[RFC 0/3] THP Shrinker

[RFC 0/3] THP Shrinker

[alexlzhu]

From: Alexander Zhu <alexlzhu@fb.com>

Transparent Hugepages use a larger page size of 2MB in comparison to
normal sized pages that are 4kb. A larger page size allows for fewer TLB
cache misses and thus more efficient use of the CPU. Using a larger page
size also results in more memory waste, which can hurt performance in some
use cases. THPs are currently enabled in the Linux Kernel by applications
in limited virtual address ranges via the madvise system call.  The THP
shrinker tries to find a balance between increased use of THPs, and
increased use of memory. It shrinks the size of memory by removing the
underutilized THPs that are identified by the thp_utilization scanner. 

In our experiments we have noticed that the least utilized THPs are almost
entirely unutilized.

Sample Output: 

Utilized[0-50]: 1331 680884
Utilized[51-101]: 9 3983
Utilized[102-152]: 3 1187
Utilized[153-203]: 0 0
Utilized[204-255]: 2 539
Utilized[256-306]: 5 1135
Utilized[307-357]: 1 192
Utilized[358-408]: 0 0
Utilized[409-459]: 1 57
Utilized[460-512]: 400 13
Last Scan Time: 223.98
Last Scan Duration: 70.65

Above is a sample obtained from one of our test machines when THP is always
enabled. Of the 1331 THPs in this thp_utilization sample that have from
0-50 utilized subpages, we see that there are 680884 free pages. This
comes out to 680884 / (512 * 1331) = 99.91% zero pages in the least
utilized bucket. This represents 680884 * 4KB = 2.7GB memory waste.

Also note that the vast majority of pages are either in the least utilized
[0-50] or most utilized [460-512] buckets. The least utilized THPs are 
responsible for almost all of the memory waste when THP is always 
enabled. Thus by clearing out THPs in the lowest utilization bucket
we extract most of the improvement in CPU efficiency. We have seen 
similar results on our production hosts.

This patchset introduces the THP shrinker we have developed to identify
and split the least utilized THPs. It includes the thp_utilization 
changes that groups anonymous THPs into buckets, the split_huge_page()
changes that identify and zap zero 4KB pages within THPs and the shrinker
changes. It should be noted that the split_huge_page() changes are based
off previous work done by Yu Zhao. 

In the future, we intend to allow additional tuning to the shrinker
based on workload depending on CPU/IO/Memory pressure and the 
amount of anonymous memory. The long term goal is to eventually always 
enable THP for all applications and deprecate madvise entirely.

Alexander Zhu (3):
  mm: add thp_utilization metrics to debugfs
  mm: changes to split_huge_page() to free zero filled tail pages
  mm: THP low utilization shrinker

 Documentation/admin-guide/mm/transhuge.rst    |   9 +
 include/linux/huge_mm.h                       |   9 +
 include/linux/list_lru.h                      |  24 ++
 include/linux/mm_types.h                      |   5 +
 include/linux/rmap.h                          |   2 +-
 include/linux/vm_event_item.h                 |   2 +
 mm/huge_memory.c                              | 333 +++++++++++++++++-
 mm/list_lru.c                                 |  49 +++
 mm/migrate.c                                  |  60 +++-
 mm/migrate_device.c                           |   4 +-
 mm/page_alloc.c                               |   6 +
 mm/vmstat.c                                   |   2 +
 .../selftests/vm/split_huge_page_test.c       |  58 ++-
 tools/testing/selftests/vm/vm_util.c          |  23 ++
 tools/testing/selftests/vm/vm_util.h          |   1 +
 15 files changed, 569 insertions(+), 18 deletions(-)

-- 
2.30.2

[RFC 1/3] mm: add thp_utilization metrics to debugfs

[alexlzhu]

From: Alexander Zhu <alexlzhu@fb.com>

This change introduces a tool that scans through all of physical
memory for anonymous THPs and groups them into buckets based
on utilization. It also includes an interface under
/sys/kernel/debug/thp_utilization.

Sample Output:

Utilized[0-50]: 1331 680884
Utilized[51-101]: 9 3983
Utilized[102-152]: 3 1187
Utilized[153-203]: 0 0
Utilized[204-255]: 2 539
Utilized[256-306]: 5 1135
Utilized[307-357]: 1 192
Utilized[358-408]: 0 0
Utilized[409-459]: 1 57
Utilized[460-512]: 400 13
Last Scan Time: 223.98
Last Scan Duration: 70.65

This indicates that there are 1331 THPs that have between 0 and 50
utilized (non zero) pages. In total there are 680884 zero pages in
this utilization bucket. THPs in the [0-50] bucket compose 76% of total
THPs, and are responsible for 99% of total zero pages across all
THPs. In other words, the least utilized THPs are responsible for almost
all of the memory waste when THP is always enabled. Similar results
have been observed across production workloads.

The last two lines indicate the timestamp and duration of the most recent
scan through all of physical memory. Here we see that the last scan
occurred 223.98 seconds after boot time and took 70.65 seconds.

Utilization of a THP is defined as the percentage of nonzero
pages in the THP. The worker thread will scan through all
of physical memory and obtain utilization of all anonymous
THPs. It will gather this information by periodically scanning
through all of physical memory for anonymous THPs, group them
into buckets based on utilization, and report utilization
information through debugfs under /sys/kernel/debug/thp_utilization.

Signed-off-by: Alexander Zhu <alexlzhu@fb.com>
---
 Documentation/admin-guide/mm/transhuge.rst |   9 +
 include/linux/huge_mm.h                    |   2 +
 mm/huge_memory.c                           | 198 +++++++++++++++++++++
 3 files changed, 209 insertions(+)

diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
index c9c37f16eef8..d883ff9fddc7 100644
--- a/Documentation/admin-guide/mm/transhuge.rst
+++ b/Documentation/admin-guide/mm/transhuge.rst
@@ -297,6 +297,15 @@ To identify what applications are mapping file transparent huge pages, it
 is necessary to read ``/proc/PID/smaps`` and count the FileHugeMapped fields
 for each mapping.
 
+The utilization of transparent hugepages can be viewed by reading
+``/sys/kernel/debug/thp_utilization``. The utilization of a THP is defined
+as the ratio of non zero filled 4kb pages to the total number of pages in a
+THP. The buckets are labelled by the range of total utilized 4kb pages with
+one line per utilization bucket. Each line contains the total number of
+THPs in that bucket and the total number of zero filled 4kb pages summed
+over all THPs in that bucket. The last two lines show the timestamp and
+duration respectively of the most recent scan over all of physical memory.
+
 Note that reading the smaps file is expensive and reading it
 frequently will incur overhead.
 
diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index 768e5261fdae..c9086239deb7 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -179,6 +179,8 @@ bool hugepage_vma_check(struct vm_area_struct *vma,
 unsigned long thp_get_unmapped_area(struct file *filp, unsigned long addr,
 		unsigned long len, unsigned long pgoff, unsigned long flags);
 
+int thp_number_utilized_pages(struct page *page);
+
 void prep_transhuge_page(struct page *page);
 void free_transhuge_page(struct page *page);
 
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 8a7c1b344abe..8be1e320e70c 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -45,6 +45,21 @@
 #define CREATE_TRACE_POINTS
 #include <trace/events/thp.h>
 
+/*
+ * The number of utilization buckets THPs will be grouped in
+ * under /sys/kernel/debug/thp_utilization.
+ */
+#define THP_UTIL_BUCKET_NR 10
+/*
+ * The maximum number of hugepages to scan through on each periodic
+ * run of the scanner that generates /sys/kernel/debug/thp_utilization.
+ * We scan through physical memory in chunks of size PMD_SIZE and
+ * record the timestamp and duration of each scan. In practice we have
+ * found that scanning THP_UTIL_SCAN_SIZE hugepages per second is sufficient
+ * for obtaining useful utilization metrics and does not have a noticeable
+ * impact on CPU.
+ */
+#define THP_UTIL_SCAN_SIZE 256
 /*
  * By default, transparent hugepage support is disabled in order to avoid
  * risking an increased memory footprint for applications that are not
@@ -70,6 +85,25 @@ static atomic_t huge_zero_refcount;
 struct page *huge_zero_page __read_mostly;
 unsigned long huge_zero_pfn __read_mostly = ~0UL;
 
+static void thp_utilization_workfn(struct work_struct *work);
+static DECLARE_DELAYED_WORK(thp_utilization_work, thp_utilization_workfn);
+
+struct thp_scan_info_bucket {
+	int nr_thps;
+	int nr_zero_pages;
+};
+
+struct thp_scan_info {
+	struct thp_scan_info_bucket buckets[THP_UTIL_BUCKET_NR];
+	struct zone *scan_zone;
+	struct timespec64 last_scan_duration;
+	struct timespec64 last_scan_time;
+	unsigned long pfn;
+};
+
+static struct thp_scan_info thp_scan_debugfs;
+static struct thp_scan_info thp_scan;
+
 bool hugepage_vma_check(struct vm_area_struct *vma,
 			unsigned long vm_flags,
 			bool smaps, bool in_pf)
@@ -486,6 +520,7 @@ static int __init hugepage_init(void)
 	if (err)
 		goto err_slab;
 
+	schedule_delayed_work(&thp_utilization_work, HZ);
 	err = register_shrinker(&huge_zero_page_shrinker, "thp-zero");
 	if (err)
 		goto err_hzp_shrinker;
@@ -600,6 +635,11 @@ static inline bool is_transparent_hugepage(struct page *page)
 	       page[1].compound_dtor == TRANSHUGE_PAGE_DTOR;
 }
 
+static inline bool is_anon_transparent_hugepage(struct page *page)
+{
+	return PageAnon(page) && is_transparent_hugepage(page);
+}
+
 static unsigned long __thp_get_unmapped_area(struct file *filp,
 		unsigned long addr, unsigned long len,
 		loff_t off, unsigned long flags, unsigned long size)
@@ -650,6 +690,38 @@ unsigned long thp_get_unmapped_area(struct file *filp, unsigned long addr,
 }
 EXPORT_SYMBOL_GPL(thp_get_unmapped_area);
 
+int thp_number_utilized_pages(struct page *page)
+{
+	struct folio *folio;
+	unsigned long page_offset, value;
+	int thp_nr_utilized_pages = HPAGE_PMD_NR;
+	int step_size = sizeof(unsigned long);
+	bool is_all_zeroes;
+	void *kaddr;
+	int i;
+
+	if (!page || !is_anon_transparent_hugepage(page))
+		return -1;
+
+	folio = page_folio(page);
+	for (i = 0; i < folio_nr_pages(folio); i++) {
+		kaddr = kmap_local_folio(folio, i);
+		is_all_zeroes = true;
+		for (page_offset = 0; page_offset < PAGE_SIZE; page_offset += step_size) {
+			value = *(unsigned long *)(kaddr + page_offset);
+			if (value != 0) {
+				is_all_zeroes = false;
+				break;
+			}
+		}
+		if (is_all_zeroes)
+			thp_nr_utilized_pages--;
+
+		kunmap_local(kaddr);
+	}
+	return thp_nr_utilized_pages;
+}
+
 static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf,
 			struct page *page, gfp_t gfp)
 {
@@ -3135,6 +3207,42 @@ static int __init split_huge_pages_debugfs(void)
 	return 0;
 }
 late_initcall(split_huge_pages_debugfs);
+
+static int thp_utilization_show(struct seq_file *seqf, void *pos)
+{
+	int i;
+	int start;
+	int end;
+
+	for (i = 0; i < THP_UTIL_BUCKET_NR; i++) {
+		start = i * HPAGE_PMD_NR / THP_UTIL_BUCKET_NR;
+		end = (i + 1 == THP_UTIL_BUCKET_NR)
+			   ? HPAGE_PMD_NR
+			   : ((i + 1) * HPAGE_PMD_NR / THP_UTIL_BUCKET_NR - 1);
+		/* The last bucket will need to contain 100 */
+		seq_printf(seqf, "Utilized[%d-%d]: %d %d\n", start, end,
+			   thp_scan_debugfs.buckets[i].nr_thps,
+			   thp_scan_debugfs.buckets[i].nr_zero_pages);
+	}
+	seq_printf(seqf, "Last Scan Time: %lu.%02lu\n",
+		   (unsigned long)thp_scan_debugfs.last_scan_time.tv_sec,
+		   (thp_scan_debugfs.last_scan_time.tv_nsec / (NSEC_PER_SEC / 100)));
+
+	seq_printf(seqf, "Last Scan Duration: %lu.%02lu\n",
+		   (unsigned long)thp_scan_debugfs.last_scan_duration.tv_sec,
+		   (thp_scan_debugfs.last_scan_duration.tv_nsec / (NSEC_PER_SEC / 100)));
+
+	return 0;
+}
+DEFINE_SHOW_ATTRIBUTE(thp_utilization);
+
+static int __init thp_utilization_debugfs(void)
+{
+	debugfs_create_file("thp_utilization", 0200, NULL, NULL,
+			    &thp_utilization_fops);
+	return 0;
+}
+late_initcall(thp_utilization_debugfs);
 #endif
 
 #ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
@@ -3220,3 +3328,93 @@ void remove_migration_pmd(struct page_vma_mapped_walk *pvmw, struct page *new)
 	trace_remove_migration_pmd(address, pmd_val(pmde));
 }
 #endif
+
+static void thp_scan_next_zone(void)
+{
+	struct timespec64 current_time;
+	int i;
+	bool update_debugfs;
+	/*
+	 * THP utilization worker thread has reached the end
+	 * of the memory zone. Proceed to the next zone.
+	 */
+	thp_scan.scan_zone = next_zone(thp_scan.scan_zone);
+	update_debugfs = !thp_scan.scan_zone;
+	thp_scan.scan_zone = update_debugfs ? (first_online_pgdat())->node_zones
+			: thp_scan.scan_zone;
+	thp_scan.pfn = (thp_scan.scan_zone->zone_start_pfn + HPAGE_PMD_NR - 1)
+			& ~(HPAGE_PMD_SIZE - 1);
+	if (!update_debugfs)
+		return;
+	/*
+	 * If the worker has scanned through all of physical
+	 * memory. Then update information displayed in /sys/kernel/debug/thp_utilization
+	 */
+	ktime_get_ts64(&current_time);
+	thp_scan_debugfs.last_scan_duration = timespec64_sub(current_time,
+							     thp_scan_debugfs.last_scan_time);
+	thp_scan_debugfs.last_scan_time = current_time;
+
+	for (i = 0; i < THP_UTIL_BUCKET_NR; i++) {
+		thp_scan_debugfs.buckets[i].nr_thps = thp_scan.buckets[i].nr_thps;
+		thp_scan_debugfs.buckets[i].nr_zero_pages = thp_scan.buckets[i].nr_zero_pages;
+		thp_scan.buckets[i].nr_thps = 0;
+		thp_scan.buckets[i].nr_zero_pages = 0;
+	}
+}
+
+static void thp_util_scan(unsigned long pfn_end)
+{
+	struct page *page = NULL;
+	int bucket, num_utilized_pages, current_pfn;
+	int i;
+	/*
+	 * Scan through each memory zone in chunks of up to THP_UTIL_SCAN_SIZE
+	 * hugepages every second looking for anonymous THPs.
+	 */
+	for (i = 0; i < THP_UTIL_SCAN_SIZE; i++) {
+		current_pfn = thp_scan.pfn;
+		thp_scan.pfn += HPAGE_PMD_NR;
+		if (current_pfn >= pfn_end)
+			return;
+
+		if (!pfn_valid(current_pfn))
+			continue;
+
+		page = pfn_to_page(current_pfn);
+		num_utilized_pages = thp_number_utilized_pages(page);
+		 /* Not a THP; skip it. */
+		if (num_utilized_pages < 0)
+			continue;
+		/* Group THPs into utilization buckets */
+		bucket = num_utilized_pages * THP_UTIL_BUCKET_NR / HPAGE_PMD_NR;
+		bucket = min(bucket, THP_UTIL_BUCKET_NR - 1);
+		thp_scan.buckets[bucket].nr_thps++;
+		thp_scan.buckets[bucket].nr_zero_pages += (HPAGE_PMD_NR - num_utilized_pages);
+	}
+}
+
+static void thp_utilization_workfn(struct work_struct *work)
+{
+	unsigned long pfn_end;
+
+	if (!thp_scan.scan_zone)
+		thp_scan.scan_zone = (first_online_pgdat())->node_zones;
+	/*
+	 * Worker function that scans through all of physical memory
+	 * for anonymous THPs.
+	 */
+	pfn_end = (thp_scan.scan_zone->zone_start_pfn +
+			thp_scan.scan_zone->spanned_pages + HPAGE_PMD_NR - 1)
+			& ~(HPAGE_PMD_SIZE - 1);
+	/* If we have reached the end of the zone or end of physical memory
+	 * move on to the next zone. Otherwise, scan the next PFNs in the
+	 * current zone.
+	 */
+	if (!populated_zone(thp_scan.scan_zone) || thp_scan.pfn >= pfn_end)
+		thp_scan_next_zone();
+	else
+		thp_util_scan(pfn_end);
+
+	schedule_delayed_work(&thp_utilization_work, HZ);
+}
-- 
2.30.2

Re: [RFC 1/3] mm: add thp_utilization metrics to debugfs

[Zi Yan]

[-- Attachment #1: Type: text/plain, Size: 12871 bytes --]

On 25 Aug 2022, at 17:30, alexlzhu@fb.com wrote:

> From: Alexander Zhu <alexlzhu@fb.com>
>
> This change introduces a tool that scans through all of physical
> memory for anonymous THPs and groups them into buckets based
> on utilization. It also includes an interface under
> /sys/kernel/debug/thp_utilization.
>
> Sample Output:
>
> Utilized[0-50]: 1331 680884
> Utilized[51-101]: 9 3983
> Utilized[102-152]: 3 1187
> Utilized[153-203]: 0 0
> Utilized[204-255]: 2 539
> Utilized[256-306]: 5 1135
> Utilized[307-357]: 1 192
> Utilized[358-408]: 0 0
> Utilized[409-459]: 1 57
> Utilized[460-512]: 400 13
> Last Scan Time: 223.98
> Last Scan Duration: 70.65

How large is the memory? Just wonder the scanning speed.
Also, it might be better to explicitly add the time unit, second,
in the output.

>
> This indicates that there are 1331 THPs that have between 0 and 50
> utilized (non zero) pages. In total there are 680884 zero pages in
> this utilization bucket. THPs in the [0-50] bucket compose 76% of total
> THPs, and are responsible for 99% of total zero pages across all
> THPs. In other words, the least utilized THPs are responsible for almost
> all of the memory waste when THP is always enabled. Similar results
> have been observed across production workloads.
>
> The last two lines indicate the timestamp and duration of the most recent
> scan through all of physical memory. Here we see that the last scan
> occurred 223.98 seconds after boot time and took 70.65 seconds.
>
> Utilization of a THP is defined as the percentage of nonzero
> pages in the THP. The worker thread will scan through all
> of physical memory and obtain utilization of all anonymous
> THPs. It will gather this information by periodically scanning
> through all of physical memory for anonymous THPs, group them
> into buckets based on utilization, and report utilization
> information through debugfs under /sys/kernel/debug/thp_utilization.
>
> Signed-off-by: Alexander Zhu <alexlzhu@fb.com>
> ---
>  Documentation/admin-guide/mm/transhuge.rst |   9 +
>  include/linux/huge_mm.h                    |   2 +
>  mm/huge_memory.c                           | 198 +++++++++++++++++++++
>  3 files changed, 209 insertions(+)
>
> diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
> index c9c37f16eef8..d883ff9fddc7 100644
> --- a/Documentation/admin-guide/mm/transhuge.rst
> +++ b/Documentation/admin-guide/mm/transhuge.rst
> @@ -297,6 +297,15 @@ To identify what applications are mapping file transparent huge pages, it
>  is necessary to read ``/proc/PID/smaps`` and count the FileHugeMapped fields
>  for each mapping.
>
> +The utilization of transparent hugepages can be viewed by reading
> +``/sys/kernel/debug/thp_utilization``. The utilization of a THP is defined
> +as the ratio of non zero filled 4kb pages to the total number of pages in a
> +THP. The buckets are labelled by the range of total utilized 4kb pages with
> +one line per utilization bucket. Each line contains the total number of
> +THPs in that bucket and the total number of zero filled 4kb pages summed
> +over all THPs in that bucket. The last two lines show the timestamp and
> +duration respectively of the most recent scan over all of physical memory.
> +
>  Note that reading the smaps file is expensive and reading it
>  frequently will incur overhead.
>
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index 768e5261fdae..c9086239deb7 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -179,6 +179,8 @@ bool hugepage_vma_check(struct vm_area_struct *vma,
>  unsigned long thp_get_unmapped_area(struct file *filp, unsigned long addr,
>  		unsigned long len, unsigned long pgoff, unsigned long flags);
>
> +int thp_number_utilized_pages(struct page *page);
> +
>  void prep_transhuge_page(struct page *page);
>  void free_transhuge_page(struct page *page);
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 8a7c1b344abe..8be1e320e70c 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -45,6 +45,21 @@
>  #define CREATE_TRACE_POINTS
>  #include <trace/events/thp.h>
>
> +/*
> + * The number of utilization buckets THPs will be grouped in
> + * under /sys/kernel/debug/thp_utilization.
> + */
> +#define THP_UTIL_BUCKET_NR 10
> +/*
> + * The maximum number of hugepages to scan through on each periodic
> + * run of the scanner that generates /sys/kernel/debug/thp_utilization.
> + * We scan through physical memory in chunks of size PMD_SIZE and
> + * record the timestamp and duration of each scan. In practice we have
> + * found that scanning THP_UTIL_SCAN_SIZE hugepages per second is sufficient
> + * for obtaining useful utilization metrics and does not have a noticeable
> + * impact on CPU.
> + */
> +#define THP_UTIL_SCAN_SIZE 256
>  /*
>   * By default, transparent hugepage support is disabled in order to avoid
>   * risking an increased memory footprint for applications that are not
> @@ -70,6 +85,25 @@ static atomic_t huge_zero_refcount;
>  struct page *huge_zero_page __read_mostly;
>  unsigned long huge_zero_pfn __read_mostly = ~0UL;
>
> +static void thp_utilization_workfn(struct work_struct *work);
> +static DECLARE_DELAYED_WORK(thp_utilization_work, thp_utilization_workfn);
> +
> +struct thp_scan_info_bucket {
> +	int nr_thps;
> +	int nr_zero_pages;
> +};
> +
> +struct thp_scan_info {
> +	struct thp_scan_info_bucket buckets[THP_UTIL_BUCKET_NR];
> +	struct zone *scan_zone;
> +	struct timespec64 last_scan_duration;
> +	struct timespec64 last_scan_time;
> +	unsigned long pfn;
> +};
> +
> +static struct thp_scan_info thp_scan_debugfs;
> +static struct thp_scan_info thp_scan;
> +
>  bool hugepage_vma_check(struct vm_area_struct *vma,
>  			unsigned long vm_flags,
>  			bool smaps, bool in_pf)
> @@ -486,6 +520,7 @@ static int __init hugepage_init(void)
>  	if (err)
>  		goto err_slab;
>
> +	schedule_delayed_work(&thp_utilization_work, HZ);
>  	err = register_shrinker(&huge_zero_page_shrinker, "thp-zero");
>  	if (err)
>  		goto err_hzp_shrinker;
> @@ -600,6 +635,11 @@ static inline bool is_transparent_hugepage(struct page *page)
>  	       page[1].compound_dtor == TRANSHUGE_PAGE_DTOR;
>  }
>
> +static inline bool is_anon_transparent_hugepage(struct page *page)
> +{
> +	return PageAnon(page) && is_transparent_hugepage(page);
> +}
> +
>  static unsigned long __thp_get_unmapped_area(struct file *filp,
>  		unsigned long addr, unsigned long len,
>  		loff_t off, unsigned long flags, unsigned long size)
> @@ -650,6 +690,38 @@ unsigned long thp_get_unmapped_area(struct file *filp, unsigned long addr,
>  }
>  EXPORT_SYMBOL_GPL(thp_get_unmapped_area);
>
> +int thp_number_utilized_pages(struct page *page)
> +{
> +	struct folio *folio;
> +	unsigned long page_offset, value;
> +	int thp_nr_utilized_pages = HPAGE_PMD_NR;
> +	int step_size = sizeof(unsigned long);
> +	bool is_all_zeroes;
> +	void *kaddr;
> +	int i;
> +
> +	if (!page || !is_anon_transparent_hugepage(page))
> +		return -1;
> +
> +	folio = page_folio(page);
> +	for (i = 0; i < folio_nr_pages(folio); i++) {
> +		kaddr = kmap_local_folio(folio, i);
> +		is_all_zeroes = true;
> +		for (page_offset = 0; page_offset < PAGE_SIZE; page_offset += step_size) {
> +			value = *(unsigned long *)(kaddr + page_offset);

Is it possible to use cache-bypassing read to avoid cache
pollution? You are scanning for 256*2M at a time. Wouldn’t that
wipe out all the useful data in the cache?

> +			if (value != 0) {
> +				is_all_zeroes = false;
> +				break;
> +			}
> +		}
> +		if (is_all_zeroes)
> +			thp_nr_utilized_pages--;
> +
> +		kunmap_local(kaddr);
> +	}
> +	return thp_nr_utilized_pages;
> +}
> +
>  static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf,
>  			struct page *page, gfp_t gfp)
>  {
> @@ -3135,6 +3207,42 @@ static int __init split_huge_pages_debugfs(void)
>  	return 0;
>  }
>  late_initcall(split_huge_pages_debugfs);
> +
> +static int thp_utilization_show(struct seq_file *seqf, void *pos)
> +{
> +	int i;
> +	int start;
> +	int end;
> +
> +	for (i = 0; i < THP_UTIL_BUCKET_NR; i++) {
> +		start = i * HPAGE_PMD_NR / THP_UTIL_BUCKET_NR;
> +		end = (i + 1 == THP_UTIL_BUCKET_NR)
> +			   ? HPAGE_PMD_NR
> +			   : ((i + 1) * HPAGE_PMD_NR / THP_UTIL_BUCKET_NR - 1);
> +		/* The last bucket will need to contain 100 */
> +		seq_printf(seqf, "Utilized[%d-%d]: %d %d\n", start, end,
> +			   thp_scan_debugfs.buckets[i].nr_thps,
> +			   thp_scan_debugfs.buckets[i].nr_zero_pages);
> +	}
> +	seq_printf(seqf, "Last Scan Time: %lu.%02lu\n",
> +		   (unsigned long)thp_scan_debugfs.last_scan_time.tv_sec,
> +		   (thp_scan_debugfs.last_scan_time.tv_nsec / (NSEC_PER_SEC / 100)));
> +
> +	seq_printf(seqf, "Last Scan Duration: %lu.%02lu\n",
> +		   (unsigned long)thp_scan_debugfs.last_scan_duration.tv_sec,
> +		   (thp_scan_debugfs.last_scan_duration.tv_nsec / (NSEC_PER_SEC / 100)));
> +
> +	return 0;
> +}
> +DEFINE_SHOW_ATTRIBUTE(thp_utilization);
> +
> +static int __init thp_utilization_debugfs(void)
> +{
> +	debugfs_create_file("thp_utilization", 0200, NULL, NULL,
> +			    &thp_utilization_fops);
> +	return 0;
> +}
> +late_initcall(thp_utilization_debugfs);
>  #endif
>
>  #ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
> @@ -3220,3 +3328,93 @@ void remove_migration_pmd(struct page_vma_mapped_walk *pvmw, struct page *new)
>  	trace_remove_migration_pmd(address, pmd_val(pmde));
>  }
>  #endif
> +
> +static void thp_scan_next_zone(void)
> +{
> +	struct timespec64 current_time;
> +	int i;
> +	bool update_debugfs;
> +	/*
> +	 * THP utilization worker thread has reached the end
> +	 * of the memory zone. Proceed to the next zone.
> +	 */
> +	thp_scan.scan_zone = next_zone(thp_scan.scan_zone);
> +	update_debugfs = !thp_scan.scan_zone;
> +	thp_scan.scan_zone = update_debugfs ? (first_online_pgdat())->node_zones
> +			: thp_scan.scan_zone;
> +	thp_scan.pfn = (thp_scan.scan_zone->zone_start_pfn + HPAGE_PMD_NR - 1)
> +			& ~(HPAGE_PMD_SIZE - 1);
> +	if (!update_debugfs)
> +		return;
> +	/*
> +	 * If the worker has scanned through all of physical
> +	 * memory. Then update information displayed in /sys/kernel/debug/thp_utilization
> +	 */
> +	ktime_get_ts64(&current_time);
> +	thp_scan_debugfs.last_scan_duration = timespec64_sub(current_time,
> +							     thp_scan_debugfs.last_scan_time);
> +	thp_scan_debugfs.last_scan_time = current_time;
> +
> +	for (i = 0; i < THP_UTIL_BUCKET_NR; i++) {
> +		thp_scan_debugfs.buckets[i].nr_thps = thp_scan.buckets[i].nr_thps;
> +		thp_scan_debugfs.buckets[i].nr_zero_pages = thp_scan.buckets[i].nr_zero_pages;
> +		thp_scan.buckets[i].nr_thps = 0;
> +		thp_scan.buckets[i].nr_zero_pages = 0;
> +	}
> +}
> +
> +static void thp_util_scan(unsigned long pfn_end)
> +{
> +	struct page *page = NULL;
> +	int bucket, num_utilized_pages, current_pfn;
> +	int i;
> +	/*
> +	 * Scan through each memory zone in chunks of up to THP_UTIL_SCAN_SIZE
> +	 * hugepages every second looking for anonymous THPs.
> +	 */
> +	for (i = 0; i < THP_UTIL_SCAN_SIZE; i++) {
> +		current_pfn = thp_scan.pfn;
> +		thp_scan.pfn += HPAGE_PMD_NR;
> +		if (current_pfn >= pfn_end)
> +			return;
> +
> +		if (!pfn_valid(current_pfn))
> +			continue;
> +
> +		page = pfn_to_page(current_pfn);
> +		num_utilized_pages = thp_number_utilized_pages(page);
> +		 /* Not a THP; skip it. */
> +		if (num_utilized_pages < 0)
> +			continue;
> +		/* Group THPs into utilization buckets */
> +		bucket = num_utilized_pages * THP_UTIL_BUCKET_NR / HPAGE_PMD_NR;
> +		bucket = min(bucket, THP_UTIL_BUCKET_NR - 1);
> +		thp_scan.buckets[bucket].nr_thps++;
> +		thp_scan.buckets[bucket].nr_zero_pages += (HPAGE_PMD_NR - num_utilized_pages);
> +	}
> +}
> +
> +static void thp_utilization_workfn(struct work_struct *work)
> +{
> +	unsigned long pfn_end;
> +
> +	if (!thp_scan.scan_zone)
> +		thp_scan.scan_zone = (first_online_pgdat())->node_zones;
> +	/*
> +	 * Worker function that scans through all of physical memory
> +	 * for anonymous THPs.
> +	 */
> +	pfn_end = (thp_scan.scan_zone->zone_start_pfn +
> +			thp_scan.scan_zone->spanned_pages + HPAGE_PMD_NR - 1)
> +			& ~(HPAGE_PMD_SIZE - 1);
> +	/* If we have reached the end of the zone or end of physical memory
> +	 * move on to the next zone. Otherwise, scan the next PFNs in the
> +	 * current zone.
> +	 */
> +	if (!populated_zone(thp_scan.scan_zone) || thp_scan.pfn >= pfn_end)
> +		thp_scan_next_zone();
> +	else
> +		thp_util_scan(pfn_end);
> +
> +	schedule_delayed_work(&thp_utilization_work, HZ);
> +}
> -- 
> 2.30.2

--
Best Regards,
Yan, Zi

[-- Attachment #2: OpenPGP digital signature --]
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Re: [RFC 1/3] mm: add thp_utilization metrics to debugfs

[Alex Zhu (Kernel)]

> On Aug 26, 2022, at 5:11 PM, Zi Yan <ziy@nvidia.com> wrote:
> 
> On 25 Aug 2022, at 17:30, alexlzhu@fb.com wrote:
> 
> How large is the memory? Just wonder the scanning speed.
> Also, it might be better to explicitly add the time unit, second,
> in the output.

The size of memory was 65GB on the test machine I obtained these 
numbers on. I’ll take note of adding the time unit. Thanks!


> Is it possible to use cache-bypassing read to avoid cache
> pollution? You are scanning for 256*2M at a time. Wouldn’t that
> wipe out all the useful data in the cache?

I have only found non-temporal writes in arch/x86/, not non-temporal reads (with MOVNTDQA). I suppose we should figure out why nobody ever bothered using non-temporal reads on x86 before trying to make this code use them.

A quick search of the internet suggests they non-temporal reads are not being used on x86 because people could not show a performance improvement by using them, but maybe somebody here has more insight?

[RFC 2/3] mm: changes to split_huge_page() to free zero filled tail pages

[alexlzhu]

From: Alexander Zhu <alexlzhu@fb.com>

Currently, when /sys/kernel/mm/transparent_hugepage/enabled=always is set
there are a large number of transparent hugepages that are almost entirely
zero filled.  This is mentioned in a number of previous patchsets
including:
https://lore.kernel.org/all/20210731063938.1391602-1-yuzhao@google.com/
https://lore.kernel.org/all/
1635422215-99394-1-git-send-email-ningzhang@linux.alibaba.com/

Currently, split_huge_page() does not have a way to identify zero filled
pages within the THP. Thus these zero pages get remapped and continue to
create memory waste. In this patch, we identify and free tail pages that
are zero filled in split_huge_page(). In this way, we avoid mapping these
pages back into page table entries and can free up unused memory within
THPs. This is based off the previously mentioned patchset by Yu Zhao.
However, we chose to free zero tail pages whenever they are encountered
instead of only on reclaim or migration. We also add a self test to verify
the RssAnon value to make sure zero pages are not remapped.

Signed-off-by: Alexander Zhu <alexlzhu@fb.com>
---
 include/linux/rmap.h                          |  2 +-
 include/linux/vm_event_item.h                 |  2 +
 mm/huge_memory.c                              | 43 +++++++++++--
 mm/migrate.c                                  | 60 ++++++++++++++++---
 mm/migrate_device.c                           |  4 +-
 mm/vmstat.c                                   |  2 +
 .../selftests/vm/split_huge_page_test.c       | 58 +++++++++++++++++-
 tools/testing/selftests/vm/vm_util.c          | 23 +++++++
 tools/testing/selftests/vm/vm_util.h          |  1 +
 9 files changed, 180 insertions(+), 15 deletions(-)

diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index bf80adca980b..f45481ab60ba 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -369,7 +369,7 @@ int folio_mkclean(struct folio *);
 int pfn_mkclean_range(unsigned long pfn, unsigned long nr_pages, pgoff_t pgoff,
 		      struct vm_area_struct *vma);
 
-void remove_migration_ptes(struct folio *src, struct folio *dst, bool locked);
+void remove_migration_ptes(struct folio *src, struct folio *dst, bool locked, bool unmap_clean);
 
 int page_mapped_in_vma(struct page *page, struct vm_area_struct *vma);
 
diff --git a/include/linux/vm_event_item.h b/include/linux/vm_event_item.h
index 404024486fa5..1d81e60ee12e 100644
--- a/include/linux/vm_event_item.h
+++ b/include/linux/vm_event_item.h
@@ -104,6 +104,8 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
 #ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
 		THP_SPLIT_PUD,
 #endif
+		THP_SPLIT_FREE,
+		THP_SPLIT_UNMAP,
 		THP_ZERO_PAGE_ALLOC,
 		THP_ZERO_PAGE_ALLOC_FAILED,
 		THP_SWPOUT,
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 8be1e320e70c..0f774a7c0727 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2414,7 +2414,7 @@ static void unmap_page(struct page *page)
 		try_to_unmap(folio, ttu_flags | TTU_IGNORE_MLOCK);
 }
 
-static void remap_page(struct folio *folio, unsigned long nr)
+static void remap_page(struct folio *folio, unsigned long nr, bool unmap_clean)
 {
 	int i = 0;
 
@@ -2422,7 +2422,7 @@ static void remap_page(struct folio *folio, unsigned long nr)
 	if (!folio_test_anon(folio))
 		return;
 	for (;;) {
-		remove_migration_ptes(folio, folio, true);
+		remove_migration_ptes(folio, folio, true, unmap_clean);
 		i += folio_nr_pages(folio);
 		if (i >= nr)
 			break;
@@ -2536,6 +2536,8 @@ static void __split_huge_page(struct page *page, struct list_head *list,
 	struct address_space *swap_cache = NULL;
 	unsigned long offset = 0;
 	unsigned int nr = thp_nr_pages(head);
+	LIST_HEAD(pages_to_free);
+	int nr_pages_to_free = 0;
 	int i;
 
 	/* complete memcg works before add pages to LRU */
@@ -2598,7 +2600,7 @@ static void __split_huge_page(struct page *page, struct list_head *list,
 	}
 	local_irq_enable();
 
-	remap_page(folio, nr);
+	remap_page(folio, nr, true);
 
 	if (PageSwapCache(head)) {
 		swp_entry_t entry = { .val = page_private(head) };
@@ -2612,6 +2614,32 @@ static void __split_huge_page(struct page *page, struct list_head *list,
 			continue;
 		unlock_page(subpage);
 
+		/*
+		 * If a tail page has only two references left, one inherited
+		 * from the isolation of its head and the other from
+		 * lru_add_page_tail() which we are about to drop, it means this
+		 * tail page was concurrently zapped. Then we can safely free it
+		 * and save page reclaim or migration the trouble of trying it.
+		 */
+		if (list && page_ref_freeze(subpage, 2)) {
+			VM_BUG_ON_PAGE(PageLRU(subpage), subpage);
+			VM_BUG_ON_PAGE(PageCompound(subpage), subpage);
+			VM_BUG_ON_PAGE(page_mapped(subpage), subpage);
+
+			ClearPageActive(subpage);
+			ClearPageUnevictable(subpage);
+			list_move(&subpage->lru, &pages_to_free);
+			nr_pages_to_free++;
+			continue;
+		}
+		/*
+		 * If a tail page has only one reference left, it will be freed
+		 * by the call to free_page_and_swap_cache below. Since zero
+		 * subpages are no longer remapped, there will only be one
+		 * reference left in cases outside of reclaim or migration.
+		 */
+		if (page_ref_count(subpage) == 1)
+			nr_pages_to_free++;
 		/*
 		 * Subpages may be freed if there wasn't any mapping
 		 * like if add_to_swap() is running on a lru page that
@@ -2621,6 +2649,13 @@ static void __split_huge_page(struct page *page, struct list_head *list,
 		 */
 		free_page_and_swap_cache(subpage);
 	}
+
+	if (!nr_pages_to_free)
+		return;
+
+	mem_cgroup_uncharge_list(&pages_to_free);
+	free_unref_page_list(&pages_to_free);
+	count_vm_events(THP_SPLIT_FREE, nr_pages_to_free);
 }
 
 /* Racy check whether the huge page can be split */
@@ -2783,7 +2818,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
 		if (mapping)
 			xas_unlock(&xas);
 		local_irq_enable();
-		remap_page(folio, folio_nr_pages(folio));
+		remap_page(folio, folio_nr_pages(folio), false);
 		ret = -EBUSY;
 	}
 
diff --git a/mm/migrate.c b/mm/migrate.c
index 6a1597c92261..c87e81e60a1b 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -167,13 +167,50 @@ void putback_movable_pages(struct list_head *l)
 	}
 }
 
+static bool try_to_unmap_clean(struct page_vma_mapped_walk *pvmw, struct page *page)
+{
+	void *addr;
+	bool dirty;
+
+	VM_BUG_ON_PAGE(PageCompound(page), page);
+	VM_BUG_ON_PAGE(!PageAnon(page), page);
+	VM_BUG_ON_PAGE(!PageLocked(page), page);
+	VM_BUG_ON_PAGE(pte_present(*pvmw->pte), page);
+
+	if (PageMlocked(page) || (pvmw->vma->vm_flags & VM_LOCKED))
+		return false;
+
+	/*
+	 * The pmd entry mapping the old thp was flushed and the pte mapping
+	 * this subpage has been non present. Therefore, this subpage is
+	 * inaccessible. We don't need to remap it if it contains only zeros.
+	 */
+	addr = kmap_local_page(page);
+	dirty = memchr_inv(addr, 0, PAGE_SIZE);
+	kunmap_local(addr);
+
+	if (dirty)
+		return false;
+
+	pte_clear_not_present_full(pvmw->vma->vm_mm, pvmw->address, pvmw->pte, false);
+	dec_mm_counter(pvmw->vma->vm_mm, mm_counter(page));
+	count_vm_event(THP_SPLIT_UNMAP);
+	return true;
+}
+
+struct rmap_walk_arg {
+	struct folio *folio;
+	bool unmap_clean;
+};
+
 /*
  * Restore a potential migration pte to a working pte entry
  */
 static bool remove_migration_pte(struct folio *folio,
-		struct vm_area_struct *vma, unsigned long addr, void *old)
+		struct vm_area_struct *vma, unsigned long addr, void *arg)
 {
-	DEFINE_FOLIO_VMA_WALK(pvmw, old, vma, addr, PVMW_SYNC | PVMW_MIGRATION);
+	struct rmap_walk_arg *rmap_walk_arg = arg;
+	DEFINE_FOLIO_VMA_WALK(pvmw, rmap_walk_arg->folio, vma, addr, PVMW_SYNC | PVMW_MIGRATION);
 
 	while (page_vma_mapped_walk(&pvmw)) {
 		rmap_t rmap_flags = RMAP_NONE;
@@ -196,6 +233,8 @@ static bool remove_migration_pte(struct folio *folio,
 			continue;
 		}
 #endif
+		if (rmap_walk_arg->unmap_clean && try_to_unmap_clean(&pvmw, new))
+			continue;
 
 		folio_get(folio);
 		pte = pte_mkold(mk_pte(new, READ_ONCE(vma->vm_page_prot)));
@@ -267,13 +306,20 @@ static bool remove_migration_pte(struct folio *folio,
  * Get rid of all migration entries and replace them by
  * references to the indicated page.
  */
-void remove_migration_ptes(struct folio *src, struct folio *dst, bool locked)
+void remove_migration_ptes(struct folio *src, struct folio *dst, bool locked, bool unmap_clean)
 {
+	struct rmap_walk_arg rmap_walk_arg = {
+		.folio = src,
+		.unmap_clean = unmap_clean,
+	};
+
 	struct rmap_walk_control rwc = {
 		.rmap_one = remove_migration_pte,
-		.arg = src,
+		.arg = &rmap_walk_arg,
 	};
 
+	VM_BUG_ON_FOLIO(unmap_clean && src != dst, src);
+
 	if (locked)
 		rmap_walk_locked(dst, &rwc);
 	else
@@ -849,7 +895,7 @@ static int writeout(struct address_space *mapping, struct folio *folio)
 	 * At this point we know that the migration attempt cannot
 	 * be successful.
 	 */
-	remove_migration_ptes(folio, folio, false);
+	remove_migration_ptes(folio, folio, false, false);
 
 	rc = mapping->a_ops->writepage(&folio->page, &wbc);
 
@@ -1108,7 +1154,7 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
 
 	if (page_was_mapped)
 		remove_migration_ptes(folio,
-			rc == MIGRATEPAGE_SUCCESS ? dst : folio, false);
+			rc == MIGRATEPAGE_SUCCESS ? dst : folio, false, false);
 
 out_unlock_both:
 	unlock_page(newpage);
@@ -1318,7 +1364,7 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
 
 	if (page_was_mapped)
 		remove_migration_ptes(src,
-			rc == MIGRATEPAGE_SUCCESS ? dst : src, false);
+			rc == MIGRATEPAGE_SUCCESS ? dst : src, false, false);
 
 unlock_put_anon:
 	unlock_page(new_hpage);
diff --git a/mm/migrate_device.c b/mm/migrate_device.c
index 27fb37d65476..cf5a54715a58 100644
--- a/mm/migrate_device.c
+++ b/mm/migrate_device.c
@@ -407,7 +407,7 @@ static void migrate_vma_unmap(struct migrate_vma *migrate)
 			continue;
 
 		folio = page_folio(page);
-		remove_migration_ptes(folio, folio, false);
+		remove_migration_ptes(folio, folio, false, false);
 
 		migrate->src[i] = 0;
 		folio_unlock(folio);
@@ -783,7 +783,7 @@ void migrate_vma_finalize(struct migrate_vma *migrate)
 
 		src = page_folio(page);
 		dst = page_folio(newpage);
-		remove_migration_ptes(src, dst, false);
+		remove_migration_ptes(src, dst, false, false);
 		folio_unlock(src);
 
 		if (is_zone_device_page(page))
diff --git a/mm/vmstat.c b/mm/vmstat.c
index 373d2730fcf2..c8fae2fb1cdf 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1363,6 +1363,8 @@ const char * const vmstat_text[] = {
 #ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
 	"thp_split_pud",
 #endif
+	"thp_split_free",
+	"thp_split_unmap",
 	"thp_zero_page_alloc",
 	"thp_zero_page_alloc_failed",
 	"thp_swpout",
diff --git a/tools/testing/selftests/vm/split_huge_page_test.c b/tools/testing/selftests/vm/split_huge_page_test.c
index 6aa2b8253aed..f47a6ba80773 100644
--- a/tools/testing/selftests/vm/split_huge_page_test.c
+++ b/tools/testing/selftests/vm/split_huge_page_test.c
@@ -88,6 +88,62 @@ static void write_debugfs(const char *fmt, ...)
 	}
 }
 
+void split_pmd_zero_pages(void)
+{
+	char *one_page;
+	size_t len = 4 * pmd_pagesize;
+	uint64_t thp_size, rss_anon_before, rss_anon_after;
+	size_t i;
+
+	one_page = memalign(pmd_pagesize, len);
+
+	if (!one_page) {
+		printf("Fail to allocate memory\n");
+		exit(EXIT_FAILURE);
+	}
+
+	madvise(one_page, len, MADV_HUGEPAGE);
+	for (i = 0; i < len; i++)
+		one_page[i] = (char)0;
+
+	thp_size = check_huge(one_page);
+	if (!thp_size) {
+		printf("No THP is allocated\n");
+		exit(EXIT_FAILURE);
+	}
+
+	rss_anon_before = rss_anon();
+	if (!rss_anon_before) {
+		printf("No RssAnon is allocated before split\n");
+		exit(EXIT_FAILURE);
+	}
+	/* split all THPs */
+	write_debugfs(PID_FMT, getpid(), (uint64_t)one_page,
+		      (uint64_t)one_page + len);
+
+	for (i = 0; i < len; i++)
+		if (one_page[i] != (char)0) {
+			printf("%ld byte corrupted\n", i);
+			exit(EXIT_FAILURE);
+		}
+
+	thp_size = check_huge(one_page);
+	if (thp_size) {
+		printf("Still %ld kB AnonHugePages not split\n", thp_size);
+		exit(EXIT_FAILURE);
+	}
+
+	rss_anon_after = rss_anon();
+	if (rss_anon_after >= rss_anon_before) {
+		printf("Incorrect RssAnon value. Before: %ld After: %ld\n",
+		       rss_anon_before, rss_anon_after);
+		exit(EXIT_FAILURE);
+	}
+
+	printf("Split zero filled huge pages successful\n");
+	free(one_page);
+}
+
 void split_pmd_thp(void)
 {
 	char *one_page;
@@ -123,7 +179,6 @@ void split_pmd_thp(void)
 			exit(EXIT_FAILURE);
 		}
 
-
 	thp_size = check_huge(one_page);
 	if (thp_size) {
 		printf("Still %ld kB AnonHugePages not split\n", thp_size);
@@ -305,6 +360,7 @@ int main(int argc, char **argv)
 	pageshift = ffs(pagesize) - 1;
 	pmd_pagesize = read_pmd_pagesize();
 
+	split_pmd_zero_pages();
 	split_pmd_thp();
 	split_pte_mapped_thp();
 	split_file_backed_thp();
diff --git a/tools/testing/selftests/vm/vm_util.c b/tools/testing/selftests/vm/vm_util.c
index b58ab11a7a30..c6a785a67fc9 100644
--- a/tools/testing/selftests/vm/vm_util.c
+++ b/tools/testing/selftests/vm/vm_util.c
@@ -6,6 +6,7 @@
 
 #define PMD_SIZE_FILE_PATH "/sys/kernel/mm/transparent_hugepage/hpage_pmd_size"
 #define SMAP_FILE_PATH "/proc/self/smaps"
+#define STATUS_FILE_PATH "/proc/self/status"
 #define MAX_LINE_LENGTH 500
 
 uint64_t pagemap_get_entry(int fd, char *start)
@@ -72,6 +73,28 @@ uint64_t read_pmd_pagesize(void)
 	return strtoul(buf, NULL, 10);
 }
 
+uint64_t rss_anon(void)
+{
+	uint64_t rss_anon = 0;
+	int ret;
+	FILE *fp;
+	char buffer[MAX_LINE_LENGTH];
+
+	fp = fopen(STATUS_FILE_PATH, "r");
+	if (!fp)
+		ksft_exit_fail_msg("%s: Failed to open file %s\n", __func__, STATUS_FILE_PATH);
+
+	if (!check_for_pattern(fp, "RssAnon:", buffer))
+		goto err_out;
+
+	if (sscanf(buffer, "RssAnon:%10ld kB", &rss_anon) != 1)
+		ksft_exit_fail_msg("Reading status error\n");
+
+err_out:
+	fclose(fp);
+	return rss_anon;
+}
+
 uint64_t check_huge(void *addr)
 {
 	uint64_t thp = 0;
diff --git a/tools/testing/selftests/vm/vm_util.h b/tools/testing/selftests/vm/vm_util.h
index 2e512bd57ae1..00b92ccef20d 100644
--- a/tools/testing/selftests/vm/vm_util.h
+++ b/tools/testing/selftests/vm/vm_util.h
@@ -6,4 +6,5 @@ uint64_t pagemap_get_entry(int fd, char *start);
 bool pagemap_is_softdirty(int fd, char *start);
 void clear_softdirty(void);
 uint64_t read_pmd_pagesize(void);
+uint64_t rss_anon(void);
 uint64_t check_huge(void *addr);
-- 
2.30.2

Re: [RFC 2/3] mm: changes to split_huge_page() to free zero filled tail pages

[David Hildenbrand]

On 25.08.22 23:30, alexlzhu@fb.com wrote:
> From: Alexander Zhu <alexlzhu@fb.com>
> 
> Currently, when /sys/kernel/mm/transparent_hugepage/enabled=always is set
> there are a large number of transparent hugepages that are almost entirely
> zero filled.  This is mentioned in a number of previous patchsets
> including:
> https://lore.kernel.org/all/20210731063938.1391602-1-yuzhao@google.com/
> https://lore.kernel.org/all/
> 1635422215-99394-1-git-send-email-ningzhang@linux.alibaba.com/
> 
> Currently, split_huge_page() does not have a way to identify zero filled
> pages within the THP. Thus these zero pages get remapped and continue to
> create memory waste. In this patch, we identify and free tail pages that
> are zero filled in split_huge_page(). In this way, we avoid mapping these
> pages back into page table entries and can free up unused memory within
> THPs. This is based off the previously mentioned patchset by Yu Zhao.
> However, we chose to free zero tail pages whenever they are encountered
> instead of only on reclaim or migration. We also add a self test to verify
> the RssAnon value to make sure zero pages are not remapped.
> 

Isn't this to some degree splitting the THP (PMDs->PTEs + dissolve
compound page) and then letting KSM replace the zero-filled page by the
shared zeropage?

-- 
Thanks,

David / dhildenb

Re: [RFC 2/3] mm: changes to split_huge_page() to free zero filled tail pages

[Alex Zhu (Kernel)]

> On Aug 26, 2022, at 3:18 AM, David Hildenbrand <david@redhat.com> wrote:
> 
> !-------------------------------------------------------------------|
>  This Message Is From an External Sender
> 
> |-------------------------------------------------------------------!
> 
> On 25.08.22 23:30, alexlzhu@fb.com wrote:
>> From: Alexander Zhu <alexlzhu@fb.com>
>> 
>> Currently, when /sys/kernel/mm/transparent_hugepage/enabled=always is set
>> there are a large number of transparent hugepages that are almost entirely
>> zero filled.  This is mentioned in a number of previous patchsets
>> including:
>> https://lore.kernel.org/all/20210731063938.1391602-1-yuzhao@google.com/
>> https://lore.kernel.org/all/
>> 1635422215-99394-1-git-send-email-ningzhang@linux.alibaba.com/
>> 
>> Currently, split_huge_page() does not have a way to identify zero filled
>> pages within the THP. Thus these zero pages get remapped and continue to
>> create memory waste. In this patch, we identify and free tail pages that
>> are zero filled in split_huge_page(). In this way, we avoid mapping these
>> pages back into page table entries and can free up unused memory within
>> THPs. This is based off the previously mentioned patchset by Yu Zhao.
>> However, we chose to free zero tail pages whenever they are encountered
>> instead of only on reclaim or migration. We also add a self test to verify
>> the RssAnon value to make sure zero pages are not remapped.
>> 
> 
> Isn't this to some degree splitting the THP (PMDs->PTEs + dissolve
> compound page) and then letting KSM replace the zero-filled page by the
> shared zeropage?
> 
> -- 
> Thanks,
> 
> David / dhildenb

AFAICT KSM may or may not replace the zero filled page with the shared zero page depending on whether the VMA is mergeable. Whether
or not the VMA is mergeable comes from madvise. Madvise only applies to certain memory regions. Here we have THP always enabled rather than on madvise, and the end goal is to deprecate madvise entirely.

These THPs would previously not have been split at all, as we could not identify which THPs were underutilized, and would thus
have just been memory waste when THP was always enabled. 

In split_huge_page() we chose the most straightforward approach to free (zap) the zero page immediately to get rid of the memory waste. It does not seem to me that KSM is necessary here.

Thanks,
Alex

Re: [RFC 2/3] mm: changes to split_huge_page() to free zero filled tail pages

[Rik van Riel]

[-- Attachment #1: Type: text/plain, Size: 1429 bytes --]

On Fri, 2022-08-26 at 12:18 +0200, David Hildenbrand wrote:
> On 25.08.22 23:30, alexlzhu@fb.com wrote:
> > From: Alexander Zhu <alexlzhu@fb.com>
> > 
> > Currently, split_huge_page() does not have a way to identify zero
> > filled
> > pages within the THP. Thus these zero pages get remapped and
> > continue to
> > create memory waste. In this patch, we identify and free tail pages
> > that
> > are zero filled in split_huge_page(). In this way, we avoid mapping
> > these
> > pages back into page table entries and can free up unused memory
> > within
> > THPs. 
> > 
> 
> Isn't this to some degree splitting the THP (PMDs->PTEs + dissolve
> compound page) and then letting KSM replace the zero-filled page by
> the
> shared zeropage?
> 
Many systems do not run KSM, though, and even on the systems
where it does, KSM only covers a subset of the memory in the
system.

I could see wanting to maybe consolidate the scanning between
KSM and this thing at some point, if it could be done without
too much complexity, but keeping this change to split_huge_page
looks like it might make sense even when KSM is enabled, since
it will get rid of the unnecessary memory much faster than KSM could.

Keeping a hundred MB of unnecessary memory around for longer
would simply result in more THPs getting split up, and more
memory pressure for a longer time than we need.

-- 
All Rights Reversed.

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Re: [RFC 2/3] mm: changes to split_huge_page() to free zero filled tail pages

[David Hildenbrand]

On 26.08.22 23:18, Rik van Riel wrote:
> On Fri, 2022-08-26 at 12:18 +0200, David Hildenbrand wrote:
>> On 25.08.22 23:30, alexlzhu@fb.com wrote:
>>> From: Alexander Zhu <alexlzhu@fb.com>
>>>
>>> Currently, split_huge_page() does not have a way to identify zero
>>> filled
>>> pages within the THP. Thus these zero pages get remapped and
>>> continue to
>>> create memory waste. In this patch, we identify and free tail pages
>>> that
>>> are zero filled in split_huge_page(). In this way, we avoid mapping
>>> these
>>> pages back into page table entries and can free up unused memory
>>> within
>>> THPs. 
>>>
>>
>> Isn't this to some degree splitting the THP (PMDs->PTEs + dissolve
>> compound page) and then letting KSM replace the zero-filled page by
>> the
>> shared zeropage?
>>
> Many systems do not run KSM, though, and even on the systems
> where it does, KSM only covers a subset of the memory in the
> system.

Right, however there seems to be a push from some folks to enable it
more widely.

> 
> I could see wanting to maybe consolidate the scanning between
> KSM and this thing at some point, if it could be done without
> too much complexity, but keeping this change to split_huge_page
> looks like it might make sense even when KSM is enabled, since
> it will get rid of the unnecessary memory much faster than KSM could.
> 
> Keeping a hundred MB of unnecessary memory around for longer
> would simply result in more THPs getting split up, and more
> memory pressure for a longer time than we need.

Right. I was wondering if we want to map the shared zeropage instead of
the "detected to be zero" page, similar to how KSM would do it. For
example, with userfaultfd there would be an observable difference.

(maybe that's already done in this patch set)

-- 
Thanks,

David / dhildenb

Re: [RFC 2/3] mm: changes to split_huge_page() to free zero filled tail pages

[Rik van Riel]

[-- Attachment #1: Type: text/plain, Size: 1315 bytes --]

On Mon, 2022-08-29 at 12:02 +0200, David Hildenbrand wrote:
> On 26.08.22 23:18, Rik van Riel wrote:
> > On Fri, 2022-08-26 at 12:18 +0200, David Hildenbrand wrote:
> > > On 25.08.22 23:30, alexlzhu@fb.com wrote:
> > > > From: Alexander Zhu <alexlzhu@fb.com>
> > 
> > I could see wanting to maybe consolidate the scanning between
> > KSM and this thing at some point, if it could be done without
> > too much complexity, but keeping this change to split_huge_page
> > looks like it might make sense even when KSM is enabled, since
> > it will get rid of the unnecessary memory much faster than KSM
> > could.
> > 
> > Keeping a hundred MB of unnecessary memory around for longer
> > would simply result in more THPs getting split up, and more
> > memory pressure for a longer time than we need.
> 
> Right. I was wondering if we want to map the shared zeropage instead
> of
> the "detected to be zero" page, similar to how KSM would do it. For
> example, with userfaultfd there would be an observable difference.
> 
> (maybe that's already done in this patch set)
> 
The patch does not currently do that, but I suppose it could?

What exactly are the userfaultfd differences here, and how does
dropping 4kB pages break things vs. using the shared zeropage?

-- 
All Rights Reversed.

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Re: [RFC 2/3] mm: changes to split_huge_page() to free zero filled tail pages

[David Hildenbrand]

On 29.08.22 15:17, Rik van Riel wrote:
> On Mon, 2022-08-29 at 12:02 +0200, David Hildenbrand wrote:
>> On 26.08.22 23:18, Rik van Riel wrote:
>>> On Fri, 2022-08-26 at 12:18 +0200, David Hildenbrand wrote:
>>>> On 25.08.22 23:30, alexlzhu@fb.com wrote:
>>>>> From: Alexander Zhu <alexlzhu@fb.com>
>>>
>>> I could see wanting to maybe consolidate the scanning between
>>> KSM and this thing at some point, if it could be done without
>>> too much complexity, but keeping this change to split_huge_page
>>> looks like it might make sense even when KSM is enabled, since
>>> it will get rid of the unnecessary memory much faster than KSM
>>> could.
>>>
>>> Keeping a hundred MB of unnecessary memory around for longer
>>> would simply result in more THPs getting split up, and more
>>> memory pressure for a longer time than we need.
>>
>> Right. I was wondering if we want to map the shared zeropage instead
>> of
>> the "detected to be zero" page, similar to how KSM would do it. For
>> example, with userfaultfd there would be an observable difference.
>>
>> (maybe that's already done in this patch set)
>>
> The patch does not currently do that, but I suppose it could?
> 

It would be interesting to know why KSM decided to replace the mapped
page with the shared zeropage instead of dropping the page and letting
the next read fault populate the shared zeropage. That code predates
userfaultfd IIRC.

> What exactly are the userfaultfd differences here, and how does
> dropping 4kB pages break things vs. using the shared zeropage?

Once userfaultfd (missing mode) is enabled on a VMA:

1) khugepaged will no longer collapse pte_none(pteval), independent of
khugepaged_max_ptes_none setting -- see __collapse_huge_page_isolate.
[it will also not collapse zeropages, but I recall that that's not
actually required]

So it will not close holes, because the user space fault handler is in
charge of making a decision when something will get mapped there and
with which content.


2) Page faults will no longer populate a THP -- the user space handler
is notified instead and has to decide how the fault will be resolved
(place pages).


If you unmap something (resulting in pte_none()) where previously
something used to be mapped in a page table, you might suddenly inform
the user space fault handler about a page fault that it doesn't expect,
because it previously placed a page and did not zap that page itself
(MADV_DONTNEED).

So at least with userfaultfd I think we have to be careful. Not sure if
there are other corner cases (again, KSM behavior is interesting)

-- 
Thanks,

David / dhildenb

Re: [RFC 2/3] mm: changes to split_huge_page() to free zero filled tail pages

[Alex Zhu (Kernel)]

> If you unmap something (resulting in pte_none()) where previously
> something used to be mapped in a page table, you might suddenly inform
> the user space fault handler about a page fault that it doesn't expect,
> because it previously placed a page and did not zap that page itself
> (MADV_DONTNEED).
> 
> So at least with userfaultfd I think we have to be careful. Not sure if
> there are other corner cases (again, KSM behavior is interesting)
> 
> -- 
> Thanks,
> 
> David / dhildenb

We can implement it such that if userfaultfd is enabled on a VMA then instead of unmapping the zero page, 
we will map to a read only zero page.

The original patch from Yu Zhao frees zero pages only on reclaim, I am not sure
it needs to be this restricted though. In use cases where immediately freeing
zero pages does not work we can dedupe similar to how KSM does it. 

[RFC 3/3] mm: THP low utilization shrinker

[alexlzhu]

From: Alexander Zhu <alexlzhu@fb.com>

This patch introduces a shrinker that will remove THPs in the lowest
utilization bucket. As previously mentioned, we have observed that
almost all of the memory waste when THPs are always enabled
is contained in the lowest utilization bucket. The shrinker will
add these THPs to a list_lru and split anonymous THPs based off
information from kswapd. It requires the changes from
thp_utilization to identify the least utilized THPs, and the
changes to split_huge_page to identify and free zero pages
within THPs.

Signed-off-by: Alexander Zhu <alexlzhu@fb.com>
---
 include/linux/huge_mm.h  |  7 +++
 include/linux/list_lru.h | 24 +++++++++++
 include/linux/mm_types.h |  5 +++
 mm/huge_memory.c         | 92 ++++++++++++++++++++++++++++++++++++++--
 mm/list_lru.c            | 49 +++++++++++++++++++++
 mm/page_alloc.c          |  6 +++
 6 files changed, 180 insertions(+), 3 deletions(-)

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index c9086239deb7..13bd470173d2 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -192,6 +192,8 @@ static inline int split_huge_page(struct page *page)
 }
 void deferred_split_huge_page(struct page *page);
 
+void add_underutilized_thp(struct page *page);
+
 void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
 		unsigned long address, bool freeze, struct folio *folio);
 
@@ -302,6 +304,11 @@ static inline struct list_head *page_deferred_list(struct page *page)
 	return &page[2].deferred_list;
 }
 
+static inline struct list_head *page_underutilized_thp_list(struct page *page)
+{
+	return &page[3].underutilized_thp_list;
+}
+
 #else /* CONFIG_TRANSPARENT_HUGEPAGE */
 #define HPAGE_PMD_SHIFT ({ BUILD_BUG(); 0; })
 #define HPAGE_PMD_MASK ({ BUILD_BUG(); 0; })
diff --git a/include/linux/list_lru.h b/include/linux/list_lru.h
index b35968ee9fb5..c2cf146ea880 100644
--- a/include/linux/list_lru.h
+++ b/include/linux/list_lru.h
@@ -89,6 +89,18 @@ void memcg_reparent_list_lrus(struct mem_cgroup *memcg, struct mem_cgroup *paren
  */
 bool list_lru_add(struct list_lru *lru, struct list_head *item);
 
+/**
+ * list_lru_add_page: add an element to the lru list's tail
+ * @list_lru: the lru pointer
+ * @page: the page containing the item
+ * @item: the item to be deleted.
+ *
+ * This function works the same as list_lru_add in terms of list
+ * manipulation. Used for non slab objects contained in the page.
+ *
+ * Return value: true if the list was updated, false otherwise
+ */
+bool list_lru_add_page(struct list_lru *lru, struct page *page, struct list_head *item);
 /**
  * list_lru_del: delete an element to the lru list
  * @list_lru: the lru pointer
@@ -102,6 +114,18 @@ bool list_lru_add(struct list_lru *lru, struct list_head *item);
  */
 bool list_lru_del(struct list_lru *lru, struct list_head *item);
 
+/**
+ * list_lru_del_page: delete an element to the lru list
+ * @list_lru: the lru pointer
+ * @page: the page containing the item
+ * @item: the item to be deleted.
+ *
+ * This function works the same as list_lru_del in terms of list
+ * manipulation. Used for non slab objects contained in the page.
+ *
+ * Return value: true if the list was updated, false otherwise
+ */
+bool list_lru_del_page(struct list_lru *lru, struct page *page, struct list_head *item);
 /**
  * list_lru_count_one: return the number of objects currently held by @lru
  * @lru: the lru pointer.
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index cf97f3884fda..05667a2030c0 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -151,6 +151,11 @@ struct page {
 			/* For both global and memcg */
 			struct list_head deferred_list;
 		};
+		struct { /* Third tail page of compound page */
+			unsigned long _compound_pad_3; /* compound_head */
+			unsigned long _compound_pad_4;
+			struct list_head underutilized_thp_list;
+		};
 		struct {	/* Page table pages */
 			unsigned long _pt_pad_1;	/* compound_head */
 			pgtable_t pmd_huge_pte; /* protected by page->ptl */
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 0f774a7c0727..03dc42eba0ba 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -8,6 +8,7 @@
 #include <linux/mm.h>
 #include <linux/sched.h>
 #include <linux/sched/mm.h>
+#include <linux/sched/clock.h>
 #include <linux/sched/coredump.h>
 #include <linux/sched/numa_balancing.h>
 #include <linux/highmem.h>
@@ -85,6 +86,8 @@ static atomic_t huge_zero_refcount;
 struct page *huge_zero_page __read_mostly;
 unsigned long huge_zero_pfn __read_mostly = ~0UL;
 
+struct list_lru huge_low_util_page_lru;
+
 static void thp_utilization_workfn(struct work_struct *work);
 static DECLARE_DELAYED_WORK(thp_utilization_work, thp_utilization_workfn);
 
@@ -269,6 +272,46 @@ static struct shrinker huge_zero_page_shrinker = {
 	.seeks = DEFAULT_SEEKS,
 };
 
+static enum lru_status low_util_free_page(struct list_head *item,
+					  struct list_lru_one *lru,
+					  spinlock_t *lock,
+					  void *cb_arg)
+{
+	struct page *head = compound_head(list_entry(item,
+									struct page,
+									underutilized_thp_list));
+
+	if (get_page_unless_zero(head)) {
+		lock_page(head);
+		list_lru_isolate(lru, item);
+		split_huge_page(head);
+		unlock_page(head);
+		put_page(head);
+	}
+
+	return LRU_REMOVED_RETRY;
+}
+
+static unsigned long shrink_huge_low_util_page_count(struct shrinker *shrink,
+						     struct shrink_control *sc)
+{
+	return list_lru_shrink_count(&huge_low_util_page_lru, sc);
+}
+
+static unsigned long shrink_huge_low_util_page_scan(struct shrinker *shrink,
+						    struct shrink_control *sc)
+{
+	return list_lru_shrink_walk(&huge_low_util_page_lru, sc, low_util_free_page, NULL);
+}
+
+static struct shrinker huge_low_util_page_shrinker = {
+	.count_objects = shrink_huge_low_util_page_count,
+	.scan_objects = shrink_huge_low_util_page_scan,
+	.seeks = DEFAULT_SEEKS,
+	.flags = SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_AWARE |
+		SHRINKER_NONSLAB,
+};
+
 #ifdef CONFIG_SYSFS
 static ssize_t enabled_show(struct kobject *kobj,
 			    struct kobj_attribute *attr, char *buf)
@@ -521,13 +564,18 @@ static int __init hugepage_init(void)
 		goto err_slab;
 
 	schedule_delayed_work(&thp_utilization_work, HZ);
+	err = register_shrinker(&huge_low_util_page_shrinker, "thp-low-util");
+	if (err)
+		goto err_low_util_shrinker;
 	err = register_shrinker(&huge_zero_page_shrinker, "thp-zero");
 	if (err)
 		goto err_hzp_shrinker;
 	err = register_shrinker(&deferred_split_shrinker, "thp-deferred_split");
 	if (err)
 		goto err_split_shrinker;
-
+	err = list_lru_init_memcg(&huge_low_util_page_lru, &huge_low_util_page_shrinker);
+	if (err)
+		goto err_low_util_list_lru;
 	/*
 	 * By default disable transparent hugepages on smaller systems,
 	 * where the extra memory used could hurt more than TLB overhead
@@ -543,11 +591,16 @@ static int __init hugepage_init(void)
 		goto err_khugepaged;
 
 	return 0;
+
 err_khugepaged:
+	list_lru_destroy(&huge_low_util_page_lru);
+err_low_util_list_lru:
 	unregister_shrinker(&deferred_split_shrinker);
 err_split_shrinker:
 	unregister_shrinker(&huge_zero_page_shrinker);
 err_hzp_shrinker:
+	unregister_shrinker(&huge_low_util_page_shrinker);
+err_low_util_shrinker:
 	khugepaged_destroy();
 err_slab:
 	hugepage_exit_sysfs(hugepage_kobj);
@@ -622,6 +675,7 @@ void prep_transhuge_page(struct page *page)
 	 */
 
 	INIT_LIST_HEAD(page_deferred_list(page));
+	INIT_LIST_HEAD(page_underutilized_thp_list(page));
 	set_compound_page_dtor(page, TRANSHUGE_PAGE_DTOR);
 }
 
@@ -2491,8 +2545,7 @@ static void __split_huge_page_tail(struct page *head, int tail,
 			 (1L << PG_dirty)));
 
 	/* ->mapping in first tail page is compound_mapcount */
-	VM_BUG_ON_PAGE(tail > 2 && page_tail->mapping != TAIL_MAPPING,
-			page_tail);
+	VM_BUG_ON_PAGE(tail > 3 && page_tail->mapping != TAIL_MAPPING, page_tail);
 	page_tail->mapping = head->mapping;
 	page_tail->index = head->index + tail;
 	page_tail->private = 0;
@@ -2698,6 +2751,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
 	struct folio *folio = page_folio(page);
 	struct page *head = &folio->page;
 	struct deferred_split *ds_queue = get_deferred_split_queue(head);
+	struct list_head *underutilized_thp_list = page_underutilized_thp_list(head);
 	XA_STATE(xas, &head->mapping->i_pages, head->index);
 	struct anon_vma *anon_vma = NULL;
 	struct address_space *mapping = NULL;
@@ -2796,6 +2850,8 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
 			list_del(page_deferred_list(head));
 		}
 		spin_unlock(&ds_queue->split_queue_lock);
+		if (!list_empty(underutilized_thp_list))
+			list_lru_del_page(&huge_low_util_page_lru, head, underutilized_thp_list);
 		if (mapping) {
 			int nr = thp_nr_pages(head);
 
@@ -2838,6 +2894,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
 void free_transhuge_page(struct page *page)
 {
 	struct deferred_split *ds_queue = get_deferred_split_queue(page);
+	struct list_head *underutilized_thp_list = page_underutilized_thp_list(page);
 	unsigned long flags;
 
 	spin_lock_irqsave(&ds_queue->split_queue_lock, flags);
@@ -2846,6 +2903,12 @@ void free_transhuge_page(struct page *page)
 		list_del(page_deferred_list(page));
 	}
 	spin_unlock_irqrestore(&ds_queue->split_queue_lock, flags);
+	if (!list_empty(underutilized_thp_list))
+		list_lru_del_page(&huge_low_util_page_lru, page, underutilized_thp_list);
+
+	if (PageLRU(page))
+		__clear_page_lru_flags(page);
+
 	free_compound_page(page);
 }
 
@@ -2886,6 +2949,26 @@ void deferred_split_huge_page(struct page *page)
 	spin_unlock_irqrestore(&ds_queue->split_queue_lock, flags);
 }
 
+void add_underutilized_thp(struct page *page)
+{
+	VM_BUG_ON_PAGE(!PageTransHuge(page), page);
+
+	if (PageSwapCache(page))
+		return;
+
+	/*
+	 * Need to take a reference on the page to prevent the page from getting free'd from
+	 * under us while we are adding the THP to the shrinker.
+	 */
+	if (!get_page_unless_zero(page))
+		return;
+
+	if (!is_huge_zero_page(page) && is_anon_transparent_hugepage(page))
+		list_lru_add_page(&huge_low_util_page_lru, page, page_underutilized_thp_list(page));
+
+	put_page(page);
+}
+
 static unsigned long deferred_split_count(struct shrinker *shrink,
 		struct shrink_control *sc)
 {
@@ -3424,6 +3507,9 @@ static void thp_util_scan(unsigned long pfn_end)
 		/* Group THPs into utilization buckets */
 		bucket = num_utilized_pages * THP_UTIL_BUCKET_NR / HPAGE_PMD_NR;
 		bucket = min(bucket, THP_UTIL_BUCKET_NR - 1);
+		if (bucket == 0)
+			add_underutilized_thp(page);
+
 		thp_scan.buckets[bucket].nr_thps++;
 		thp_scan.buckets[bucket].nr_zero_pages += (HPAGE_PMD_NR - num_utilized_pages);
 	}
diff --git a/mm/list_lru.c b/mm/list_lru.c
index a05e5bef3b40..7e8b324cc840 100644
--- a/mm/list_lru.c
+++ b/mm/list_lru.c
@@ -140,6 +140,32 @@ bool list_lru_add(struct list_lru *lru, struct list_head *item)
 }
 EXPORT_SYMBOL_GPL(list_lru_add);
 
+bool list_lru_add_page(struct list_lru *lru, struct page *page, struct list_head *item)
+{
+	int nid = page_to_nid(page);
+	struct list_lru_node *nlru = &lru->node[nid];
+	struct list_lru_one *l;
+	struct mem_cgroup *memcg;
+
+	spin_lock(&nlru->lock);
+	if (list_empty(item)) {
+		memcg = page_memcg(page);
+		memcg_list_lru_alloc(memcg, lru, GFP_KERNEL);
+		l = list_lru_from_memcg_idx(lru, nid, memcg_kmem_id(memcg));
+		list_add_tail(item, &l->list);
+		/* Set shrinker bit if the first element was added */
+		if (!l->nr_items++)
+			set_shrinker_bit(memcg, nid,
+					 lru_shrinker_id(lru));
+		nlru->nr_items++;
+		spin_unlock(&nlru->lock);
+		return true;
+	}
+	spin_unlock(&nlru->lock);
+	return false;
+}
+EXPORT_SYMBOL_GPL(list_lru_add_page);
+
 bool list_lru_del(struct list_lru *lru, struct list_head *item)
 {
 	int nid = page_to_nid(virt_to_page(item));
@@ -160,6 +186,29 @@ bool list_lru_del(struct list_lru *lru, struct list_head *item)
 }
 EXPORT_SYMBOL_GPL(list_lru_del);
 
+bool list_lru_del_page(struct list_lru *lru, struct page *page, struct list_head *item)
+{
+	int nid = page_to_nid(page);
+	struct list_lru_node *nlru = &lru->node[nid];
+	struct list_lru_one *l;
+	struct mem_cgroup *memcg;
+
+	spin_lock(&nlru->lock);
+	if (!list_empty(item)) {
+		memcg = page_memcg(page);
+		memcg_list_lru_alloc(memcg, lru, GFP_KERNEL);
+		l = list_lru_from_memcg_idx(lru, nid, memcg_kmem_id(memcg));
+		list_del_init(item);
+		l->nr_items--;
+		nlru->nr_items--;
+		spin_unlock(&nlru->lock);
+		return true;
+	}
+	spin_unlock(&nlru->lock);
+	return false;
+}
+EXPORT_SYMBOL_GPL(list_lru_del_page);
+
 void list_lru_isolate(struct list_lru_one *list, struct list_head *item)
 {
 	list_del_init(item);
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index e5486d47406e..a2a33b4d71db 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1327,6 +1327,12 @@ static int free_tail_pages_check(struct page *head_page, struct page *page)
 		 * deferred_list.next -- ignore value.
 		 */
 		break;
+	case 3:
+		/*
+		 * the third tail page: ->mapping is
+		 * underutilized_thp_list.next -- ignore value.
+		 */
+		break;
 	default:
 		if (page->mapping != TAIL_MAPPING) {
 			bad_page(page, "corrupted mapping in tail page");
-- 
2.30.2

Re: [RFC 3/3] mm: THP low utilization shrinker

[Zi Yan]

[-- Attachment #1: Type: text/plain, Size: 14751 bytes --]

On 25 Aug 2022, at 17:30, alexlzhu@fb.com wrote:

> From: Alexander Zhu <alexlzhu@fb.com>
>
> This patch introduces a shrinker that will remove THPs in the lowest
> utilization bucket. As previously mentioned, we have observed that
> almost all of the memory waste when THPs are always enabled
> is contained in the lowest utilization bucket. The shrinker will
> add these THPs to a list_lru and split anonymous THPs based off
> information from kswapd. It requires the changes from
> thp_utilization to identify the least utilized THPs, and the
> changes to split_huge_page to identify and free zero pages
> within THPs.

How stale could the information in the utilization bucket be? Is it
possible that THP shrinker splits a THP used to have a lot of
zero-filled subpages but now have all subpages filled with useful
values? In Patch 2, split_huge_page() only unmap zero-filled subpages,
but for THP shrinker, should it verify the utilization before it
splits the page?

>
> Signed-off-by: Alexander Zhu <alexlzhu@fb.com>
> ---
>  include/linux/huge_mm.h  |  7 +++
>  include/linux/list_lru.h | 24 +++++++++++
>  include/linux/mm_types.h |  5 +++
>  mm/huge_memory.c         | 92 ++++++++++++++++++++++++++++++++++++++--
>  mm/list_lru.c            | 49 +++++++++++++++++++++
>  mm/page_alloc.c          |  6 +++
>  6 files changed, 180 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index c9086239deb7..13bd470173d2 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -192,6 +192,8 @@ static inline int split_huge_page(struct page *page)
>  }
>  void deferred_split_huge_page(struct page *page);
>
> +void add_underutilized_thp(struct page *page);
> +
>  void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
>  		unsigned long address, bool freeze, struct folio *folio);
>
> @@ -302,6 +304,11 @@ static inline struct list_head *page_deferred_list(struct page *page)
>  	return &page[2].deferred_list;
>  }
>
> +static inline struct list_head *page_underutilized_thp_list(struct page *page)
> +{
> +	return &page[3].underutilized_thp_list;
> +}
> +
>  #else /* CONFIG_TRANSPARENT_HUGEPAGE */
>  #define HPAGE_PMD_SHIFT ({ BUILD_BUG(); 0; })
>  #define HPAGE_PMD_MASK ({ BUILD_BUG(); 0; })
> diff --git a/include/linux/list_lru.h b/include/linux/list_lru.h
> index b35968ee9fb5..c2cf146ea880 100644
> --- a/include/linux/list_lru.h
> +++ b/include/linux/list_lru.h
> @@ -89,6 +89,18 @@ void memcg_reparent_list_lrus(struct mem_cgroup *memcg, struct mem_cgroup *paren
>   */
>  bool list_lru_add(struct list_lru *lru, struct list_head *item);
>
> +/**
> + * list_lru_add_page: add an element to the lru list's tail
> + * @list_lru: the lru pointer
> + * @page: the page containing the item
> + * @item: the item to be deleted.
> + *
> + * This function works the same as list_lru_add in terms of list
> + * manipulation. Used for non slab objects contained in the page.
> + *
> + * Return value: true if the list was updated, false otherwise
> + */
> +bool list_lru_add_page(struct list_lru *lru, struct page *page, struct list_head *item);
>  /**
>   * list_lru_del: delete an element to the lru list
>   * @list_lru: the lru pointer
> @@ -102,6 +114,18 @@ bool list_lru_add(struct list_lru *lru, struct list_head *item);
>   */
>  bool list_lru_del(struct list_lru *lru, struct list_head *item);
>
> +/**
> + * list_lru_del_page: delete an element to the lru list
> + * @list_lru: the lru pointer
> + * @page: the page containing the item
> + * @item: the item to be deleted.
> + *
> + * This function works the same as list_lru_del in terms of list
> + * manipulation. Used for non slab objects contained in the page.
> + *
> + * Return value: true if the list was updated, false otherwise
> + */
> +bool list_lru_del_page(struct list_lru *lru, struct page *page, struct list_head *item);
>  /**
>   * list_lru_count_one: return the number of objects currently held by @lru
>   * @lru: the lru pointer.
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index cf97f3884fda..05667a2030c0 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -151,6 +151,11 @@ struct page {
>  			/* For both global and memcg */
>  			struct list_head deferred_list;
>  		};
> +		struct { /* Third tail page of compound page */
> +			unsigned long _compound_pad_3; /* compound_head */
> +			unsigned long _compound_pad_4;
> +			struct list_head underutilized_thp_list;
> +		};
>  		struct {	/* Page table pages */
>  			unsigned long _pt_pad_1;	/* compound_head */
>  			pgtable_t pmd_huge_pte; /* protected by page->ptl */
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 0f774a7c0727..03dc42eba0ba 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -8,6 +8,7 @@
>  #include <linux/mm.h>
>  #include <linux/sched.h>
>  #include <linux/sched/mm.h>
> +#include <linux/sched/clock.h>
>  #include <linux/sched/coredump.h>
>  #include <linux/sched/numa_balancing.h>
>  #include <linux/highmem.h>
> @@ -85,6 +86,8 @@ static atomic_t huge_zero_refcount;
>  struct page *huge_zero_page __read_mostly;
>  unsigned long huge_zero_pfn __read_mostly = ~0UL;
>
> +struct list_lru huge_low_util_page_lru;
> +
>  static void thp_utilization_workfn(struct work_struct *work);
>  static DECLARE_DELAYED_WORK(thp_utilization_work, thp_utilization_workfn);
>
> @@ -269,6 +272,46 @@ static struct shrinker huge_zero_page_shrinker = {
>  	.seeks = DEFAULT_SEEKS,
>  };
>
> +static enum lru_status low_util_free_page(struct list_head *item,
> +					  struct list_lru_one *lru,
> +					  spinlock_t *lock,
> +					  void *cb_arg)
> +{
> +	struct page *head = compound_head(list_entry(item,
> +									struct page,
> +									underutilized_thp_list));
> +
> +	if (get_page_unless_zero(head)) {
> +		lock_page(head);
> +		list_lru_isolate(lru, item);
> +		split_huge_page(head);
> +		unlock_page(head);
> +		put_page(head);
> +	}
> +
> +	return LRU_REMOVED_RETRY;
> +}
> +
> +static unsigned long shrink_huge_low_util_page_count(struct shrinker *shrink,
> +						     struct shrink_control *sc)
> +{
> +	return list_lru_shrink_count(&huge_low_util_page_lru, sc);
> +}
> +
> +static unsigned long shrink_huge_low_util_page_scan(struct shrinker *shrink,
> +						    struct shrink_control *sc)
> +{
> +	return list_lru_shrink_walk(&huge_low_util_page_lru, sc, low_util_free_page, NULL);
> +}
> +
> +static struct shrinker huge_low_util_page_shrinker = {
> +	.count_objects = shrink_huge_low_util_page_count,
> +	.scan_objects = shrink_huge_low_util_page_scan,
> +	.seeks = DEFAULT_SEEKS,
> +	.flags = SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_AWARE |
> +		SHRINKER_NONSLAB,
> +};
> +
>  #ifdef CONFIG_SYSFS
>  static ssize_t enabled_show(struct kobject *kobj,
>  			    struct kobj_attribute *attr, char *buf)
> @@ -521,13 +564,18 @@ static int __init hugepage_init(void)
>  		goto err_slab;
>
>  	schedule_delayed_work(&thp_utilization_work, HZ);
> +	err = register_shrinker(&huge_low_util_page_shrinker, "thp-low-util");
> +	if (err)
> +		goto err_low_util_shrinker;
>  	err = register_shrinker(&huge_zero_page_shrinker, "thp-zero");
>  	if (err)
>  		goto err_hzp_shrinker;
>  	err = register_shrinker(&deferred_split_shrinker, "thp-deferred_split");
>  	if (err)
>  		goto err_split_shrinker;
> -
> +	err = list_lru_init_memcg(&huge_low_util_page_lru, &huge_low_util_page_shrinker);
> +	if (err)
> +		goto err_low_util_list_lru;
>  	/*
>  	 * By default disable transparent hugepages on smaller systems,
>  	 * where the extra memory used could hurt more than TLB overhead
> @@ -543,11 +591,16 @@ static int __init hugepage_init(void)
>  		goto err_khugepaged;
>
>  	return 0;
> +
>  err_khugepaged:
> +	list_lru_destroy(&huge_low_util_page_lru);
> +err_low_util_list_lru:
>  	unregister_shrinker(&deferred_split_shrinker);
>  err_split_shrinker:
>  	unregister_shrinker(&huge_zero_page_shrinker);
>  err_hzp_shrinker:
> +	unregister_shrinker(&huge_low_util_page_shrinker);
> +err_low_util_shrinker:
>  	khugepaged_destroy();
>  err_slab:
>  	hugepage_exit_sysfs(hugepage_kobj);
> @@ -622,6 +675,7 @@ void prep_transhuge_page(struct page *page)
>  	 */
>
>  	INIT_LIST_HEAD(page_deferred_list(page));
> +	INIT_LIST_HEAD(page_underutilized_thp_list(page));
>  	set_compound_page_dtor(page, TRANSHUGE_PAGE_DTOR);
>  }
>
> @@ -2491,8 +2545,7 @@ static void __split_huge_page_tail(struct page *head, int tail,
>  			 (1L << PG_dirty)));
>
>  	/* ->mapping in first tail page is compound_mapcount */
> -	VM_BUG_ON_PAGE(tail > 2 && page_tail->mapping != TAIL_MAPPING,
> -			page_tail);
> +	VM_BUG_ON_PAGE(tail > 3 && page_tail->mapping != TAIL_MAPPING, page_tail);
>  	page_tail->mapping = head->mapping;
>  	page_tail->index = head->index + tail;
>  	page_tail->private = 0;
> @@ -2698,6 +2751,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
>  	struct folio *folio = page_folio(page);
>  	struct page *head = &folio->page;
>  	struct deferred_split *ds_queue = get_deferred_split_queue(head);
> +	struct list_head *underutilized_thp_list = page_underutilized_thp_list(head);
>  	XA_STATE(xas, &head->mapping->i_pages, head->index);
>  	struct anon_vma *anon_vma = NULL;
>  	struct address_space *mapping = NULL;
> @@ -2796,6 +2850,8 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
>  			list_del(page_deferred_list(head));
>  		}
>  		spin_unlock(&ds_queue->split_queue_lock);
> +		if (!list_empty(underutilized_thp_list))
> +			list_lru_del_page(&huge_low_util_page_lru, head, underutilized_thp_list);
>  		if (mapping) {
>  			int nr = thp_nr_pages(head);
>
> @@ -2838,6 +2894,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
>  void free_transhuge_page(struct page *page)
>  {
>  	struct deferred_split *ds_queue = get_deferred_split_queue(page);
> +	struct list_head *underutilized_thp_list = page_underutilized_thp_list(page);
>  	unsigned long flags;
>
>  	spin_lock_irqsave(&ds_queue->split_queue_lock, flags);
> @@ -2846,6 +2903,12 @@ void free_transhuge_page(struct page *page)
>  		list_del(page_deferred_list(page));
>  	}
>  	spin_unlock_irqrestore(&ds_queue->split_queue_lock, flags);
> +	if (!list_empty(underutilized_thp_list))
> +		list_lru_del_page(&huge_low_util_page_lru, page, underutilized_thp_list);
> +
> +	if (PageLRU(page))
> +		__clear_page_lru_flags(page);
> +
>  	free_compound_page(page);
>  }
>
> @@ -2886,6 +2949,26 @@ void deferred_split_huge_page(struct page *page)
>  	spin_unlock_irqrestore(&ds_queue->split_queue_lock, flags);
>  }
>
> +void add_underutilized_thp(struct page *page)
> +{
> +	VM_BUG_ON_PAGE(!PageTransHuge(page), page);
> +
> +	if (PageSwapCache(page))
> +		return;
> +
> +	/*
> +	 * Need to take a reference on the page to prevent the page from getting free'd from
> +	 * under us while we are adding the THP to the shrinker.
> +	 */
> +	if (!get_page_unless_zero(page))
> +		return;
> +
> +	if (!is_huge_zero_page(page) && is_anon_transparent_hugepage(page))
> +		list_lru_add_page(&huge_low_util_page_lru, page, page_underutilized_thp_list(page));
> +
> +	put_page(page);
> +}
> +
>  static unsigned long deferred_split_count(struct shrinker *shrink,
>  		struct shrink_control *sc)
>  {
> @@ -3424,6 +3507,9 @@ static void thp_util_scan(unsigned long pfn_end)
>  		/* Group THPs into utilization buckets */
>  		bucket = num_utilized_pages * THP_UTIL_BUCKET_NR / HPAGE_PMD_NR;
>  		bucket = min(bucket, THP_UTIL_BUCKET_NR - 1);
> +		if (bucket == 0)
> +			add_underutilized_thp(page);
> +
>  		thp_scan.buckets[bucket].nr_thps++;
>  		thp_scan.buckets[bucket].nr_zero_pages += (HPAGE_PMD_NR - num_utilized_pages);
>  	}
> diff --git a/mm/list_lru.c b/mm/list_lru.c
> index a05e5bef3b40..7e8b324cc840 100644
> --- a/mm/list_lru.c
> +++ b/mm/list_lru.c
> @@ -140,6 +140,32 @@ bool list_lru_add(struct list_lru *lru, struct list_head *item)
>  }
>  EXPORT_SYMBOL_GPL(list_lru_add);
>
> +bool list_lru_add_page(struct list_lru *lru, struct page *page, struct list_head *item)
> +{
> +	int nid = page_to_nid(page);
> +	struct list_lru_node *nlru = &lru->node[nid];
> +	struct list_lru_one *l;
> +	struct mem_cgroup *memcg;
> +
> +	spin_lock(&nlru->lock);
> +	if (list_empty(item)) {
> +		memcg = page_memcg(page);
> +		memcg_list_lru_alloc(memcg, lru, GFP_KERNEL);
> +		l = list_lru_from_memcg_idx(lru, nid, memcg_kmem_id(memcg));
> +		list_add_tail(item, &l->list);
> +		/* Set shrinker bit if the first element was added */
> +		if (!l->nr_items++)
> +			set_shrinker_bit(memcg, nid,
> +					 lru_shrinker_id(lru));
> +		nlru->nr_items++;
> +		spin_unlock(&nlru->lock);
> +		return true;
> +	}
> +	spin_unlock(&nlru->lock);
> +	return false;
> +}
> +EXPORT_SYMBOL_GPL(list_lru_add_page);
> +
>  bool list_lru_del(struct list_lru *lru, struct list_head *item)
>  {
>  	int nid = page_to_nid(virt_to_page(item));
> @@ -160,6 +186,29 @@ bool list_lru_del(struct list_lru *lru, struct list_head *item)
>  }
>  EXPORT_SYMBOL_GPL(list_lru_del);
>
> +bool list_lru_del_page(struct list_lru *lru, struct page *page, struct list_head *item)
> +{
> +	int nid = page_to_nid(page);
> +	struct list_lru_node *nlru = &lru->node[nid];
> +	struct list_lru_one *l;
> +	struct mem_cgroup *memcg;
> +
> +	spin_lock(&nlru->lock);
> +	if (!list_empty(item)) {
> +		memcg = page_memcg(page);
> +		memcg_list_lru_alloc(memcg, lru, GFP_KERNEL);
> +		l = list_lru_from_memcg_idx(lru, nid, memcg_kmem_id(memcg));
> +		list_del_init(item);
> +		l->nr_items--;
> +		nlru->nr_items--;
> +		spin_unlock(&nlru->lock);
> +		return true;
> +	}
> +	spin_unlock(&nlru->lock);
> +	return false;
> +}
> +EXPORT_SYMBOL_GPL(list_lru_del_page);
> +
>  void list_lru_isolate(struct list_lru_one *list, struct list_head *item)
>  {
>  	list_del_init(item);
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index e5486d47406e..a2a33b4d71db 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1327,6 +1327,12 @@ static int free_tail_pages_check(struct page *head_page, struct page *page)
>  		 * deferred_list.next -- ignore value.
>  		 */
>  		break;
> +	case 3:
> +		/*
> +		 * the third tail page: ->mapping is
> +		 * underutilized_thp_list.next -- ignore value.
> +		 */
> +		break;
>  	default:
>  		if (page->mapping != TAIL_MAPPING) {
>  			bad_page(page, "corrupted mapping in tail page");
> -- 
> 2.30.2


--
Best Regards,
Yan, Zi

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Re: [RFC 3/3] mm: THP low utilization shrinker

[Alex Zhu (Kernel)]

> How stale could the information in the utilization bucket be?

The staleness would be capped by the duration of the scan, 70s in in the 
example.  

> Is it possible that THP shrinker splits a THP used to have a lot of
> zero-filled subpages but now have all subpages filled with useful
> values?

This is possible, but we free only the zero pages, which cannot have
any useful values. How often it happens that THPs move utilization buckets 
should be workload dependent. 

> In Patch 2, split_huge_page() only unmap zero-filled subpages,
> but for THP shrinker, should it verify the utilization before it
> splits the page?

I think we should add this check to verify that it is still in the lowest bucket before having the
shrinker split the page. The utilization could have changed and this way we do not
need to worry about workloads where THPs move utilization buckets. Thanks!