From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:48861) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1XqK27-0006G0-U1 for qemu-devel@nongnu.org; Mon, 17 Nov 2014 05:59:52 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1XqK22-0007hI-VR for qemu-devel@nongnu.org; Mon, 17 Nov 2014 05:59:47 -0500 Received: from mx1.redhat.com ([209.132.183.28]:48103) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1XqK22-0007h7-OV for qemu-devel@nongnu.org; Mon, 17 Nov 2014 05:59:42 -0500 Message-ID: <5469D518.10408@redhat.com> Date: Mon, 17 Nov 2014 11:59:36 +0100 From: Max Reitz MIME-Version: 1.0 References: <1416219514-22530-1-git-send-email-armbru@redhat.com> <1416219514-22530-3-git-send-email-armbru@redhat.com> <5469CE54.3010607@redhat.com> <87k32uf5zs.fsf@blackfin.pond.sub.org> In-Reply-To: <87k32uf5zs.fsf@blackfin.pond.sub.org> Content-Type: text/plain; charset=iso-8859-15; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH v3 for-2.2 2/3] raw-posix: SEEK_HOLE suffices, get rid of FIEMAP List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Markus Armbruster Cc: kwolf@redhat.com, famz@redhat.com, qemu-devel@nongnu.org, tony@bakeyournoodle.com, stefanha@redhat.com, pbonzini@redhat.com On 2014-11-17 at 11:58, Markus Armbruster wrote: > Max Reitz writes: > >> 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 >>> Reviewed-by: Max Reitz >>> Reviewed-by: Eric Blake >>> --- >>> 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. > You're right! The appended patch should be squashed in, either on > commit, or in a respin. > > > diff --git a/block/raw-posix.c b/block/raw-posix.c > index 0b5d5a8..414e6d1 100644 > --- a/block/raw-posix.c > +++ b/block/raw-posix.c > @@ -148,9 +148,6 @@ typedef struct BDRVRawState { > bool has_write_zeroes:1; > bool discard_zeroes:1; > bool needs_alignment; > -#ifdef CONFIG_FIEMAP > - bool skip_fiemap; > -#endif > } BDRVRawState; > > typedef struct BDRVRawReopenState { With or without thank hunk (better with): Reviewed-by: Max Reitz