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()'.
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.
Thanks,
-liubo
--
To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
