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

Stefan Roesch
2022-09-08T00:30:11Z

On 9/2/22 8:04 AM, Filipe Manana wrote:
> >
> On Fri, Sep 2, 2022 at 3:57 PM Jens Axboe <axboe@kernel.dk> wrote:
>>
>> 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.
>
> Oh right, EWOULDBLOCK is defined as EAGAIN, same values.
> It would be best to use the same everywhere, avoiding confusion...
>

I changed it to EAGAIN in the patch series.
>>
>> --
>> Jens Axboe
>
>
>