From: Filipe Manana <fdman...@suse.com>

Commit 951555856b88 ("Btrfs: send, don't bug on inconsistent snapshots")
removed some BUG_ON() statements (replacing them with returning errors
to user space and logging error messages) when a snapshot is in an
inconsistent state due to failures to update a delayed inode item (ENOMEM
or ENOSPC) after adding/updating/deleting references, xattrs or file
extent items.

However there is a case, when no errors happen, where a file extent item
can be modified without having the corresponding inode item updated. This
case happens during balance under very specific timings, when relocation
is in the stage where it updates data pointers and a leaf that contains
file extent items is COWed. When that happens file extent items get their
disk_bytenr field updated to a new value that reflects the post relocation
logical address of the extent, without updating their respective inode
items (as there is nothing that needs to be updated on them). This is
performed at relocation.c:replace_file_extents() through
relocation.c:btrfs_reloc_cow_block().

So make an incremental send deal with this case and don't do any processing
for a file extent item that got its disk_bytenr field updated by relocation,
since the extent's data is the same as the one pointed by the file extent
item in the parent snapshot.

After the recent commit mentioned above this case resulted in EIO errors
returned to user space (and an error message logged to dmesg/syslog) when
doing an incremental send, while before it, it resulted in hitting a
BUG_ON leading to the following trace:

[  952.206705] ------------[ cut here ]------------
[  952.206714] kernel BUG at ../fs/btrfs/send.c:5653!
[  952.206719] Internal error: Oops - BUG: 0 [#1] SMP
[  952.209854] Modules linked in: st dm_mod nls_utf8 isofs fuse nf_log_ipv6 
xt_pkttype xt_physdev br_netfilter nf_log_ipv4 nf_log_common xt_LOG xt_limit 
ebtable_filter ebtables af_packet bridge stp llc ip6t_REJECT xt_tcpudp 
nf_conntrack_ipv6 nf_defrag_ipv6 ip6table_raw ipt_REJECT iptable_raw xt_CT 
iptable_filter ip6table_mangle nf_conntrack_netbios_ns nf_conntrack_broadcast 
nf_conntrack_ipv4 nf_defrag_ipv4 ip_tables xt_conntrack nf_conntrack 
ip6table_filter ip6_tables x_tables xfs libcrc32c nls_iso8859_1 nls_cp437 vfat 
fat joydev aes_ce_blk ablk_helper cryptd snd_intel8x0 aes_ce_cipher 
snd_ac97_codec ac97_bus snd_pcm ghash_ce sha2_ce sha1_ce snd_timer snd 
virtio_net soundcore btrfs xor sr_mod cdrom hid_generic usbhid raid6_pq 
virtio_blk virtio_scsi bochs_drm drm_kms_helper syscopyarea sysfillrect 
sysimgblt fb_sys_fops ttm virtio_mmio xhci_pci xhci_hcd usbcore usb_common 
virtio_pci virtio_ring virtio drm sg efivarfs
[  952.228333] Supported: Yes
[  952.228908] CPU: 0 PID: 12779 Comm: snapperd Not tainted 4.4.14-50-default #1
[  952.230329] Hardware name: QEMU KVM Virtual Machine, BIOS 0.0.0 02/06/2015
[  952.231683] task: ffff800058e94100 ti: ffff8000d866c000 task.ti: 
ffff8000d866c000
[  952.233279] PC is at changed_cb+0x9f4/0xa48 [btrfs]
[  952.234375] LR is at changed_cb+0x58/0xa48 [btrfs]
[  952.236552] pc : [<ffff7ffffc39de7c>] lr : [<ffff7ffffc39d4e0>] pstate: 
80000145
[  952.238049] sp : ffff8000d866fa20
[  952.238732] x29: ffff8000d866fa20 x28: 0000000000000019
[  952.239840] x27: 00000000000028d5 x26: 00000000000024a2
[  952.241008] x25: 0000000000000002 x24: ffff8000e66e92f0
[  952.242131] x23: ffff8000b8c76800 x22: ffff800092879140
[  952.243238] x21: 0000000000000002 x20: ffff8000d866fb78
[  952.244348] x19: ffff8000b8f8c200 x18: 0000000000002710
[  952.245607] x17: 0000ffff90d42480 x16: ffff800000237dc0
[  952.246719] x15: 0000ffff90de7510 x14: ab000c000a2faf08
[  952.247835] x13: 0000000000577c2b x12: ab000c000b696665
[  952.248981] x11: 2e65726f632f6966 x10: 652d34366d72612f
[  952.250101] x9 : 32627572672f746f x8 : ab000c00092f1671
[  952.251352] x7 : 8000000000577c2b x6 : ffff800053eadf45
[  952.252468] x5 : 0000000000000000 x4 : ffff80005e169494
[  952.253582] x3 : 0000000000000004 x2 : ffff8000d866fb78
[  952.254695] x1 : 000000000003e2a3 x0 : 000000000003e2a4
[  952.255803]
[  952.256150] Process snapperd (pid: 12779, stack limit = 0xffff8000d866c020)
[  952.257516] Stack: (0xffff8000d866fa20 to 0xffff8000d8670000)
[  952.258654] fa20: ffff8000d866fae0 ffff7ffffc308fc0 ffff800092879140 
ffff8000e66e92f0
[  952.260219] fa40: 0000000000000035 ffff800055de6000 ffff8000b8c76800 
ffff8000d866fb78
[  952.261745] fa60: 0000000000000002 00000000000024a2 00000000000028d5 
0000000000000019
[  952.263269] fa80: ffff8000d866fae0 ffff7ffffc3090f0 ffff8000d866fae0 
ffff7ffffc309128
[  952.264797] faa0: ffff800092879140 ffff8000e66e92f0 0000000000000035 
ffff800055de6000
[  952.268261] fac0: ffff8000b8c76800 ffff8000d866fb78 0000000000000002 
0000000000001000
[  952.269822] fae0: ffff8000d866fbc0 ffff7ffffc39ecfc ffff8000b8f8c200 
ffff8000b8f8c368
[  952.271368] fb00: ffff8000b8f8c378 ffff800055de6000 0000000000000001 
ffff8000ecb17500
[  952.272893] fb20: ffff8000b8c76800 ffff800092879140 ffff800062b6d000 
ffff80007a9e2470
[  952.274420] fb40: ffff8000b8f8c208 0000000005784000 ffff8000580a8000 
ffff8000b8f8c200
[  952.276088] fb60: ffff7ffffc39d488 00000002b8f8c368 0000000000000000 
000000000003e2a4
[  952.280275] fb80: 000000000000006c ffff7ffffc39ec00 000000000003e2a4 
000000000000006c
[  952.283219] fba0: ffff8000b8f8c300 0000000000000100 0000000000000001 
ffff8000ecb17500
[  952.286166] fbc0: ffff8000d866fcd0 ffff7ffffc3643c0 ffff8000f8842700 
0000ffff8ffe9278
[  952.289136] fbe0: 0000000040489426 ffff800055de6000 0000ffff8ffe9278 
0000000040489426
[  952.292083] fc00: 000000000000011d 000000000000001d ffff80007a9e4598 
ffff80007a9e43e8
[  952.294959] fc20: ffff8000b8c7693f 0000000000003b24 0000000000000019 
ffff8000b8f8c218
[  952.301161] fc40: 00000001d866fc70 ffff8000b8c76800 0000000000000128 
ffffffffffffff84
[  952.305749] fc60: ffff800058e941ff 0000000000003a58 ffff8000d866fcb0 
ffff8000000f7390
[  952.308875] fc80: 000000000000012a 0000000000010290 ffff8000d866fc00 
000000000000007b
[  952.311915] fca0: 0000000000010290 ffff800046c1b100 74732d7366727462 
000001006d616572
[  952.314937] fcc0: ffff8000fffc4100 cb88537fdc8ba60e ffff8000d866fe10 
ffff8000002499e8
[  952.318008] fce0: 0000000040489426 ffff8000f8842700 0000ffff8ffe9278 
ffff80007a9e4598
[  952.321321] fd00: 0000ffff8ffe9278 0000000040489426 000000000000011d 
000000000000001d
[  952.324280] fd20: ffff80000072c000 ffff8000d866c000 ffff8000d866fda0 
ffff8000000e997c
[  952.327156] fd40: ffff8000fffc4180 00000000000031ed ffff8000fffc4180 
ffff800046c1b7d4
[  952.329895] fd60: 0000000000000140 0000ffff907ea170 000000000000011d 
00000000000000dc
[  952.334641] fd80: ffff80000072c000 ffff8000d866c000 0000000000000000 
0000000000000002
[  952.338002] fda0: ffff8000d866fdd0 ffff8000000ebacc ffff800046c1b080 
ffff800046c1b7d4
[  952.340724] fdc0: ffff8000d866fdf0 ffff8000000db67c 0000000000000040 
ffff800000e69198
[  952.343415] fde0: 0000ffff8ffea790 00000000000031ed ffff8000d866fe20 
ffff800000254000
[  952.346101] fe00: 000000000000001d 0000000000000004 ffff8000d866fe90 
ffff800000249d3c
[  952.348980] fe20: ffff8000f8842700 0000000000000000 ffff8000f8842701 
0000000000000008
[  952.351696] fe40: ffff8000d866fe70 0000000000000008 ffff8000d866fe90 
ffff800000249cf8
[  952.354387] fe60: ffff8000f8842700 0000ffff8ffe9170 ffff8000f8842701 
0000000000000008
[  952.357083] fe80: 0000ffff8ffe9278 ffff80008ff85500 0000ffff8ffe90c0 
ffff800000085c84
[  952.359800] fea0: 0000000000000000 0000ffff8ffe9170 ffffffffffffffff 
0000ffff90d473bc
[  952.365351] fec0: 0000000000000000 0000000000000015 0000000000000008 
0000000040489426
[  952.369550] fee0: 0000ffff8ffe9278 0000ffff907ea790 0000ffff907ea170 
0000ffff907ea790
[  952.372416] ff00: 0000ffff907ea170 0000000000000000 000000000000001d 
0000000000000004
[  952.375223] ff20: 0000ffff90a32220 00000000003d0f00 0000ffff907ea0a0 
0000ffff8ffe8f30
[  952.378099] ff40: 0000ffff9100f554 0000ffff91147000 0000ffff91117bc0 
0000ffff90d473b0
[  952.381115] ff60: 0000ffff9100f620 0000ffff880069b0 0000ffff8ffe9170 
0000ffff8ffe91a0
[  952.384003] ff80: 0000ffff8ffe9160 0000ffff8ffe9140 0000ffff88006990 
0000ffff8ffe9278
[  952.386860] ffa0: 0000ffff88008a60 0000ffff8ffe9480 0000ffff88014ca0 
0000ffff8ffe90c0
[  952.389654] ffc0: 0000ffff910be8e8 0000ffff8ffe90c0 0000ffff90d473bc 
0000000000000000
[  952.410986] ffe0: 0000000000000008 000000000000001d 6e2079747265706f 
72616d223d656d61
[  952.415497] Call trace:
[  952.417403] [<ffff7ffffc39de7c>] changed_cb+0x9f4/0xa48 [btrfs]
[  952.420023] [<ffff7ffffc308fc0>] btrfs_compare_trees+0x500/0x6b0 [btrfs]
[  952.422759] [<ffff7ffffc39ecfc>] btrfs_ioctl_send+0xb4c/0xe10 [btrfs]
[  952.425601] [<ffff7ffffc3643c0>] btrfs_ioctl+0x374/0x29a4 [btrfs]
[  952.428031] [<ffff8000002499e8>] do_vfs_ioctl+0x33c/0x600
[  952.430360] [<ffff800000249d3c>] SyS_ioctl+0x90/0xa4
[  952.432552] [<ffff800000085c84>] el0_svc_naked+0x38/0x3c
[  952.434803] Code: 2a1503e0 17fffdac b9404282 17ffff28 (d4210000)
[  952.437457] ---[ end trace 9afd7090c466cf15 ]---

Signed-off-by: Filipe Manana <fdman...@suse.com>
---
 fs/btrfs/send.c | 58 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 58 insertions(+)

diff --git a/fs/btrfs/send.c b/fs/btrfs/send.c
index efe129f..3ece9d4 100644
--- a/fs/btrfs/send.c
+++ b/fs/btrfs/send.c
@@ -5800,6 +5800,64 @@ static int changed_extent(struct send_ctx *sctx,
        int ret = 0;
 
        if (sctx->cur_ino != sctx->cmp_key->objectid) {
+
+               if (result == BTRFS_COMPARE_TREE_CHANGED) {
+                       struct extent_buffer *leaf_l;
+                       struct extent_buffer *leaf_r;
+                       struct btrfs_file_extent_item *ei_l;
+                       struct btrfs_file_extent_item *ei_r;
+
+                       leaf_l = sctx->left_path->nodes[0];
+                       leaf_r = sctx->right_path->nodes[0];
+                       ei_l = btrfs_item_ptr(leaf_l,
+                                             sctx->left_path->slots[0],
+                                             struct btrfs_file_extent_item);
+                       ei_r = btrfs_item_ptr(leaf_r,
+                                             sctx->right_path->slots[0],
+                                             struct btrfs_file_extent_item);
+
+                       /*
+                        * We may have found an extent item that has changed
+                        * only its disk_bytenr field and the corresponding
+                        * inode item was not updated. This case happens due to
+                        * very specific timings during relocation when a leaf
+                        * that contains file extent items is COWed while
+                        * relocation is ongoing and its in the stage where it
+                        * updates data pointers. So when this happens we can
+                        * safely ignore it since we know it's the same extent,
+                        * but just at different logical and physical locations
+                        * (when an extent is fully replaced with a new one, we
+                        * know the generation number must have changed too,
+                        * since snapshot creation implies committing the 
current
+                        * transaction, and the inode item must have been 
updated
+                        * as well).
+                        * This replacement of the disk_bytenr happens at
+                        * relocation.c:replace_file_extents() through
+                        * relocation.c:btrfs_reloc_cow_block().
+                        */
+                       if (btrfs_file_extent_generation(leaf_l, ei_l) ==
+                           btrfs_file_extent_generation(leaf_r, ei_r) &&
+                           btrfs_file_extent_ram_bytes(leaf_l, ei_l) ==
+                           btrfs_file_extent_ram_bytes(leaf_r, ei_r) &&
+                           btrfs_file_extent_compression(leaf_l, ei_l) ==
+                           btrfs_file_extent_compression(leaf_r, ei_r) &&
+                           btrfs_file_extent_encryption(leaf_l, ei_l) ==
+                           btrfs_file_extent_encryption(leaf_r, ei_r) &&
+                           btrfs_file_extent_other_encoding(leaf_l, ei_l) ==
+                           btrfs_file_extent_other_encoding(leaf_r, ei_r) &&
+                           btrfs_file_extent_type(leaf_l, ei_l) ==
+                           btrfs_file_extent_type(leaf_r, ei_r) &&
+                           btrfs_file_extent_disk_bytenr(leaf_l, ei_l) !=
+                           btrfs_file_extent_disk_bytenr(leaf_r, ei_r) &&
+                           btrfs_file_extent_disk_num_bytes(leaf_l, ei_l) ==
+                           btrfs_file_extent_disk_num_bytes(leaf_r, ei_r) &&
+                           btrfs_file_extent_offset(leaf_l, ei_l) ==
+                           btrfs_file_extent_offset(leaf_r, ei_r) &&
+                           btrfs_file_extent_num_bytes(leaf_l, ei_l) ==
+                           btrfs_file_extent_num_bytes(leaf_r, ei_r))
+                               return 0;
+               }
+
                inconsistent_snapshot_error(sctx, result, "extent");
                return -EIO;
        }
-- 
2.7.0.rc3

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