Josef Bacik 於 2020-01-07 00:05 寫到:
On 1/5/20 10:45 PM, ethanwu wrote:
Josef Bacik 於 2020-01-04 00:31 寫到:
On 1/3/20 4:44 AM, ethanwu wrote:
Btrfs has two types of data backref.
For BTRFS_EXTENT_DATA_REF_KEY type of backref, we don't have the
exact block number. Therefore, we need to call resolve_indirect_refs
which uses btrfs_search_slot to locate the leaf block. After that,
we need to walk through the leafs to search for the EXTENT_DATA
items
that have disk bytenr matching the extent item(add_all_parents).
The only conditions we'll stop searching are
1. We find different object id or type is not EXTENT_DATA
2. We've already got all the refs we want(total_refs)
Take the following EXTENT_ITEM as example:
item 11 key (40831553536 EXTENT_ITEM 4194304) itemoff 15460 itemsize
95
extent refs 24 gen 7302 flags DATA
extent data backref root 257 objectid 260 offset 65536 count 5
#backref entry 1
extent data backref root 258 objectid 265 offset 0 count 9
#backref entry 2
shared data backref parent 394985472 count 10 #backref entry 3
If we want to search for backref entry 1, total_refs here would be
24 rather
than its count 5.
The reason to use 24 is because some EXTENT_DATA in backref entry 3
block
394985472 also points to EXTENT_ITEM 40831553536, if this block also
belongs to
root 257 and lies between these 5 items of backref entry 1,
and we use total_refs = 5, we'll end up missing some refs from
backref
entry 1.
This seems like the crux of the problem here. The backref stuff is
just blindly looking for counts, without keeping track of which
counts
matter. So for full refs we should only be looking down paths where
generation > the snapshot generation. And then for the shared refs
it
should be anything that comes from that shared block. That would be
the proper way to fix the problem, not put some arbitrary limit on
how
far into the inode we can search.
I am not sure if generation could be used to skip blocks for
full(indirect) backref.
For exmple:
create a data extent in subvol id 257 at generation 10
At generation 11, take snapshot(suppose the snapshot id is 258) from
subvol 257.
When we send snapshot 258, all the tree blocks it searches comes from
subvol 257,
since snapshot only copy root node from its source,
none of tree blocks in subvol 257 has generation(all <= 10) > snapshot
generation(11)
Or do I miss something?
Nope I was saying it wrong, sorry about that. What I should say is
for "backref entry 1" we should _only_ walk down paths that belong to
root 257, and then for root 258 we _only_ walk down paths that belong
to 258, and then we do our normal dance for indirect refs.
That's not to say what you are doing here is wrong, we really won't
have anything past the given extent size so we can definitely break
out earlier. But what I worry about is say 394985472 _was_ in
between
the leaves while searching down for backref entry #1, we'd end up
with
duplicate entries and not catch some of the other entries. This
feels
This patch doesn't adjust the total_refs. Is there any example that
this patch will ruin the backref walking?
No I'm talking about a general failure of the current code, your patch
doesn't make it better or worse.
like we need to fix the backref logic to know if it's looking for
direct refs, and thus only go down paths with generation > snapshot
generation, or shared refs and thus only count things that directly
point to the parent block. Thanks,
Ok, I agree, my patch doesn't solve the original problem:
When resolving indirect refs, we could take entries that don't
belong to the backref entry we are searching for right now.
If this need to be fixed, I think it could be done by the following
way
item 11 key (40831553536 EXTENT_ITEM 4194304) itemoff 15460 itemsize
extent refs 24 gen 7302 flags DATA
shared data backref parent 394985472 count 10 #backref entry
1
extent data backref root 257 objectid 260 offset 1048576
count 3 #backref entry 2
extent data backref root 256 objectid 260 offset 65536 count
6 #backref entry 3
extent data backref root 257 objectid 260 offset 65536 count
5 #backref entry 4
When searching for entry 4, the EXTENT_DATA entries that match the
EXTENT_ITEM bytenr
will be in one of the following situations:
1. shared block that just happens to be part of root 257. For every
leaf we run into,
check its bytenr to see if it is a shared data backref entry, if
so skip it.
We may need an extra list or rb tree to store this information.
We don't need to worry about this case, because if we have a normal
ref then the whole path down to that bytenr belongs wholey to that
root. The full backref will only be in paths that were not touched by
the referencing root.
Thank you for the review,
I don't fully understand the way shared data backref is used in btrfs.
It took me a while to check the backref code and do some experiment.
One way shared backref will be used is balance, all the items used
by relocation tree use shared backref.
After running balance, if any EXTENT_ITEM is moved during balance,
all the back reference of that newly-located EXTENT_ITEM will become
shared, and the block owner is exactly the original root.
We could then produce normal reference by just COWIng the tree block,
and leaving some of the shared backrefs unchanged,(dd an 128MB extent
and cow every other 4K blocks, so these items span across many
leafs and COWing one block leaves the other shared backrefs untouched)
In the end, we have two types of back reference from the same root,
and yet the owner of all these blocks are the same root.
Therefore, I think this condition is still needed.
2. same subvol, inode but different offset. Right now in
add_all_parents, we only
check if bytenr matches. Adding extra check to see if backref
offset is the same
(here backref entry 1: 65536 != entry 3: 1048576)
Yeah we definitely need to do this because clone can change the offset
and point at the same bytenr, so we for sure want to only match on
matching offsets.
3. This might happen if subvol 257 is a snapshot from subvol 256,
check leaf owner, if
not 257 skip it.
Yup, this is the "only walk down paths owned by the root" thing.
4. None of the above, it's type 4 backref entry, this is what we want,
add it!
In this way, we only count entries that matter, and total_refs could
be
changed from total refs of that extent item to number of each entry.
Then, we could break from loop as soon as possible.
Will this look better?
Yup this is what I want, because then we are for sure always getting
exactly the right refs. I would do
1) Make a path->only_this_root (or something better named) that _only_
walks down paths that are owned by the original root. We use this for
non-full backref walking (backref entry 2, 3, and 4 in the example
above). If we have a normal extent backref that references a real
root then we know for sure if we walk down to that bytenr from that
root the whole path will be owned by that root.
2) Match on the offset in the extent data ref. We don't want to find
unrelated clones, we want exactly the right match.
3) Keep track of how many refs for that backref. For the backref
entry #4 example we'd use the above strategies and break as soon as we
found 5 entries.
4) Take a good hard look at the indirect backref resolution code. I
feel like it's probably mostly ok, we just fix the above things for
normal backref lookups and it'll just work, so we'll definitely only
find references that come from that indirect backref. But I haven't
looked too closely so I could be wrong.
I think we're on the same page now, hopefully it's not too much extra
work. But it will be by far more robust and reliable. Thanks,
Thanks for the steps provided, I've been looking the backref code, and
so far haven't found any thing that might break the idea. I'll start
working and do some test.
Thanks,
ethanwu
Josef
