[PATCH 3/8] mm, truncate: Remove all exceptional entries from pagevec under one lock

Mel Gorman
2017-10-12T09:31:50Z

During truncate each entry in a pagevec is checked to see if it is an
exceptional entry and if so, the shadow entry is cleaned up. This is
potentially expensive as multiple entries for a mapping locks/unlocks the
tree lock. This batches the operation such that any exceptional entries
removed from a pagevec only acquire the mapping tree lock once. The corner
case where this is more expensive is where there is only one exceptional
entry but this is unlikely due to temporal locality and how it affects
LRU ordering. Note that for truncations of small files created recently,
this patch should show no gain because it only batches the handling of
exceptional entries.

sparsetruncate (large)
4.14.0-rc4 4.14.0-rc4
pickhelper-v1r1 batchshadow-v1r1
Min Time 38.00 ( 0.00%) 27.00 ( 28.95%)
1st-qrtle Time 40.00 ( 0.00%) 28.00 ( 30.00%)
2nd-qrtle Time 44.00 ( 0.00%) 41.00 ( 6.82%)
3rd-qrtle Time 146.00 ( 0.00%) 147.00 ( -0.68%)
Max-90% Time 153.00 ( 0.00%) 153.00 ( 0.00%)
Max-95% Time 155.00 ( 0.00%) 156.00 ( -0.65%)
Max-99% Time 181.00 ( 0.00%) 171.00 ( 5.52%)
Amean Time 93.04 ( 0.00%) 88.43 ( 4.96%)
Best99%Amean Time 92.08 ( 0.00%) 86.13 ( 6.46%)
Best95%Amean Time 89.19 ( 0.00%) 83.13 ( 6.80%)
Best90%Amean Time 85.60 ( 0.00%) 79.15 ( 7.53%)
Best75%Amean Time 72.95 ( 0.00%) 65.09 ( 10.78%)
Best50%Amean Time 39.86 ( 0.00%) 28.20 ( 29.25%)
Best25%Amean Time 39.44 ( 0.00%) 27.70 ( 29.77%)

bonnie
4.14.0-rc4 4.14.0-rc4
pickhelper-v1r1 batchshadow-v1r1
Hmean SeqCreate ops 71.92 ( 0.00%) 76.78 ( 6.76%)
Hmean SeqCreate read 42.42 ( 0.00%) 45.01 ( 6.10%)
Hmean SeqCreate del 26519.88 ( 0.00%) 27191.87 ( 2.53%)
Hmean RandCreate ops 71.92 ( 0.00%) 76.95 ( 7.00%)
Hmean RandCreate read 44.44 ( 0.00%) 49.23 ( 10.78%)
Hmean RandCreate del 24948.62 ( 0.00%) 24764.97 ( -0.74%)

Truncation of a large number of files shows a substantial gain with 99% of files
being trruncated 6.46% faster. bonnie shows a modest gain of 2.53%

Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
---
mm/truncate.c | 86 ++++++++++++++++++++++++++++++++++++++++++-----------------
1 file changed, 61 insertions(+), 25 deletions(-)

diff --git a/mm/truncate.c b/mm/truncate.c
index 3dfa2d5e642e..af1eaa5b9450 100644
--- a/mm/truncate.c
+++ b/mm/truncate.c
@@ -25,44 +25,77 @@
#include <linux/rmap.h>
#include "internal.h"

-static void clear_shadow_entry(struct address_space *mapping, pgoff_t index,
- void *entry)
+/*
+ * Regular page slots are stabilized by the page lock even without the tree
+ * itself locked. These unlocked entries need verification under the tree
+ * lock.
+ */
+static inline void __clear_shadow_entry(struct address_space *mapping,
+ pgoff_t index, void *entry)
{
struct radix_tree_node *node;
void **slot;

- spin_lock_irq(&mapping->tree_lock);
- /*
- * Regular page slots are stabilized by the page lock even
- * without the tree itself locked. These unlocked entries
- * need verification under the tree lock.
- */
if (!__radix_tree_lookup(&mapping->page_tree, index, &node, &slot))
- goto unlock;
+ return;
if (*slot != entry)
- goto unlock;
+ return;
__radix_tree_replace(&mapping->page_tree, node, slot, NULL,
workingset_update_node, mapping);
mapping->nrexceptional--;
-unlock:
+}
+
+static void clear_shadow_entry(struct address_space *mapping, pgoff_t index,
+ void *entry)
+{
+ spin_lock_irq(&mapping->tree_lock);
+ __clear_shadow_entry(mapping, index, entry);
spin_unlock_irq(&mapping->tree_lock);
}

/*
- * Unconditionally remove exceptional entry. Usually called from truncate path.
+ * Unconditionally remove exceptional entries. Usually called from truncate
+ * path. Note that the pagevec may be altered by this function by removing
+ * exceptional entries similar to what pagevec_remove_exceptionals does.
*/
-static void truncate_exceptional_entry(struct address_space *mapping,
- pgoff_t index, void *entry)
+static void truncate_exceptional_pvec_entries(struct address_space *mapping,
+ struct pagevec *pvec, pgoff_t *indices, int ei)
{
+ int i, j;
+ bool dax;
+
+ /* Return immediately if caller indicates there are no entries */
+ if (ei == PAGEVEC_SIZE)
+ return;
+
/* Handled by shmem itself */
if (shmem_mapping(mapping))
return;

- if (dax_mapping(mapping)) {
- dax_delete_mapping_entry(mapping, index);
- return;
+ dax = dax_mapping(mapping);
+ if (!dax)
+ spin_lock_irq(&mapping->tree_lock);
+
+ for (i = ei, j = ei; i < pagevec_count(pvec); i++) {
+ struct page *page = pvec->pages[i];
+ pgoff_t index = indices[i];
+
+ if (!radix_tree_exceptional_entry(page)) {
+ pvec->pages[j++] = page;
+ continue;
+ }
+
+ if (unlikely(dax)) {
+ dax_delete_mapping_entry(mapping, index);
+ continue;
+ }
+
+ __clear_shadow_entry(mapping, index, page);
}
- clear_shadow_entry(mapping, index, entry);
+
+ if (!dax)
+ spin_unlock_irq(&mapping->tree_lock);
+ pvec->nr = j;
}

/*
@@ -301,6 +334,7 @@ void truncate_inode_pages_range(struct address_space *mapping,
*/
struct page *pages[PAGEVEC_SIZE];
int batch_count = 0;
+ int ei = PAGEVEC_SIZE;

for (i = 0; i < pagevec_count(&pvec); i++) {
struct page *page = pvec.pages[i];
@@ -311,8 +345,8 @@ void truncate_inode_pages_range(struct address_space *mapping,
break;

if (radix_tree_exceptional_entry(page)) {
- truncate_exceptional_entry(mapping, index,
- page);
+ if (ei == PAGEVEC_SIZE)
+ ei = i;
continue;
}

@@ -334,12 +368,11 @@ void truncate_inode_pages_range(struct address_space *mapping,
delete_from_page_cache_batch(mapping, batch_count, pages);
for (i = 0; i < batch_count; i++)
unlock_page(pages[i]);
- pagevec_remove_exceptionals(&pvec);
+ truncate_exceptional_pvec_entries(mapping, &pvec, indices, ei);
pagevec_release(&pvec);
cond_resched();
index++;
}
-
if (partial_start) {
struct page *page = find_lock_page(mapping, start - 1);
if (page) {
@@ -381,6 +414,8 @@ void truncate_inode_pages_range(struct address_space *mapping,

index = start;
for ( ; ; ) {
+ int ei = PAGEVEC_SIZE;
+
cond_resched();
if (!pagevec_lookup_entries(&pvec, mapping, index,
min(end - index, (pgoff_t)PAGEVEC_SIZE), indices)) {
@@ -397,6 +432,7 @@ void truncate_inode_pages_range(struct address_space *mapping,
pagevec_release(&pvec);
break;
}
+
for (i = 0; i < pagevec_count(&pvec); i++) {
struct page *page = pvec.pages[i];

@@ -409,8 +445,8 @@ void truncate_inode_pages_range(struct address_space *mapping,
}

if (radix_tree_exceptional_entry(page)) {
- truncate_exceptional_entry(mapping, index,
- page);
+ if (ei != PAGEVEC_SIZE)
+ ei = i;
continue;
}

@@ -420,7 +456,7 @@ void truncate_inode_pages_range(struct address_space *mapping,
truncate_inode_page(mapping, page);
unlock_page(page);
}
- pagevec_remove_exceptionals(&pvec);
+ truncate_exceptional_pvec_entries(mapping, &pvec, indices, ei);
pagevec_release(&pvec);
index++;
}
--
2.14.0