[PATCH v2 03/26] btrfs: use async_chunk::async_cow to replace the confusing pending pointer

Qu Wenruo
2021-08-29T05:25:09Z

For structure async_chunk, we use a very strange member layout to grab
structure async_cow who owns this async_chunk.

At initialization, it goes like this:

async_chunk[i].pending = &ctx->num_chunks;

Then at async_cow_free() we do a super weird freeing:

/*
* Since the pointer to 'pending' is at the beginning of the array of
* async_chunk's, freeing it ensures the whole array has been freed.
*/
if (atomic_dec_and_test(async_chunk->pending))
kvfree(async_chunk->pending);

This is absolutely an abuse of kvfree().

Replace async_chunk::pending with async_chunk::async_cow, so that we can
grab the async_cow structure directly, without this strange dancing.

And with this change, there is no requirement for any specific member
location.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
fs/btrfs/inode.c | 16 +++++++---------
1 file changed, 7 insertions(+), 9 deletions(-)

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 0517f31a3bed..02448857b72f 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -455,11 +455,10 @@ struct async_chunk {
struct list_head extents;
struct cgroup_subsys_state *blkcg_css;
struct btrfs_work work;
- atomic_t *pending;
+ struct async_cow *async_cow;
};

struct async_cow {
- /* Number of chunks in flight; must be first in the structure */
atomic_t num_chunks;
struct async_chunk chunks[];
};
@@ -1325,18 +1324,17 @@ static noinline void async_cow_submit(struct btrfs_work *work)
static noinline void async_cow_free(struct btrfs_work *work)
{
struct async_chunk *async_chunk;
+ struct async_cow *async_cow;

async_chunk = container_of(work, struct async_chunk, work);
if (async_chunk->inode)
btrfs_add_delayed_iput(async_chunk->inode);
if (async_chunk->blkcg_css)
css_put(async_chunk->blkcg_css);
- /*
- * Since the pointer to 'pending' is at the beginning of the array of
- * async_chunk's, freeing it ensures the whole array has been freed.
- */
- if (atomic_dec_and_test(async_chunk->pending))
- kvfree(async_chunk->pending);
+
+ async_cow = async_chunk->async_cow;
+ if (atomic_dec_and_test(&async_cow->num_chunks))
+ kvfree(async_cow);
}

static int cow_file_range_async(struct btrfs_inode *inode,
@@ -1397,7 +1395,7 @@ static int cow_file_range_async(struct btrfs_inode *inode,
* lightweight reference for the callback lifetime
*/
ihold(&inode->vfs_inode);
- async_chunk[i].pending = &ctx->num_chunks;
+ async_chunk[i].async_cow = ctx;
async_chunk[i].inode = &inode->vfs_inode;
async_chunk[i].start = start;
async_chunk[i].end = cur_end;
--
2.32.0