      Slab:             660044 kB
>      SReclaimable:     517708 kB
> @@ -1095,6 +1096,8 @@ Mapped
>                files which have been mmapped, such as libraries
>  Shmem
>                Total memory used by shared memory (shmem) and tmpfs
> +Memmap
> +              Memory used for per-page metadata
>  KReclaimable
>                Kernel allocations that the kernel will attempt to reclaim
>                under memory pressure. Includes SReclaimable (below), and other
> diff --git a/fs/proc/meminfo.c b/fs/proc/meminfo.c
> index 45af9a989d40..3d3db55cfeab 100644
> --- a/fs/proc/meminfo.c
> +++ b/fs/proc/meminfo.c
> @@ -39,6 +39,7 @@ static int meminfo_proc_show(struct seq_file *m, void *v)
>         long available;
>         unsigned long pages[NR_LRU_LISTS];
>         unsigned long sreclaimable, sunreclaim;
> +       unsigned long nr_memmap;
>         int lru;
>
>         si_meminfo(&i);
> @@ -57,6 +58,8 @@ static int meminfo_proc_show(struct seq_file *m, void *v)
>         sreclaimable = global_node_page_state_pages(NR_SLAB_RECLAIMABLE_B);
>         sunreclaim = global_node_page_state_pages(NR_SLAB_UNRECLAIMABLE_B);
>
> +       nr_memmap = global_node_page_state_pages(NR_MEMMAP);
> +
>         show_val_kb(m, "MemTotal:       ", i.totalram);
>         show_val_kb(m, "MemFree:        ", i.freeram);
>         show_val_kb(m, "MemAvailable:   ", available);
> @@ -104,6 +107,7 @@ static int meminfo_proc_show(struct seq_file *m, void *v)
>         show_val_kb(m, "Mapped:         ",
>                     global_node_page_state(NR_FILE_MAPPED));
>         show_val_kb(m, "Shmem:          ", i.sharedram);
> +       show_val_kb(m, "Memmap:         ", nr_memmap);
>         show_val_kb(m, "KReclaimable:   ", sreclaimable +
>                     global_node_page_state(NR_KERNEL_MISC_RECLAIMABLE));
>         show_val_kb(m, "Slab:           ", sreclaimable + sunreclaim);
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index a497f189d988..59b244092325 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -214,6 +214,10 @@ enum node_stat_item {
>         PGDEMOTE_KSWAPD,
>         PGDEMOTE_DIRECT,
>         PGDEMOTE_KHUGEPAGED,
> +       NR_MEMMAP,              /* Page metadata size (struct page and page_ext)
> +                                * in pages
> +                                */
> +       NR_MEMMAP_BOOT,         /* NR_MEMMAP for bootmem */
>         NR_VM_NODE_STAT_ITEMS
>  };
>
> diff --git a/include/linux/vmstat.h b/include/linux/vmstat.h
> index 343906a98d6e..c3785fdd3668 100644
> --- a/include/linux/vmstat.h
> +++ b/include/linux/vmstat.h
> @@ -632,4 +632,8 @@ static inline void lruvec_stat_sub_folio(struct folio *folio,
>  {
>         lruvec_stat_mod_folio(folio, idx, -folio_nr_pages(folio));
>  }
> +
> +void __meminit mod_node_early_perpage_metadata(int nid, long delta);
> +void __meminit store_early_perpage_metadata(void);
> +
>  #endif /* _LINUX_VMSTAT_H */
> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> index da177e49d956..2da8689aeb93 100644
> --- a/mm/hugetlb_vmemmap.c
> +++ b/mm/hugetlb_vmemmap.c
> @@ -184,10 +184,13 @@ static int vmemmap_remap_range(unsigned long start, unsigned long end,
>   */
>  static inline void free_vmemmap_page(struct page *page)
>  {
> -       if (PageReserved(page))
> +       if (PageReserved(page)) {
>                 free_bootmem_page(page);
> -       else
> +               mod_node_page_state(page_pgdat(page), NR_MEMMAP_BOOT, -1);
> +       } else {
>                 __free_page(page);
> +               mod_node_page_state(page_pgdat(page), NR_MEMMAP, -1);
> +       }
>  }
>
>  /* Free a list of the vmemmap pages */
> @@ -338,6 +341,7 @@ static int vmemmap_remap_free(unsigned long start, unsigned long end,
>                 copy_page(page_to_virt(walk.reuse_page),
>                           (void *)walk.reuse_addr);
>                 list_add(&walk.reuse_page->lru, vmemmap_pages);
> +               mod_node_page_state(NODE_DATA(nid), NR_MEMMAP, 1);
>         }
>
>         /*
> @@ -384,14 +388,19 @@ static int alloc_vmemmap_page_list(unsigned long start, unsigned long end,
>         unsigned long nr_pages = (end - start) >> PAGE_SHIFT;
>         int nid = page_to_nid((struct page *)start);
>         struct page *page, *next;
> +       int i;
>
> -       while (nr_pages--) {
> +       for (i = 0; i < nr_pages; i++) {
>                 page = alloc_pages_node(nid, gfp_mask, 0);
> -               if (!page)
> +               if (!page) {
> +                       mod_node_page_state(NODE_DATA(nid), NR_MEMMAP, i);
>                         goto out;
> +               }
>                 list_add(&page->lru, list);
>         }
>
> +       mod_node_page_state(NODE_DATA(nid), NR_MEMMAP, nr_pages);
> +
>         return 0;
>  out:
>         list_for_each_entry_safe(page, next, list, lru)
> diff --git a/mm/mm_init.c b/mm/mm_init.c
> index 2c19f5515e36..b61372431b7d 100644
> --- a/mm/mm_init.c
> +++ b/mm/mm_init.c
> @@ -27,6 +27,7 @@
>  #include <linux/swap.h>
>  #include <linux/cma.h>
>  #include <linux/crash_dump.h>
> +#include <linux/vmstat.h>
>  #include "internal.h"
>  #include "slab.h"
>  #include "shuffle.h"
> @@ -1656,6 +1657,8 @@ static void __init alloc_node_mem_map(struct pglist_data *pgdat)
>                 panic("Failed to allocate %ld bytes for node %d memory map\n",
>                       size, pgdat->node_id);
>         pgdat->node_mem_map = map + offset;
> +       mod_node_early_perpage_metadata(pgdat->node_id,
> +                                       DIV_ROUND_UP(size, PAGE_SIZE));
>         pr_debug("%s: node %d, pgdat %08lx, node_mem_map %08lx\n",
>                  __func__, pgdat->node_id, (unsigned long)pgdat,
>                  (unsigned long)pgdat->node_mem_map);
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 150d4f23b010..236cfdf5a8fa 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5635,6 +5635,7 @@ void __init setup_per_cpu_pageset(void)
>         for_each_online_pgdat(pgdat)
>                 pgdat->per_cpu_nodestats =
>                         alloc_percpu(struct per_cpu_nodestat);
> +       store_early_perpage_metadata();
>  }
>
>  __meminit void zone_pcp_init(struct zone *zone)
> diff --git a/mm/page_ext.c b/mm/page_ext.c
> index 4548fcc66d74..c1e324a1427e 100644
> --- a/mm/page_ext.c
> +++ b/mm/page_ext.c
> @@ -201,6 +201,8 @@ static int __init alloc_node_page_ext(int nid)
>                 return -ENOMEM;
>         NODE_DATA(nid)->node_page_ext = base;
>         total_usage += table_size;
> +       mod_node_page_state(NODE_DATA(nid), NR_MEMMAP_BOOT,
> +                           DIV_ROUND_UP(table_size, PAGE_SIZE));
>         return 0;
>  }
>
> @@ -255,12 +257,15 @@ static void *__meminit alloc_page_ext(size_t size, int nid)
>         void *addr = NULL;
>
>         addr = alloc_pages_exact_nid(nid, size, flags);
> -       if (addr) {
> +       if (addr)
>                 kmemleak_alloc(addr, size, 1, flags);
> -               return addr;
> -       }
> +       else
> +               addr = vzalloc_node(size, nid);
>
> -       addr = vzalloc_node(size, nid);
> +       if (addr) {
> +               mod_node_page_state(NODE_DATA(nid), NR_MEMMAP,
> +                                   DIV_ROUND_UP(size, PAGE_SIZE));
> +       }
>
>         return addr;
>  }
> @@ -303,18 +308,27 @@ static int __meminit init_section_page_ext(unsigned long pfn, int nid)
>
>  static void free_page_ext(void *addr)
>  {
> +       size_t table_size;
> +       struct page *page;
> +       struct pglist_data *pgdat;
> +
> +       table_size = page_ext_size * PAGES_PER_SECTION;
> +
>         if (is_vmalloc_addr(addr)) {
> +               page = vmalloc_to_page(addr);
> +               pgdat = page_pgdat(page);
>                 vfree(addr);
>         } else {
> -               struct page *page = virt_to_page(addr);
> -               size_t table_size;
> -
> -               table_size = page_ext_size * PAGES_PER_SECTION;
> -
> +               page = virt_to_page(addr);
> +               pgdat = page_pgdat(page);
>                 BUG_ON(PageReserved(page));
>                 kmemleak_free(addr);
>                 free_pages_exact(addr, table_size);
>         }
> +
> +       mod_node_page_state(pgdat, NR_MEMMAP,
> +                           -1L * (DIV_ROUND_UP(table_size, PAGE_SIZE)));
> +
>  }
>
>  static void __free_page_ext(unsigned long pfn)
> diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
> index a2cbe44c48e1..1dda6c53370b 100644
> --- a/mm/sparse-vmemmap.c
> +++ b/mm/sparse-vmemmap.c
> @@ -469,5 +469,13 @@ struct page * __meminit __populate_section_memmap(unsigned long pfn,
>         if (r < 0)
>                 return NULL;
>
> +       if (system_state == SYSTEM_BOOTING) {
> +               mod_node_early_perpage_metadata(nid, DIV_ROUND_UP(end - start,
> +                                                                 PAGE_SIZE));
> +       } else {
> +               mod_node_page_state(NODE_DATA(nid), NR_MEMMAP,
> +                                   DIV_ROUND_UP(end - start, PAGE_SIZE));
> +       }
> +
>         return pfn_to_page(pfn);
>  }
> diff --git a/mm/sparse.c b/mm/sparse.c
> index 338cf946dee8..eb2aeb4e226b 100644
> --- a/mm/sparse.c
> +++ b/mm/sparse.c
> @@ -14,7 +14,7 @@
>  #include <linux/swap.h>
>  #include <linux/swapops.h>
>  #include <linux/bootmem_info.h>
> -
> +#include <linux/vmstat.h>
>  #include "internal.h"
>  #include <asm/dma.h>
>
> @@ -465,6 +465,9 @@ static void __init sparse_buffer_init(unsigned long size, int nid)
>          */
>         sparsemap_buf = memmap_alloc(size, section_map_size(), addr, nid, true);
>         sparsemap_buf_end = sparsemap_buf + size;
> +#ifndef CONFIG_SPARSEMEM_VMEMMAP
> +       mod_node_early_perpage_metadata(nid, DIV_ROUND_UP(size, PAGE_SIZE));
> +#endif
>  }
>
>  static void __init sparse_buffer_fini(void)
> @@ -641,6 +644,8 @@ static void depopulate_section_memmap(unsigned long pfn, unsigned long nr_pages,
>         unsigned long start = (unsigned long) pfn_to_page(pfn);
>         unsigned long end = start + nr_pages * sizeof(struct page);
>
> +       mod_node_page_state(page_pgdat(pfn_to_page(pfn)), NR_MEMMAP,
> +                           -1L * (DIV_ROUND_UP(end - start, PAGE_SIZE)));
>         vmemmap_free(start, end, altmap);
>  }
>  static void free_map_bootmem(struct page *memmap)
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index db79935e4a54..79466450040e 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -1252,7 +1252,8 @@ const char * const vmstat_text[] = {
>         "pgdemote_kswapd",
>         "pgdemote_direct",
>         "pgdemote_khugepaged",
> -
> +       "nr_memmap",
> +       "nr_memmap_boot",
>         /* enum writeback_stat_item counters */
>         "nr_dirty_threshold",
>         "nr_dirty_background_threshold",
> @@ -2279,4 +2280,27 @@ static int __init extfrag_debug_init(void)
>  }
>
>  module_init(extfrag_debug_init);
> +
>  #endif
> +
> +/*
> + * Page metadata size (struct page and page_ext) in pages
> + */
> +static unsigned long early_perpage_metadata[MAX_NUMNODES] __meminitdata;
> +
> +void __meminit mod_node_early_perpage_metadata(int nid, long delta)
> +{
> +       early_perpage_metadata[nid] += delta;
> +}
> +
> +void __meminit store_early_perpage_metadata(void)
> +{
> +       int nid;
> +       struct pglist_data *pgdat;
> +
> +       for_each_online_pgdat(pgdat) {
> +               nid = pgdat->node_id;
> +               mod_node_page_state(NODE_DATA(nid), NR_MEMMAP_BOOT,
> +                                   early_perpage_metadata[nid]);
> +       }
> +}
> --
> 2.44.0.rc0.258.g7320e95886-goog
>

Re: [PATCH v9 1/1] mm: report per-page metadata information

[Pasha Tatashin]

On Mon, Feb 26, 2024 at 1:12 PM Wei Xu <weixugc@google.com> wrote:
>
> On Tue, Feb 20, 2024 at 1:46 PM Sourav Panda <souravpanda@google.com> wrote:
> >
> > Adds two new per-node fields, namely nr_memmap and nr_memmap_boot,
> > to /sys/devices/system/node/nodeN/vmstat and a global Memmap field
> > to /proc/meminfo. This information can be used by users to see how
> > much memory is being used by per-page metadata, which can vary
> > depending on build configuration, machine architecture, and system
> > use.
>
> /proc/vmstat also has the system-wide nr_memmap and nr_memmap_boot.
> Given that nr_memmap in /proc/vmstat provides the same info (in
> different units) as Memmap in /proc/meminfo, it would be better to
> remove Memmap from /proc/meminfo to avoid duplication and confusion.

There are many items both in meminfo and in vmstat. Given that
/proc/meminfo covers all kmem memory, it is beneficial to keep the
kmem part of memmap in meminfo as another classification item.

Pasha

Re: [PATCH v9 1/1] mm: report per-page metadata information

[Pasha Tatashin]

On Tue, Feb 20, 2024 at 4:46 PM Sourav Panda <souravpanda@google.com> wrote:
>
> Adds two new per-node fields, namely nr_memmap and nr_memmap_boot,
> to /sys/devices/system/node/nodeN/vmstat and a global Memmap field
> to /proc/meminfo. This information can be used by users to see how
> much memory is being used by per-page metadata, which can vary
> depending on build configuration, machine architecture, and system
> use.
>
> Per-page metadata is the amount of memory that Linux needs in order to
> manage memory at the page granularity. The majority of such memory is
> used by "struct page" and "page_ext" data structures. In contrast to
> most other memory consumption statistics, per-page metadata might not
> be included in MemTotal. For example, MemTotal does not include memblock
> allocations but includes buddy allocations. In this patch, exported
> field nr_memmap in /sys/devices/system/node/nodeN/vmstat would
> exclusively track buddy allocations while nr_memmap_boot would
> exclusively track memblock allocations. Furthermore, Memmap in
> /proc/meminfo would exclusively track buddy allocations allowing it to
> be compared against MemTotal.
>
> This memory depends on build configurations, machine architectures, and
> the way system is used:
>
> Build configuration may include extra fields into "struct page",
> and enable / disable "page_ext"
> Machine architecture defines base page sizes. For example 4K x86,
> 8K SPARC, 64K ARM64 (optionally), etc. The per-page metadata
> overhead is smaller on machines with larger page sizes.
> System use can change per-page overhead by using vmemmap
> optimizations with hugetlb pages, and emulated pmem devdax pages.
> Also, boot parameters can determine whether page_ext is needed
> to be allocated. This memory can be part of MemTotal or be outside
> MemTotal depending on whether the memory was hot-plugged, booted with,
> or hugetlb memory was returned back to the system.
>
> Utility for userspace:
>
> Application Optimization: Depending on the kernel version and command
> line options, the kernel would relinquish a different number of pages
> (that contain struct pages) when a hugetlb page is reserved (e.g., 0, 6
> or 7 for a 2MB hugepage). The userspace application would want to know
> the exact savings achieved through page metadata deallocation without
> dealing with the intricacies of the kernel.
>
> Observability: Struct page overhead can only be calculated on-paper at
> boot time (e.g., 1.5% machine capacity). Beyond boot once hugepages are
> reserved or memory is hotplugged, the computation becomes complex.
> Per-page metrics will help explain part of the system memory overhead,
> which shall help guide memory optimizations and memory cgroup sizing.
>
> Debugging: Tracking the changes or absolute value in struct pages can
> help detect anomalies as they can be correlated with other metrics in
> the machine (e.g., memtotal, number of huge pages, etc).
>
> page_ext overheads: Some kernel features such as page_owner
> page_table_check that use page_ext can be optionally enabled via kernel
> parameters. Having the total per-page metadata information helps users
> precisely measure impact.
>
> Suggested-by: Pasha Tatashin <pasha.tatashin@soleen.com>
> Signed-off-by: Sourav Panda <souravpanda@google.com>

Reviewed-by: Pasha Tatashin <pasha.tatashin@soleen.com>

Re: [PATCH v9 1/1] mm: report per-page metadata information

[Pasha Tatashin]

On Wed, Mar 13, 2024 at 6:40 PM Pasha Tatashin
<pasha.tatashin@soleen.com> wrote:
>
> On Tue, Feb 20, 2024 at 4:46 PM Sourav Panda <souravpanda@google.com> wrote:
> >
> > Adds two new per-node fields, namely nr_memmap and nr_memmap_boot,
> > to /sys/devices/system/node/nodeN/vmstat and a global Memmap field
> > to /proc/meminfo. This information can be used by users to see how
> > much memory is being used by per-page metadata, which can vary
> > depending on build configuration, machine architecture, and system
> > use.
> >
> > Per-page metadata is the amount of memory that Linux needs in order to
> > manage memory at the page granularity. The majority of such memory is
> > used by "struct page" and "page_ext" data structures. In contrast to
> > most other memory consumption statistics, per-page metadata might not
> > be included in MemTotal. For example, MemTotal does not include memblock
> > allocations but includes buddy allocations. In this patch, exported
> > field nr_memmap in /sys/devices/system/node/nodeN/vmstat would
> > exclusively track buddy allocations while nr_memmap_boot would
> > exclusively track memblock allocations. Furthermore, Memmap in
> > /proc/meminfo would exclusively track buddy allocations allowing it to
> > be compared against MemTotal.
> >
> > This memory depends on build configurations, machine architectures, and
> > the way system is used:
> >
> > Build configuration may include extra fields into "struct page",
> > and enable / disable "page_ext"
> > Machine architecture defines base page sizes. For example 4K x86,
> > 8K SPARC, 64K ARM64 (optionally), etc. The per-page metadata
> > overhead is smaller on machines with larger page sizes.
> > System use can change per-page overhead by using vmemmap
> > optimizations with hugetlb pages, and emulated pmem devdax pages.
> > Also, boot parameters can determine whether page_ext is needed
> > to be allocated. This memory can be part of MemTotal or be outside
> > MemTotal depending on whether the memory was hot-plugged, booted with,
> > or hugetlb memory was returned back to the system.
> >
> > Utility for userspace:
> >
> > Application Optimization: Depending on the kernel version and command
> > line options, the kernel would relinquish a different number of pages
> > (that contain struct pages) when a hugetlb page is reserved (e.g., 0, 6
> > or 7 for a 2MB hugepage). The userspace application would want to know
> > the exact savings achieved through page metadata deallocation without
> > dealing with the intricacies of the kernel.
> >
> > Observability: Struct page overhead can only be calculated on-paper at
> > boot time (e.g., 1.5% machine capacity). Beyond boot once hugepages are
> > reserved or memory is hotplugged, the computation becomes complex.
> > Per-page metrics will help explain part of the system memory overhead,
> > which shall help guide memory optimizations and memory cgroup sizing.
> >
> > Debugging: Tracking the changes or absolute value in struct pages can
> > help detect anomalies as they can be correlated with other metrics in
> > the machine (e.g., memtotal, number of huge pages, etc).
> >
> > page_ext overheads: Some kernel features such as page_owner
> > page_table_check that use page_ext can be optionally enabled via kernel
> > parameters. Having the total per-page metadata information helps users
> > precisely measure impact.

Hi Andrew,

Can you please give this patch another look, does it require more
reviews before you can take it in?

Thank you,
Pasha

Re: [PATCH v9 1/1] mm: report per-page metadata information

[Andrew Morton]

On Tue, 20 Feb 2024 13:45:58 -0800 Sourav Panda <souravpanda@google.com> wrote:

> Adds two new per-node fields, namely nr_memmap and nr_memmap_boot,
> to /sys/devices/system/node/nodeN/vmstat and a global Memmap field
> to /proc/meminfo. This information can be used by users to see how
> much memory is being used by per-page metadata, which can vary
> depending on build configuration, machine architecture, and system
> use.

I yield to no man in my admiration of changelogging but boy, that's a
lot of changelogging.  Would it be possible to consolidate the [0/N]
coverletter and the [1/N] changelog into a single thing please?

>  Documentation/filesystems/proc.rst |  3 +++
>  fs/proc/meminfo.c                  |  4 ++++
>  include/linux/mmzone.h             |  4 ++++
>  include/linux/vmstat.h             |  4 ++++
>  mm/hugetlb_vmemmap.c               | 17 ++++++++++++----
>  mm/mm_init.c                       |  3 +++
>  mm/page_alloc.c                    |  1 +
>  mm/page_ext.c                      | 32 +++++++++++++++++++++---------
>  mm/sparse-vmemmap.c                |  8 ++++++++
>  mm/sparse.c                        |  7 ++++++-
>  mm/vmstat.c                        | 26 +++++++++++++++++++++++-
>  11 files changed, 94 insertions(+), 15 deletions(-)

And yet we offer the users basically no documentation.  The new sysfs
file should be documented under Documentation/ABI somewhere and
perhaps we could prepare some more expansive user-facing documentation
elsewhere?

I'd like to hear others' views on the overall usefulness/utility of this
change, please?

Re: [PATCH v9 1/1] mm: report per-page metadata information

[David Rientjes]

On Tue, 19 Mar 2024, Andrew Morton wrote:

> On Tue, 20 Feb 2024 13:45:58 -0800 Sourav Panda <souravpanda@google.com> wrote:
> 
> > Adds two new per-node fields, namely nr_memmap and nr_memmap_boot,
> > to /sys/devices/system/node/nodeN/vmstat and a global Memmap field
> > to /proc/meminfo. This information can be used by users to see how
> > much memory is being used by per-page metadata, which can vary
> > depending on build configuration, machine architecture, and system
> > use.
> 
> I yield to no man in my admiration of changelogging but boy, that's a
> lot of changelogging.  Would it be possible to consolidate the [0/N]
> coverletter and the [1/N] changelog into a single thing please?
> 
> >  Documentation/filesystems/proc.rst |  3 +++
> >  fs/proc/meminfo.c                  |  4 ++++
> >  include/linux/mmzone.h             |  4 ++++
> >  include/linux/vmstat.h             |  4 ++++
> >  mm/hugetlb_vmemmap.c               | 17 ++++++++++++----
> >  mm/mm_init.c                       |  3 +++
> >  mm/page_alloc.c                    |  1 +
> >  mm/page_ext.c                      | 32 +++++++++++++++++++++---------
> >  mm/sparse-vmemmap.c                |  8 ++++++++
> >  mm/sparse.c                        |  7 ++++++-
> >  mm/vmstat.c                        | 26 +++++++++++++++++++++++-
> >  11 files changed, 94 insertions(+), 15 deletions(-)
> 
> And yet we offer the users basically no documentation.  The new sysfs
> file should be documented under Documentation/ABI somewhere and
> perhaps we could prepare some more expansive user-facing documentation
> elsewhere?
> 

Sourav, is it possible to refresh this series into a v10 on top of the 
latest upstream kernel with a single condensed changelog that details the 
current behavior, what extension this is adding, and how it is generally 
useful?

As noted here, the cover letter has great material that discusses the 
rationale for this change but would be lost if only this patch is merged.  
So typically the cover letter material gets concatenated into the 
changelog, but in this case there's a lot of overlap.

A single patch that includes a succinct changelog would be awesome.

And then the requested documentation in Documentation/ABI either included 
in the same patch or as a second patch in the series?

I don't think the resulting patch series will actually need a cover letter 
after that, it will be able to stand on its own.

> I'd like to hear others' views on the overall usefulness/utility of this
> change, please?
> 

Likely true for all hyperscalers, the immediate use case that this could 
be applied to is to track boot memory overhead and any regression over 
time (across kernel upgrades, firmware upgrades, etc) that may change the 
amount of total memory available.  We'd want to subtract out the boot 
overhead that we know about (like struct page here) and then alert on any 
regression where we're losing memory from reboot to reboot for any reason.

This increased visibility into boot memory overhead allows us to create a 
mechanism to track changes over time when otherwise that attribution of 
that memory is not available.

Re: [PATCH v9 1/1] mm: report per-page metadata information

[Pasha Tatashin]

> >  Documentation/filesystems/proc.rst |  3 +++
> >  fs/proc/meminfo.c                  |  4 ++++
> >  include/linux/mmzone.h             |  4 ++++
> >  include/linux/vmstat.h             |  4 ++++
> >  mm/hugetlb_vmemmap.c               | 17 ++++++++++++----
> >  mm/mm_init.c                       |  3 +++
> >  mm/page_alloc.c                    |  1 +
> >  mm/page_ext.c                      | 32 +++++++++++++++++++++---------
> >  mm/sparse-vmemmap.c                |  8 ++++++++
> >  mm/sparse.c                        |  7 ++++++-
> >  mm/vmstat.c                        | 26 +++++++++++++++++++++++-
> >  11 files changed, 94 insertions(+), 15 deletions(-)
>
> And yet we offer the users basically no documentation.  The new sysfs
> file should be documented under Documentation/ABI somewhere and

There are no new sysfs files in this change. The new Memmap field in
/proc/meminfo is documented.

> perhaps we could prepare some more expansive user-facing documentation
> elsewhere?
>
> I'd like to hear others' views on the overall usefulness/utility of this
> change, please?

Sourav, could you please consolidate the cover letter and the patch
into one email, sync it with the upstream kernel, and send the new
version putting the necessary background information into the stat
area in the patch.

Thanks,
Pasha