Re: [PATCH] Btrfs: do not abort transaction when failing to insert hole extent

[Liu Bo <obuil.liubo@gmail.com>]

On Fri, Apr 6, 2018 at 6:21 AM, David Sterba <dsterba@suse.cz> wrote:
> On Thu, Apr 05, 2018 at 11:58:16AM -0700, Liu Bo wrote:
>> On Thu, Apr 5, 2018 at 9:48 AM, David Sterba <dsterba@suse.cz> wrote:
>> > On Sat, Mar 31, 2018 at 06:11:55AM +0800, Liu Bo wrote:
>> >> This is running in a typical write path, not inside a critical path
>> >> where we have to abort the running transaction, so it's OK to return
>> >> errors to callers and eventually to userspace.
>> >
>> > I'm not sure this is entierly correct, several other places do not abort
>> > after btrfs_drop_extents as there's nothing that would leave the
>> > structres in some half-state.
>> >
>> >> Signed-off-by: Liu Bo <bo.liu@linux.alibaba.com>
>> >> ---
>> >>  fs/btrfs/inode.c | 5 +----
>> >>  1 file changed, 1 insertion(+), 4 deletions(-)
>> >>
>> >> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
>> >> index c7b75dd..b9310f8 100644
>> >> --- a/fs/btrfs/inode.c
>> >> +++ b/fs/btrfs/inode.c
>> >> @@ -4939,16 +4939,13 @@ static int maybe_insert_hole(struct btrfs_root *root, struct inode *inode,
>> >>
>> >>       ret = btrfs_drop_extents(trans, root, inode, offset, offset + len, 1);
>> >>       if (ret) {
>> >> -             btrfs_abort_transaction(trans, ret);
>> >>               btrfs_end_transaction(trans);
>> >>               return ret;
>> >>       }
>> >>
>> >>       ret = btrfs_insert_file_extent(trans, root, btrfs_ino(BTRFS_I(inode)),
>> >>                       offset, 0, 0, len, 0, len, 0, 0, 0);
>> >
>> > But here the extents have been already dropped and missing to insert the
>> > items does not seem to lead to a consistent state.
>> >
>> > It's possible that I'm missing something. In a call path that can be
>> > safely rolled back even with a started transaction, we don't need to
>> > abort in all cases. But if the rollback requires some non-trivial
>> > modifications, I don't see options how to avoid the abort.
>> >
>> > __btrfs_drop_extents does a lot of state changes and can itself fail
>> > in the middle of dropping the range, aborting looks like the safest
>> > option.
>> >
>>
>> As maybe_insert_hole is only called by btrfs_cont_expand here, which
>> means it's a really hole, I don't expect drop_extents would drop
>> anything, we can remove this drop_extents and put an assert after
>> btrfs_insert_file_extent for checking EEXIST.
>
> Sounds good.
>

Let me make a v2 and have a fstests run.

thanks,
liubo

>> It's different from punch hole where we need to explicitly drop an
>> actual extent and replace it with a hole range.
>
> Right, that's what I didn't see at first.