Re: [Qemu-devel] [PATCH v3 for-2.2 2/3] raw-posix: SEEK_HOLE suffices, get rid of FIEMAP

[Max Reitz <mreitz@redhat.com>]

On 2014-11-17 at 11:18, Markus Armbruster wrote:
> Commit 5500316 (May 2012) implemented raw_co_is_allocated() as
> follows:
>
> 1. If defined(CONFIG_FIEMAP), use the FS_IOC_FIEMAP ioctl
>
> 2. Else if defined(SEEK_HOLE) && defined(SEEK_DATA), use lseek()
>
> 3. Else pretend there are no holes
>
> Later on, raw_co_is_allocated() was generalized to
> raw_co_get_block_status().
>
> Commit 4f11aa8 (May 2014) changed it to try the three methods in order
> until success, because "there may be implementations which support
> [SEEK_HOLE/SEEK_DATA] but not [FIEMAP] (e.g., NFSv4.2) as well as vice
> versa."
>
> Unfortunately, we used FIEMAP incorrectly: we lacked FIEMAP_FLAG_SYNC.
> Commit 38c4d0a (Sep 2014) added it.  Because that's a significant
> speed hit, the next commit 7c159037 put SEEK_HOLE/SEEK_DATA first.
>
> As you see, the obvious use of FIEMAP is wrong, and the correct use is
> slow.  I guess this puts it somewhere between -7 "The obvious use is
> wrong" and -10 "It's impossible to get right" on Rusty Russel's Hard
> to Misuse scale[*].
>
> "Fortunately", the FIEMAP code is used only when
>
> * SEEK_HOLE/SEEK_DATA aren't defined, but CONFIG_FIEMAP is
>
>    Uncommon.  SEEK_HOLE had no XFS implementation between 2011 (when it
>    was introduced for ext4 and btrfs) and 2012.
>
> * SEEK_HOLE/SEEK_DATA and CONFIG_FIEMAP are defined, but lseek() fails
>
>    Unlikely.
>
> Thus, the FIEMAP code executes rarely.  Makes it a nice hidey-hole for
> bugs.  Worse, bugs hiding there can theoretically bite even on a host
> that has SEEK_HOLE/SEEK_DATA.
>
> I don't want to worry about this crap, not even theoretically.  Get
> rid of it.
>
> [*] http://ozlabs.org/~rusty/index.cgi/tech/2008-04-01.html
>
> Signed-off-by: Markus Armbruster <armbru@redhat.com>
> Reviewed-by: Max Reitz <mreitz@redhat.com>
> Reviewed-by: Eric Blake <eblake@redhat.com>
> ---
>   block/raw-posix.c | 60 ++++---------------------------------------------------
>   1 file changed, 4 insertions(+), 56 deletions(-)

I only just now realized that you're not getting rid of FIEMAP 
completely; there's still the skip_fiemap flag in BDRVRawState. I don't 
care for now, though, thus my R-b stands.

Max