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

[Stefan Roesch <shr@fb.com>]

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
> 
> 
>