Re: [PATCH] Btrfs: let super_stripesize match with sectorsize

[Liu Bo <bo.li.liu@oracle.com>]

On Thu, Jun 16, 2016 at 01:53:59PM +0530, Chandan Rajendra wrote:
> On Wednesday, June 15, 2016 05:09:55 PM Liu Bo wrote:
> > On Wed, Jun 15, 2016 at 03:50:17PM +0530, Chandan Rajendra wrote:
> > > On Wednesday, June 15, 2016 09:12:28 AM Chandan Rajendra wrote:
> > > > Hello Liu Bo,
> > > > 
> > > > We have to fix the following check in check_super() as well,
> > > > 
> > > >        if (btrfs_super_stripesize(sb) != 4096) {
> > > >        
> > > >                 error("invalid stripesize %u",
> > > >                 btrfs_super_stripesize(sb));
> > > >                 goto error_out;
> > > >         
> > > >         }
> > > > 
> > > > i.e. btrfs_super_stripesize(sb) must be equal to
> > > > btrfs_super_sectorsize(sb).
> > > > 
> > > > However in btrfs-progs (mkfs.c to be precise) since we had stripesize
> > > > hardcoded to 4096, setting stripesize to the value of sectorsize in
> > > > mkfs.c will cause the following to occur when mkfs.btrfs is invoked for
> > > > devices with existing Btrfs filesystem instances,
> > > > 
> > > > NOTE: Assume we have changed the stripesize validation in btrfs-progs'
> > > > check_super() to,
> > > > 
> > > >         if (btrfs_super_stripesize(sb) != btrfs_super_sectorsize(sb)) {
> > > >         
> > > >                 error("invalid stripesize %u",
> > > >                 btrfs_super_stripesize(sb));
> > > >                 goto error_out;
> > > >         
> > > >         }
> > > > 
> > > > main()
> > > > 
> > > >  for each device file passed as an argument,
> > > >  
> > > >    test_dev_for_mkfs()
> > > >    
> > > >      check_mounted
> > > >      
> > > >        check_mounted_where
> > > >        
> > > >          btrfs_scan_one_device
> > > >          
> > > >            btrfs_read_dev_super
> > > >            
> > > >              check_super() call will fail for existing filesystems which
> > > > 
> > > > have stripesize set to 4k. All existing filesystem instances will fall
> > > > into
> > > > this category.
> > > > 
> > > > This error value is pushed up the call stack and this causes the device
> > > > to
> > > > not get added to the fs_devices_mnt list in check_mounted_where(). Hence
> > > > we
> > > > would fail to correctly check the mount status of the multi-device btrfs
> > > > filesystems.
> > > 
> > > We can end up in the following scenario,
> > > - /dev/loop0, /dev/loop1 and /dev/loop2 are mounted as a single
> > > 
> > >   filesystem. The filesystem was created by an older version of mkfs.btrfs
> > >   which set stripesize to 4k.
> > > 
> > > - losetup -a
> > > 
> > >    /dev/loop0: [0030]:19477 (/root/disk-imgs/file-0.img)
> > >    /dev/loop1: [0030]:16577 (/root/disk-imgs/file-1.img)
> > >    /dev/loop2: [64770]:3423229 (/root/disk-imgs/file-2.img)
> > > 
> > > - /etc/mtab lists only /dev/loop0
> > > - "losetup /dev/loop4 /root/disk-imgs/file-1.img"
> > > 
> > >    The new mkfs.btrfs invoked as 'mkfs.btrfs -f /dev/loop4' succeeds even
> > >    though /dev/loop1 has already been mounted and has
> > >    /root/disk-imgs/file-1.img as its backing file.
> > > 
> > > So IMHO the only solution is to have the stripesize check in check_super()
> > > to allow both '4k' and 'sectorsize' as valid values i.e.
> > > 
> > >         if ((btrfs_super_stripesize(sb) != 4096)
> > > 	    
> > > 	    && (btrfs_super_stripesize(sb) != btrfs_super_sectorsize(sb))) {
> > > 	    
> > >                 error("invalid stripesize %u",
> > >                 btrfs_super_stripesize(sb));
> > >                 goto error_out;
> > >         
> > >         }
> > 
> > That's a good one.
> > 
> > But if we go back to the original point, in the kernel side,
> > 1. in open_ctree(), root->stripesize = btrfs_super_stripesize();
> > 
> > 2. in find_free_extent(),
> > 	...
> > 	search_start = ALIGN(offset, root->stripesize);
> > 	...
> > 3. in btrfs_alloc_tree_block(),
> > 	...
> > 	ret = btrfs_reserve_extent(..., &ins, ...);
> > 	...
> > 	buf = btrfs_init_new_buffer(trans, root, ins.objectid, level);
> > 
> > 4. in btrfs_init_new_buffer(),
> > 	...
> > 	buf = btrfs_find_create_tree_block(root, bytenr);
> > 	...
> > 
> > Because 'num_bytes' we pass to find_free_extent() is aligned to
> > sectorsize, the free space we can find is aligned to sectorsize,
> > which means 'offset' in '1. find_free_extent()' is aligned to sectorsize.
> > 
> > If our stripesize is larger than sectorsize, say 4 * sectorsize,
> > for data allocation it's fine while for metadata block allocations it's
> > not.  It is possible that when we allocate a new metadata block, we can
> > end up with an existing eb returned by '4. in btrfs_init_new_buffer()'.
> >
> 
> Liu, I am sorry ... I am unable to visualize a scenario where the above
> described scenario could happen. Can you please provide an example?

Sure, imagine that sectorsize is 4k and stripesize is 16k,
and a tree root's eb has eb->start = 12599296 (12582912 + 16384, a typical bytenr in
btrfs) which is aligned to 4k, and when CoW happens on another eb,

__btrfs_cow_block()
  ->btrfs_alloc_tree_block()
    ->btrfs_reserve_extent()
      ->find_free_extent()
    ->btrfs_init_new_buffer()

btrfs_reserve_extent() can return 12599296 for the new eb even if what
it've found is (12582912 + 4096), but
 after 'search_start = ALIGN(offset, root->stripesize)', it gets to
12599296.

In btrfs_init_new_buffer, we search eb tree by bytenr=12599296 and
get tree root's eb, the following btrfs_tree_lock will scream.

The example is taken from
btrfs-progs/tests/fuzz-tests/images/superblock-stripsize-bogus.raw.xz


Thanks,

-liubo
> 
> > PS: There is something wrong around '2. in find_free_extent()',
> > we only do alignment on offset, but for num_bytes, we don't do that,
> > so we may end up with a overlap with other data extents or metadata
> > blocks.
> > 
> > So I think we can just replace this ALING with a warning since the offset
> > returned by searching free space tree is aligned to
> > block_group->full_stripe_len, which is either sectorsize or
> > BTRFS_STRIPE_LEN * nr_stripes (for
> > raid56), then we can just drop the check for stripesize everywhere.
> > 
> 
> -- 
> chandan
>