Re: [PATCH v2 06/12] btrfs: make btrfs_check_nocow_lock nowait compatible

[Stefan Roesch <shr@fb.com>]

On 9/8/22 3:18 AM, Filipe Manana wrote:
> > 
> On Thu, Sep 8, 2022 at 1:26 AM Stefan Roesch <shr@fb.com> wrote:
>>
>> From: Josef Bacik <josef@toxicpanda.com>
>>
>> Now all the helpers that btrfs_check_nocow_lock uses handle nowait, add
>> a nowait flag to btrfs_check_nocow_lock so it can be used by the write
>> path.
>>
>> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
>> Signed-off-by: Stefan Roesch <shr@fb.com>
>> ---
>>  fs/btrfs/ctree.h |  2 +-
>>  fs/btrfs/file.c  | 33 ++++++++++++++++++++++-----------
>>  fs/btrfs/inode.c |  2 +-
>>  3 files changed, 24 insertions(+), 13 deletions(-)
>>
>> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
>> index 536bbc8551fc..06cb25f2d3bd 100644
>> --- a/fs/btrfs/ctree.h
>> +++ b/fs/btrfs/ctree.h
>> @@ -3482,7 +3482,7 @@ int btrfs_dirty_pages(struct btrfs_inode *inode, struct page **pages,
>>                       struct extent_state **cached, bool noreserve);
>>  int btrfs_fdatawrite_range(struct inode *inode, loff_t start, loff_t end);
>>  int btrfs_check_nocow_lock(struct btrfs_inode *inode, loff_t pos,
>> -                          size_t *write_bytes);
>> +                          size_t *write_bytes, bool nowait);
>>  void btrfs_check_nocow_unlock(struct btrfs_inode *inode);
>>
>>  /* tree-defrag.c */
>> diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
>> index 0f257205c63d..cf19d381ead6 100644
>> --- a/fs/btrfs/file.c
>> +++ b/fs/btrfs/file.c
>> @@ -1481,7 +1481,7 @@ lock_and_cleanup_extent_if_need(struct btrfs_inode *inode, struct page **pages,
>>   * NOTE: Callers need to call btrfs_check_nocow_unlock() if we return > 0.
>>   */
>>  int btrfs_check_nocow_lock(struct btrfs_inode *inode, loff_t pos,
>> -                          size_t *write_bytes)
>> +                          size_t *write_bytes, bool nowait)
>>  {
>>         struct btrfs_fs_info *fs_info = inode->root->fs_info;
>>         struct btrfs_root *root = inode->root;
>> @@ -1500,16 +1500,21 @@ int btrfs_check_nocow_lock(struct btrfs_inode *inode, loff_t pos,
>>                            fs_info->sectorsize) - 1;
>>         num_bytes = lockend - lockstart + 1;
>>
>> -       btrfs_lock_and_flush_ordered_range(inode, lockstart, lockend, NULL);
>> +       if (nowait) {
>> +               if (!btrfs_try_lock_ordered_range(inode, lockstart, lockend)) {
>> +                       btrfs_drew_write_unlock(&root->snapshot_lock);
>> +                       return -EAGAIN;
>> +               }
>> +       } else {
>> +               btrfs_lock_and_flush_ordered_range(inode, lockstart, lockend, NULL);
>> +       }
>>         ret = can_nocow_extent(&inode->vfs_inode, lockstart, &num_bytes,
>> -                       NULL, NULL, NULL, false, false);
>> -       if (ret <= 0) {
>> -               ret = 0;
>> +                       NULL, NULL, NULL, nowait, false);
>> +       if (ret <= 0)
>>                 btrfs_drew_write_unlock(&root->snapshot_lock);
>> -       } else {
>> +       else
>>                 *write_bytes = min_t(size_t, *write_bytes ,
>>                                      num_bytes - pos + lockstart);
>> -       }
>>         unlock_extent(&inode->io_tree, lockstart, lockend);
>>
>>         return ret;
>> @@ -1666,16 +1671,22 @@ static noinline ssize_t btrfs_buffered_write(struct kiocb *iocb,
>>                                                   &data_reserved, pos,
>>                                                   write_bytes, false);
>>                 if (ret < 0) {
>> +                       int tmp;
>> +
>>                         /*
>>                          * If we don't have to COW at the offset, reserve
>>                          * metadata only. write_bytes may get smaller than
>>                          * requested here.
>>                          */
>> -                       if (btrfs_check_nocow_lock(BTRFS_I(inode), pos,
>> -                                                  &write_bytes) > 0)
>> -                               only_release_metadata = true;
>> -                       else
>> +                       tmp = btrfs_check_nocow_lock(BTRFS_I(inode), pos,
>> +                                                    &write_bytes, false);
>> +                       if (tmp < 0)
>> +                               ret = tmp;
>> +                       if (tmp > 0)
>> +                               ret = 0;
>> +                       if (ret)
> 
> A variable named tmp is not a great name, something like "can_nocow'
> would be a lot more clear.
> 

I renamed the variable tmp to can_nocow.

> Thanks.
> 
> 
>>                                 break;
>> +                       only_release_metadata = true;
>>                 }
>>
>>                 num_pages = DIV_ROUND_UP(write_bytes + offset, PAGE_SIZE);
>> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
>> index 36e755f73764..5426d4f4ac23 100644
>> --- a/fs/btrfs/inode.c
>> +++ b/fs/btrfs/inode.c
>> @@ -4884,7 +4884,7 @@ int btrfs_truncate_block(struct btrfs_inode *inode, loff_t from, loff_t len,
>>         ret = btrfs_check_data_free_space(inode, &data_reserved, block_start,
>>                                           blocksize, false);
>>         if (ret < 0) {
>> -               if (btrfs_check_nocow_lock(inode, block_start, &write_bytes) > 0) {
>> +               if (btrfs_check_nocow_lock(inode, block_start, &write_bytes, false) > 0) {
>>                         /* For nocow case, no need to reserve data space */
>>                         only_release_metadata = true;
>>                 } else {
>> --
>> 2.30.2
>>