In case if the file already has underlying blocks/extents allocated
then we don't need to start a journal txn and can directly return
the underlying mapping. Currently ext4_iomap_begin() is used by
both DAX & DIO path. We can check if the write request is an
overwrite & then directly return the mapping information.
This could give a significant perf boost for multi-threaded writes
specially random overwrites.
On PPC64 VM with simulated pmem(DAX) device, ~10x perf improvement
could be seen in random writes (overwrite). Also bcoz this optimizes
away the spinlock contention during jbd2 slab cache allocation
(jbd2_journal_handle). On x86 VM, ~2x perf improvement was observed.
Reported-by: Dan Williams <dan.j.williams@xxxxxxxxx>
Suggested-by: Jan Kara <jack@xxxxxxx>
Signed-off-by: Ritesh Harjani <riteshh@xxxxxxxxxxxxx>
---
fs/ext4/inode.c | 18 +++++++++++++++---
1 file changed, 15 insertions(+), 3 deletions(-)
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 10dd470876b3..6eae17758ece 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -3437,14 +3437,26 @@ static int ext4_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
map.m_len = min_t(loff_t, (offset + length - 1) >> blkbits,
EXT4_MAX_LOGICAL_BLOCK) - map.m_lblk + 1;
- if (flags & IOMAP_WRITE)
+ if (flags & IOMAP_WRITE) {
+ /*
+ * We check here if the blocks are already allocated, then we
+ * don't need to start a journal txn and we can directly return
+ * the mapping information. This could boost performance
+ * especially in multi-threaded overwrite requests.
+ */
+ if (offset + length <= i_size_read(inode)) {
+ ret = ext4_map_blocks(NULL, inode, &map, 0);
+ if (ret > 0 && (map.m_flags & EXT4_MAP_MAPPED))
+ goto out;
+ }
ret = ext4_iomap_alloc(inode, &map, flags);
- else
+ } else {
ret = ext4_map_blocks(NULL, inode, &map, 0);
+ }
if (ret < 0)
return ret;
-
+out:
ext4_set_iomap(inode, iomap, &map, offset, length);
return 0;
--
2.26.2
