On Thu, Nov 10, 2016 at 12:09:06PM -0800, Omar Sandoval wrote:
> On Thu, Nov 10, 2016 at 12:01:20PM -0800, Liu Bo wrote:
> > On Wed, Nov 09, 2016 at 03:26:50PM -0800, Omar Sandoval wrote:
> > > From: Omar Sandoval <osandov@xxxxxx>
> > >
> > > 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.
> >
> > I don't get how this could happen(even after reading Commit
> > 8dff9c853410), btrfs_get_extent in direct_IO is protected by
> > lock_extent_direct, the assumption is that a racy thread should be
> > blocked by lock_extent_direct and when it gets the lock, it finds the
> > just-inserted em when going into btrfs_get_extent if its offset is
> > within [em->start, extent_map_end(em)].
> >
> > I think we may also need to figure out why the above doesn't work as
> > expected besides fixing another special case.
> >
> > Thanks,
> >
> > -liubo
>
> lock_extent_direct() only protects the range you're doing I/O into, not
> the entire extent. If two threads are doing two non-overlapping reads in
> the same extent, then you can get this race.
More concretely, assume the extent tree on disk has:
+-------------------------+-------------------------------+
|start=0,len=8192,bytenr=0|start=8192,len=8192,bytenr=8192|
+-------------------------+-------------------------------+
And the extent map tree in memory has a single em cached for the second
extent {start=8192, len=8192, bytenr=8192}. Then, two threads try do do
direct I/O reads:
Thread 1 | Thread 2
---------------------------------------+-------------------------------
pread(offset=0, nbyte=4096) | pread(offset=4096, nbyte=4096)
lock_extent_direct(start=0, end=4095) | lock_extent_direct(start=4096, end=8191)
btrfs_get_extent(start=0, len=4096) | btrfs_get_extent(start=4096, len4096)
lookup_extent_mapping() = NULL | lookup_extent_mapping() = NULL
reads extent from B-tree | reads extent from B-tree
| write_lock(&em_tree->lock)
| add_extent_mapping(start=0, len=8192, bytenr=0)
| try_merge_map()
| em_tree now has {start=0, len=16384, bytenr=0}
| write_unlock(&em_tree->lock)
write_lock(&em_tree->lock) |
add_extent_mapping(start=0, len=8192, |
bytenr=0) = -EEXIST |
search_extent_mapping() = {start=0, |
len=16384, |
bytenr=0} |
merge_extent_mapping() does bogus math |
and overflows, returns EEXIST |
--
Omar
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