Re: [PATCH] Btrfs: deal with existing encompassing extent map in btrfs_get_extent()

[Omar Sandoval <osandov@osandov.com>]

On Thu, Nov 10, 2016 at 05:01:44PM +0100, Holger Hoffstätte wrote:
> On 11/10/16 16:37, Omar Sandoval wrote:
> > On Thu, Nov 10, 2016 at 04:11:35PM +0100, Holger Hoffstätte wrote:
> >> On 11/10/16 00:26, Omar Sandoval wrote:
> >>> From: Omar Sandoval <osandov@fb.com>
> >>>
> >>> My QEMU VM was seeing inexplicable I/O errors that I tracked down to
> >>> errors coming from the qcow2 virtual drive in the host system. The qcow2
> >>> file is a nocow file on my Btrfs drive, which QEMU opens with O_DIRECT.
> >>> Every once in awhile, pread() or pwrite() would return EEXIST, which
> >>> makes no sense. This turned out to be a bug in btrfs_get_extent().
> >>>
> >>> Commit 8dff9c853410 ("Btrfs: deal with duplciates during extent_map
> >>> insertion in btrfs_get_extent") fixed a case in btrfs_get_extent() where
> >>> two threads race on adding the same extent map to an inode's extent map
> >>> tree. However, if the added em is merged with an adjacent em in the
> >>> extent tree, then we'll end up with an existing extent that is not
> >>> identical to but instead encompasses the extent we tried to add. When we
> >>> call merge_extent_mapping() to find the nonoverlapping part of the new
> >>> em, the arithmetic overflows because there is no such thing. We then end
> >>> up trying to add a bogus em to the em_tree, which results in a EEXIST
> >>> that can bubble all the way up to userspace.
> >>>
> >>> Fix it by extending the identical extent map special case.
> >>>
> >>> Signed-off-by: Omar Sandoval <osandov@fb.com>
> >>> ---
> >>> Applies to 4.9-rc4.
> >>>
> >>> Here [1] is a reproducer for this bug that doesn't involve firing up a
> >>> QEMU VM. Also, a big shoutout to BCC [2] and BPF for making it possible
> >>> to debug this on my laptop without compiling a custom kernel and
> >>> rebooting just to add printks [3].
> >>>
> >>> 1: https://gist.github.com/osandov/d08aabe5d4dec15517e9fde17012fd3b
> >>
> >> I can't really make this reproducer fail. It builds and runs fine, but just
> >> exits with no messages (other than the one about drop_caches in dmesg).
> >> It creates the 1MB file and always returns 0. Ideas?
> >>
> >> -h
> > 
> > It's a race condition, so it doesn't happen 100% of the time. I imagine
> > it depends on the storage speed, as well. On my laptop, which is
> > dm-crypt on top of an SSD, it works about 50% of the time. Could you
> > just try running it 100 times or something and see if it fails?
> 
> $for i ($(seq 1 1000)) ./pread_eexist_repro /mnt/test/$i || echo "fail"
> 
> ..couple of thousand runs without problem, only lots of fallocating and
> cache dropping.
> 
> Oh well, I tried. :)
> 
> -h

Just out of curiousity, what kind of disk were you trying this on? I've
only been able to trigger it on my laptop and a VM running on my laptop.

-- 
Omar