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btrfs-progs: Use mirror_num start from 1 to avoid unnecessary retry 

[BUG]
If the first copy of a tree block is corrupted but the other copy is
good, btrfs-progs will report the error twice:
  checksum verify failed on 30556160 found 42A2DA71 wanted 00000000
  checksum verify failed on 30556160 found 42A2DA71 wanted 00000000

While kernel only report it once, just as expected:
  BTRFS warning (device dm-3): dm-3 checksum verify failed on 30556160 wanted 0 found 42A2DA71 level 0

[CAUSE]
We use mirror_num = 0 in read_tree_block() of btrfs-progs.

At first glance it's pretty OK, but mirror num 0 in btrfs means ANY
good copy. Real mirror num starts from 1.
In the context of read_tree_block(), since it's read_tree_block() to do
all the checks, mirror num 0 just means the first copy.

So if the first copy is corrupted, btrfs-progs will try mirror num 1
next, which is just the same as mirror num 0.
After reporting the same error on the same copy, btrfs-progs will
finally try mirror num 2, and get the good copy.

[FIX]
The fix is way simpler than all the above analyse, just starts from
mirror num 1.

Signed-off-by: Qu Wenruo <wqu@suse.com>
master
Qu Wenruo 2019-03-07 19:31:31 +08:00
parent db51d8d8f6
commit e8ae577030
1 changed files with 2 additions and 2 deletions
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4
disk-io.c
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@ -325,7 +325,7 @@ struct extent_buffer* read_tree_block(struct btrfs_fs_info *fs_info, u64 bytenr,
struct extent_buffer *eb;
u64 best_transid = 0;
u32 sectorsize = fs_info->sectorsize;
int mirror_num = 0;
int mirror_num = 1;
int good_mirror = 0;
int num_copies;
int ignore = 0;
@ -381,7 +381,7 @@ struct extent_buffer* read_tree_block(struct btrfs_fs_info *fs_info, u64 bytenr,
ignore = 1;
continue;
}
if (btrfs_header_generation(eb) > best_transid && mirror_num) {
if (btrfs_header_generation(eb) > best_transid) {
best_transid = btrfs_header_generation(eb);
good_mirror = mirror_num;
}