mm/filemap: optimize folio adding and splitting

From: Kairui Song <[email protected]>

Currently, at least 3 tree walks are needed for filemap folio adding if the folio is previously evicted. One for getting the order of current slot, one for ranged conflict check, and one for another order retrieving. If a split is needed, more walks are needed.

This series is trying to merge these walks, and speed up filemap_add_folio, I see a 7.5% - 12.5% performance gain for fio stress test.

So instead of doing multiple tree walks, do one optimism range check with lock hold, and exit if raced with another insertion. If a shadow exists, check it with a new xas_get_order helper before releasing the lock to avoid redundant tree walks for getting its order.

Drop the lock and do the allocation only if a split is needed.

In the best case, it only need to walk the tree once. If it needs to alloc and split, 3 walks are issued (One for first ranged conflict check and order retrieving, one for the second check after allocation, one for the insert after split).

Testing with 4K pages, in an 8G cgroup, with 16G brd as block device:

echo 3 > /proc/sys/vm/drop_caches

  fio -name=cached --numjobs=16 --filename=/mnt/test.img \
    --buffered=1 --ioengine=mmap --rw=randread --time_based \
    --ramp_time=30s --runtime=5m --group_reporting

Before:
bw ( MiB/s): min= 1027, max= 3520, per=100.00%, avg=2445.02, stdev=18.90, samples=8691
iops : min=263001, max=901288, avg=625924.36, stdev=4837.28, samples=8691

After (+7.3%):
bw ( MiB/s): min= 493, max= 3947, per=100.00%, avg=2625.56, stdev=25.74, samples=8651
iops : min=126454, max=1010681, avg=672142.61, stdev=6590.48, samples=8651

Test result with THP (do a THP randread then switch to 4K page in hope it issues a lot of splitting):

echo 3 > /proc/sys/vm/drop_caches

  fio -name=cached --numjobs=16 --filename=/mnt/test.img \
      --buffered=1 --ioengine=mmap -thp=1 --readonly \
      --rw=randread --time_based --ramp_time=30s --runtime=10m \
      --group_reporting

  fio -name=cached --numjobs=16 --filename=/mnt/test.img \
      --buffered=1 --ioengine=mmap \
      --rw=randread --time_based --runtime=5s --group_reporting

Before:
bw ( KiB/s): min= 4141, max=14202, per=100.00%, avg=7935.51, stdev=96.85, samples=18976
iops : min= 1029, max= 3548, avg=1979.52, stdev=24.23, samples=18976·

READ: bw=4545B/s (4545B/s), 4545B/s-4545B/s (4545B/s-4545B/s), io=64.0KiB (65.5kB), run=14419-14419msec

After (+10.4%):
bw ( KiB/s): min= 4611, max=15370, per=100.00%, avg=8928.74, stdev=105.17, samples=19146
iops : min= 1151, max= 3842, avg=2231.27, stdev=26.29, samples=19146

READ: bw=4635B/s (4635B/s), 4635B/s-4635B/s (4635B/s-4635B/s), io=64.0KiB (65.5kB), run=14137-14137msec

The performance is better for both 4K (+7.5%) and THP (+12.5%) cached read.

V2: https://lore.kernel.org/lkml/[email protected]/
Updates from V2:
- Fix the misusage of locks in test module:
https://lore.kernel.org/oe-lkp/[email protected]

V1: https://lore.kernel.org/lkml/[email protected]/
Updates from V1:
- Collect Acks.
- Add tests for new xas_get_order and combined usage of xas_get_order with xas_for_each_conflict.
- Fix a memleak for patch 4/4 and modify the function in place instead of adding a new helper.
- Update benchmark, I forgot to drop cache and disable THP for pervious test, so the result was for mixed usaged of split and add. The result is even better now.

Kairui Song (4):
mm/filemap: return early if failed to allocate memory for split mm/filemap: clean up hugetlb exclusion code lib/xarray: introduce a new helper xas_get_order mm/filemap: optimize filemap folio adding

 include/linux/xarray.h |  6 +++
 lib/test_xarray.c      | 93 ++++++++++++++++++++++++++++++++++++++++++
 lib/xarray.c           | 49 ++++++++++++++--------
 mm/filemap.c           | 74 +++++++++++++++++++++------------
 4 files changed, 179 insertions(+), 43 deletions(-)

2.44.0

From: Kairui Song <[email protected]>

xas_split_alloc could fail with NOMEM, and in such case, it should abort early instead of keep going and fail the xas_split below.

---
 mm/filemap.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

index 480ae6589803..12089c24abfb 100644
@@ -880,9 +880,12 @@ noinline int __filemap_add_folio(struct address_space *mapping,
 		unsigned int order = xa_get_order(xas.xa, xas.xa_index);
 		void *entry, *old = NULL;
 
-		if (order > folio_order(folio))
+		if (order > folio_order(folio)) {
 			xas_split_alloc(&xas, xa_load(xas.xa, xas.xa_index),
 					order, gfp);
+			if (xas_error(&xas))
+				goto error;
+		}
 		xas_lock_irq(&xas);
 		xas_for_each_conflict(&xas, entry) {
 			old = entry;

2.44.0

From: Kairui Song <[email protected]>

__filemap_add_folio only has two callers, one never passes hugetlb folio and one always passes in hugetlb folio. So move the hugetlb related cgroup charging out of it to make the code cleaner.

---
 mm/filemap.c | 21 ++++++++-------------
 1 file changed, 8 insertions(+), 13 deletions(-)

index 12089c24abfb..17a66ea544e7 100644
@@ -853,20 +853,12 @@ noinline int __filemap_add_folio(struct address_space *mapping,
 {
 	XA_STATE(xas, &mapping->i_pages, index);
 	bool huge = folio_test_hugetlb(folio);
-	bool charged = false;
-	long nr = 1;
+	long nr;
 
 	VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
 	VM_BUG_ON_FOLIO(folio_test_swapbacked(folio), folio);
 	mapping_set_update(&xas, mapping);
 
-	if (!huge) {
-		int error = mem_cgroup_charge(folio, NULL, gfp);
-		if (error)
-			return error;
-		charged = true;
-	}
-
 	VM_BUG_ON_FOLIO(index & (folio_nr_pages(folio) - 1), folio);
 	xas_set_order(&xas, index, folio_order(folio));
 	nr = folio_nr_pages(folio);

@@ -931,8 +923,6 @@ noinline int __filemap_add_folio(struct address_space *mapping,
 	trace_mm_filemap_add_to_page_cache(folio);
 	return 0;
 error:
-	if (charged)
-		mem_cgroup_uncharge(folio);
 	folio->mapping = NULL;
 	/* Leave page->index set: truncation relies upon it */
 	folio_put_refs(folio, nr);

@@ -946,11 +936,16 @@ int filemap_add_folio(struct address_space *mapping, struct folio *folio,
 	void *shadow = NULL;
 	int ret;
 
+	ret = mem_cgroup_charge(folio, NULL, gfp);
+	if (ret)
+		return ret;
+
 	__folio_set_locked(folio);
 	ret = __filemap_add_folio(mapping, folio, index, gfp, &shadow);
-	if (unlikely(ret))
+	if (unlikely(ret)) {
+		mem_cgroup_uncharge(folio);
 		__folio_clear_locked(folio);
-	else {
+	} else {
 		/*
 		 * The folio might have been evicted from cache only
 		 * recently, in which case it should be activated like

2.44.0

From: Kairui Song <[email protected]>

It can be used after xas_load to check the order of loaded entries. Compared to xa_get_order, it saves an XA_STATE and avoid a rewalk.

Also add new test for xas_get_order, to make the test work we have to export xas_get_order with EXPORT_SYMBOL_GPL.

---
 include/linux/xarray.h |  6 ++++++
 lib/test_xarray.c      | 34 +++++++++++++++++++++++++++++
 lib/xarray.c           | 49 ++++++++++++++++++++++++++----------------
 3 files changed, 71 insertions(+), 18 deletions(-)

index cb571dfcf4b1..d9d479334c9e 100644
@@ -1548,6 +1548,7 @@ void xas_create_range(struct xa_state *);
 
 #ifdef CONFIG_XARRAY_MULTI
 int xa_get_order(struct xarray *, unsigned long index);
+int xas_get_order(struct xa_state *xas);
 void xas_split(struct xa_state *, void *entry, unsigned int order);
 void xas_split_alloc(struct xa_state *, void *entry, unsigned int order, gfp_t);
 #else

@@ -1556,6 +1557,11 @@ static inline int xa_get_order(struct xarray *xa, unsigned long index)
 	return 0;
 }
 
+static inline int xas_get_order(struct xa_state *xas)
+{
+	return 0;
+}
+
 static inline void xas_split(struct xa_state *xas, void *entry,
 		unsigned int order)
 {

index ebe2af2e072d..0efde8f93490 100644
@@ -1984,6 +1984,39 @@ static noinline void check_get_order(struct xarray *xa)
 	}
 }
 
+static noinline void check_xas_get_order(struct xarray *xa)
+{
+	XA_STATE(xas, xa, 0);
+
+	unsigned int max_order = IS_ENABLED(CONFIG_XARRAY_MULTI) ? 20 : 1;
+	unsigned int order;
+	unsigned long i, j;
+
+	for (order = 0; order < max_order; order++) {
+		for (i = 0; i < 10; i++) {
+			xas_set_order(&xas, i << order, order);
+			do {
+				xas_lock(&xas);
+				xas_store(&xas, xa_mk_value(i));
+				xas_unlock(&xas);
+			} while (xas_nomem(&xas, GFP_KERNEL));
+
+			for (j = i << order; j < (i + 1) << order; j++) {
+				xas_set_order(&xas, j, 0);
+				rcu_read_lock();
+				xas_load(&xas);
+				XA_BUG_ON(xa, xas_get_order(&xas) != order);
+				rcu_read_unlock();
+			}
+
+			xas_lock(&xas);
+			xas_set_order(&xas, i << order, order);
+			xas_store(&xas, NULL);
+			xas_unlock(&xas);
+		}
+	}
+}
+
 static noinline void check_destroy(struct xarray *xa)
 {
 	unsigned long index;

@@ -2035,6 +2068,7 @@ static int xarray_checks(void)
 	check_multi_store(&array);
 	check_multi_store_advanced(&array);
 	check_get_order(&array);
+	check_xas_get_order(&array);
 	check_xa_alloc();
 	check_find(&array);
 	check_find_entry(&array);

2.44.0

I'd just leave that as "Get the order of an entry." as it doesn't have to be after calling xas_load(), it could be from any other operation that moves xas. Also this is the short description!

Good suggestion!

OK.

From: Kairui Song <[email protected]>

Instead of doing multiple tree walks, do one optimism range check with lock hold, and exit if raced with another insertion. If a shadow exists, check it with a new xas_get_order helper before releasing the lock to avoid redundant tree walks for getting its order.

Drop the lock and do the allocation only if a split is needed.

In the best case, it only need to walk the tree once. If it needs to alloc and split, 3 walks are issued (One for first ranged conflict check and order retrieving, one for the second check after allocation, one for the insert after split).

Testing with 4K pages, in an 8G cgroup, with 16G brd as block device:

echo 3 > /proc/sys/vm/drop_caches

  fio -name=cached --numjobs=16 --filename=/mnt/test.img \
    --buffered=1 --ioengine=mmap --rw=randread --time_based \
    --ramp_time=30s --runtime=5m --group_reporting

Before:
bw ( MiB/s): min= 1027, max= 3520, per=100.00%, avg=2445.02, stdev=18.90, samples=8691
iops : min=263001, max=901288, avg=625924.36, stdev=4837.28, samples=8691

After (+7.3%):
bw ( MiB/s): min= 493, max= 3947, per=100.00%, avg=2625.56, stdev=25.74, samples=8651
iops : min=126454, max=1010681, avg=672142.61, stdev=6590.48, samples=8651

Test result with THP (do a THP randread then switch to 4K page in hope it issues a lot of splitting):

echo 3 > /proc/sys/vm/drop_caches

  fio -name=cached --numjobs=16 --filename=/mnt/test.img \
      --buffered=1 --ioengine=mmap -thp=1 --readonly \
      --rw=randread --time_based --ramp_time=30s --runtime=10m \
      --group_reporting

  fio -name=cached --numjobs=16 --filename=/mnt/test.img \
      --buffered=1 --ioengine=mmap \
      --rw=randread --time_based --runtime=5s --group_reporting

Before:
bw ( KiB/s): min= 4141, max=14202, per=100.00%, avg=7935.51, stdev=96.85, samples=18976
iops : min= 1029, max= 3548, avg=1979.52, stdev=24.23, samples=18976·

READ: bw=4545B/s (4545B/s), 4545B/s-4545B/s (4545B/s-4545B/s), io=64.0KiB (65.5kB), run=14419-14419msec

After (+12.5%):
bw ( KiB/s): min= 4611, max=15370, per=100.00%, avg=8928.74, stdev=105.17, samples=19146
iops : min= 1151, max= 3842, avg=2231.27, stdev=26.29, samples=19146

READ: bw=4635B/s (4635B/s), 4635B/s-4635B/s (4635B/s-4635B/s), io=64.0KiB (65.5kB), run=14137-14137msec

The performance is better for both 4K (+7.5%) and THP (+12.5%) cached read.

---
 lib/test_xarray.c | 59 +++++++++++++++++++++++++++++++++++++++++++++++
 mm/filemap.c      | 56 ++++++++++++++++++++++++++++++++------------
 2 files changed, 100 insertions(+), 15 deletions(-)

index 0efde8f93490..8732a311f613 100644
@@ -2017,6 +2017,64 @@ static noinline void check_xas_get_order(struct xarray *xa)
 	}
 }
 
+static noinline void check_xas_conflict_get_order(struct xarray *xa)
+{
+	XA_STATE(xas, xa, 0);
+
+	void *entry;
+	int only_once;
+	unsigned int max_order = IS_ENABLED(CONFIG_XARRAY_MULTI) ? 20 : 1;
+	unsigned int order;
+	unsigned long i, j, k;
+
+	for (order = 0; order < max_order; order++) {
+		for (i = 0; i < 10; i++) {
+			xas_set_order(&xas, i << order, order);
+			do {
+				xas_lock(&xas);
+				xas_store(&xas, xa_mk_value(i));
+				xas_unlock(&xas);
+			} while (xas_nomem(&xas, GFP_KERNEL));
+
+			/*
+			 * Ensure xas_get_order works with xas_for_each_conflict.
+			 */
+			j = i << order;
+			for (k = 0; k < order; k++) {
+				only_once = 0;
+				xas_set_order(&xas, j + (1 << k), k);
+				xas_lock(&xas);
+				xas_for_each_conflict(&xas, entry) {
+					XA_BUG_ON(xa, entry != xa_mk_value(i));
+					XA_BUG_ON(xa, xas_get_order(&xas) != order);
+					only_once++;
+				}
+				XA_BUG_ON(xa, only_once != 1);
+				xas_unlock(&xas);
+			}
+
+			if (order < max_order - 1) {
+				only_once = 0;
+				xas_set_order(&xas, (i & ~1UL) << order, order + 1);
+				xas_lock(&xas);
+				xas_for_each_conflict(&xas, entry) {
+					XA_BUG_ON(xa, entry != xa_mk_value(i));
+					XA_BUG_ON(xa, xas_get_order(&xas) != order);
+					only_once++;
+				}
+				XA_BUG_ON(xa, only_once != 1);
+				xas_unlock(&xas);
+			}
+
+			xas_set_order(&xas, i << order, order);
+			xas_lock(&xas);
+			xas_store(&xas, NULL);
+			xas_unlock(&xas);
+		}
+	}
+}
+
+
 static noinline void check_destroy(struct xarray *xa)
 {
 	unsigned long index;

@@ -2069,6 +2127,7 @@ static int xarray_checks(void)
 	check_multi_store_advanced(&array);
 	check_get_order(&array);
 	check_xas_get_order(&array);
+	check_xas_conflict_get_order(&array);
 	check_xa_alloc();
 	check_find(&array);
 	check_find_entry(&array);

2.44.0