Re: [PATCH v2] fstests: generic: Check if a bull fallocate will change extent number

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

Hi Dave, I updated the patch and moved it to btrfs.

But I still has some question about the fallocate behavior.

Just as the new btrfs test case, I changed the fallocate range, not to 
cover the last part, to make the problem more obvious:

Btrfs will truncate beyond EOF even that's *not covered* by the 
fallocate range.

It's OK for a fs to modify the extent layout during fallocate, but is it 
OK to modify extent layout completely *out* of the fallocate range?

Thanks,
Qu

在 2015年09月30日 05:51, Dave Chinner 写道:
> On Tue, Sep 29, 2015 at 05:34:24PM +0800, Qu Wenruo wrote:
>> Normally, a bull fallocate call on a fully written and synced file
>> should not add an extent.
>
> Why not? Filesystems can do whatever they want with extents during
> a fallocate call. e.g. if the blocks are shared, then fallocate
> might break the block sharing so future overwrites don't get
> ENOSPC. This is a requirement set down by posix_fallocate(3)
>
> "After a successful call to posix_fallocate(), subsequent writes to
> bytes in the specified range are guaranteed not  to fail because of
> lack of disk space."
>
> Hence if you've got a file with shared blocks, a "full fallocate"
> must change the extent layout to break the sharing. As such, the
> premise of this test is wrong.
>
> That's not to say that btrfs has a bug:
>
>> Btrfs has a bug to always truncate the last page if the fallocate start
>> offset is smaller than inode size.
>
> But it' not clear that this behaviour is actually a bug if it's not
> changing the file data.
>
> Cheers,
>
> Dave.
>