Re: [PATCH] ext4: do not mark inode dirty which is already dirtied under append-write scenario

From: Liu Song
Date: Thu Aug 10 2023 - 02:59:40 EST


On 2023/8/9 20:25, Jan Kara wrote:
On Tue 11-07-23 11:42:56, Liu Song wrote:
In the append-write scenario, after ensuring that the dirty inode can be
seen by the writeback process, there is no need to execute
"mark_inode_dirty" for every write. Instead, we can rely on
"ext4_mark_inode_dirty" executed when updating i_disksize in
"mpage_map_and_submit_extent" to ensure data consistency, which can
significantly improve performance in high-frequency append-write
scenarios.

In test scenarios of Kafka version 2.6.2, using packet size of 2K
resulted in a 10% performance improvement.

Signed-off-by: Liu Song <liusong@xxxxxxxxxxxxxxxxx>
Overall the benefit looks interesting. Nice work.

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 9d9f414f99fe..d1aa775c9936 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -3128,6 +3128,57 @@ static int ext4_da_should_update_i_disksize(struct page *page,
return 1;
}
+/*
+ * Copy from generic_write_end, add conditions to execute mark_inode_dirty
+ * to avoid additional overhead caused by frequent dirty inode operations
+ */
+static int ext4_da_generic_write_end(struct file *file, struct address_space *mapping,
+ loff_t pos, unsigned len, unsigned copied, bool need_dirty,
+ struct page *page, void *fsdata)
+{
+ struct inode *inode = mapping->host;
+ loff_t old_size = inode->i_size;
+ bool i_size_changed = false;
+ int was_dirty;
+
+ copied = block_write_end(file, mapping, pos, len, copied, page, fsdata);
+
+ /*
+ * No need to use i_size_read() here, the i_size cannot change under us
+ * because we hold i_rwsem.
+ *
+ * But it's important to update i_size while still holding page lock:
+ * page writeout could otherwise come in and zero beyond i_size.
+ */
+ if (pos + copied > inode->i_size) {
+ i_size_write(inode, pos + copied);
+ i_size_changed = true;
+ }
+
+ unlock_page(page);
+ put_page(page);
+
+ if (old_size < pos)
+ pagecache_isize_extended(inode, old_size, pos);
+
I dislike the duplication of generic_write_end() but exporting a variant of
generic_write_end() not doing the dirtying (and additionally returning the
i_size_changed value) doesn't look appealing either. So I guess I'll just
live with this...

+ /*
+ * In the append-write scenario, if the inode is marked as dirty,
+ * it is ensured that the inode will be seen by the writeback process.
+ * In the ext4_writepages process, when updating i_disksize,
+ * corresponding metadata updates are also performed.
+ * Therefore, it is unnecessary to repeatedly execute mark_inode_dirty
+ * to improve performance.
+ */
Note that block_write_end() will mark the inode as dirty with I_DIRTY_PAGES
flag (which all that's needed to trigger page writeback). Since i_size is
never really written to disk by ext4, there's simply never a need to mark
inode dirty because of that. Also if inode timestamps were updated inode
was already properly marked dirty. Hence we can just mostly drop the code
below.

+ if (i_size_changed) {
+ spin_lock(&inode->i_lock);
+ was_dirty = inode->i_state & I_DIRTY;
+ spin_unlock(&inode->i_lock);
+ if (!was_dirty || need_dirty)
+ mark_inode_dirty(inode);
+ }
+ return copied;
+}
+
static int ext4_da_write_end(struct file *file,
struct address_space *mapping,
loff_t pos, unsigned len, unsigned copied,
@@ -3137,6 +3188,7 @@ static int ext4_da_write_end(struct file *file,
loff_t new_i_size;
unsigned long start, end;
int write_mode = (int)(unsigned long)fsdata;
+ bool need_dirty = false;
if (write_mode == FALL_BACK_TO_NONDELALLOC)
return ext4_write_end(file, mapping, pos,
@@ -3169,10 +3221,12 @@ static int ext4_da_write_end(struct file *file,
*/
new_i_size = pos + copied;
if (copied && new_i_size > inode->i_size &&
- ext4_da_should_update_i_disksize(page, end))
+ ext4_da_should_update_i_disksize(page, end)) {
ext4_update_i_disksize(inode, new_i_size);
+ need_dirty = true;
+ }


Thank you very much for your response, and the implementation of "ext4_da_do_write_end" is great.
I will refer to your suggestions and send a V2 version of the patch.

Thanks


So when we create our own new helper function anyway I'd just move all this
logic for delalloc write end there. Something like:

static int ext4_da_do_write_end(struct address_space *mapping,
loff_t pos, unsigned len, unsigned copied,
struct page *page)
{
struct inode *inode = mapping->host;
loff_t old_size = inode->i_size;
bool disksize_changed = false;
loff_t new_i_size;

copied = block_write_end(NULL, mapping, pos, len, copied, page, NULL);

new_i_size = pos + copied;
/*
* It's important to update i_size while still holding page lock:
* page writeout could otherwise come in and zero beyond i_size.
*
* Since we are holding inode lock, we are sure i_disksize <=
* i_size. We also know that if i_disksize < i_size, there are
* delalloc writes pending in the range upto i_size. If the end of
* the current write is <= i_size, there's no need to touch
* i_disksize since writeback will push i_disksize upto i_size
* eventually. If the end of the current write is > i_size and
* inside an allocated block (ext4_da_should_update_i_disksize()
* check), we need to update i_disksize here as certain
* ext4_writepages() paths not allocating blocks update i_disksize.
*/
if (new_i_size > inode->i_size) {
unsigned long end;

i_size_write(inode, new_i_size);
end = (new_i_size - 1) & (PAGE_SIZE - 1);
if (copied && ext4_da_should_update_i_disksize(folio, end)) {
ext4_update_i_disksize(inode, new_i_size);
disksize_changed = true;
}
}

unlock_page(page);
put_page(page);

if (old_size < pos)
pagecache_isize_extended(inode, old_size, pos);

if (disksize_changed) {
handle_t *handle;

handle = ext4_journal_start(inode, EXT4_HT_INODE, 2);
if (IS_ERR(handle))
return PTR_ERR(handle);
ext4_mark_inode_dirty(handle, inode);
ext4_journal_stop(handle);
}

return copied;
}


Honza