Re: [PATCH v1 01/10] btrfs: implement a nowait option for tree searches

Jens Axboe
2022-09-02T15:24:44Z

On 9/2/22 8:48 AM, Filipe Manana wrote:
> On Fri, Sep 2, 2022 at 12:01 AM Stefan Roesch <shr@fb.com> wrote:
>>
>> From: Josef Bacik <josef@toxicpanda.com>
>>
>> For NOWAIT IOCB's we'll need a way to tell search to not wait on locks
>> or anything. Accomplish this by adding a path->nowait flag that will
>> use trylocks and skip reading of metadata, returning -EWOULDBLOCK in
>> either of these cases. For now we only need this for reads, so only the
>> read side is handled. Add an ASSERT() to catch anybody trying to use
>> this for writes so they know they'll have to implement the write side.
>>
>> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
>> Signed-off-by: Stefan Roesch <shr@fb.com>
>> ---
>> fs/btrfs/ctree.c | 39 ++++++++++++++++++++++++++++++++++++---
>> fs/btrfs/ctree.h | 1 +
>> fs/btrfs/locking.c | 23 +++++++++++++++++++++++
>> fs/btrfs/locking.h | 1 +
>> 4 files changed, 61 insertions(+), 3 deletions(-)
>>
>> diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
>> index ebfa35fe1c38..052c768b2297 100644
>> --- a/fs/btrfs/ctree.c
>> +++ b/fs/btrfs/ctree.c
>> @@ -1447,6 +1447,11 @@ read_block_for_search(struct btrfs_root *root, struct btrfs_path *p,
>> return 0;
>> }
>>
>> + if (p->nowait) {
>> + free_extent_buffer(tmp);
>> + return -EWOULDBLOCK;
>> + }
>> +
>> if (unlock_up)
>> btrfs_unlock_up_safe(p, level + 1);
>>
>> @@ -1467,6 +1472,8 @@ read_block_for_search(struct btrfs_root *root, struct btrfs_path *p,
>> ret = -EAGAIN;
>>
>> goto out;
>> + } else if (p->nowait) {
>> + return -EWOULDBLOCK;
>> }
>>
>> if (unlock_up) {
>> @@ -1634,7 +1641,13 @@ static struct extent_buffer *btrfs_search_slot_get_root(struct btrfs_root *root,
>> * We don't know the level of the root node until we actually
>> * have it read locked
>> */
>> - b = btrfs_read_lock_root_node(root);
>> + if (p->nowait) {
>> + b = btrfs_try_read_lock_root_node(root);
>> + if (IS_ERR(b))
>> + return b;
>> + } else {
>> + b = btrfs_read_lock_root_node(root);
>> + }
>> level = btrfs_header_level(b);
>> if (level > write_lock_level)
>> goto out;
>> @@ -1910,6 +1923,13 @@ int btrfs_search_slot(struct btrfs_trans_handle *trans, struct btrfs_root *root,
>> WARN_ON(p->nodes[0] != NULL);
>> BUG_ON(!cow && ins_len);
>>
>> + /*
>> + * For now only allow nowait for read only operations. There's no
>> + * strict reason why we can't, we just only need it for reads so I'm
>> + * only implementing it for reads right now.
>> + */
>> + ASSERT(!p->nowait || !cow);
>> +
>> if (ins_len < 0) {
>> lowest_unlock = 2;
>>
>> @@ -1936,7 +1956,12 @@ int btrfs_search_slot(struct btrfs_trans_handle *trans, struct btrfs_root *root,
>>
>> if (p->need_commit_sem) {
>> ASSERT(p->search_commit_root);
>> - down_read(&fs_info->commit_root_sem);
>> + if (p->nowait) {
>> + if (!down_read_trylock(&fs_info->commit_root_sem))
>> + return -EAGAIN;
>
> Why EAGAIN here and everywhere else EWOULDBLOCK? See below.

Is EWOULDBLOCK ever different from EAGAIN? But it should be used
consistently, EAGAIN would be the return of choice for that.

--
Jens Axboe