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

[Chandan Rajendra <chandan@linux.vnet.ibm.com>]

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?

> 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