On 2016-09-12 09:27, Jeff Mahoney wrote:
On 9/12/16 2:54 PM, Austin S. Hemmelgarn wrote:
On 2016-09-12 08:33, Jeff Mahoney wrote:
On 9/9/16 8:47 PM, Austin S. Hemmelgarn wrote:
A couple of other things to comment about on this:
1. 'can_overcommit' (the function that the Arch kernel choked on) is
from the memory management subsystem. The fact that that's throwing a
null pointer says to me either your hardware has issues, or the Arch
kernel itself has problems (which would probably mean the kernel image
is corrupted).
fs/btrfs/extent-tree.c:
static int can_overcommit(struct btrfs_root *root,
struct btrfs_space_info *space_info, u64 bytes,
enum btrfs_reserve_flush_enum flush)
OK, my bad there, but that begs the question: why does a BTRFS function
not have a BTRFS prefix? The name blatantly sounds like a mm function
(and I could have sworn I can across one with an almost identical name
when I was trying to understand the mm code a couple months ago), and
the lack of a prefix combined with that heavily implies that it's a core
kernel function.
Given this, it's almost certainly the balance choking on corrupted
metadata that's causing the issue.
Because it's a static function and has a namespace limited to the
current C file. If we prefixed every function in a local namespace with
the subsystem, the code would be unreadable. At any rate, the full
symbol name in the Oops is:
can_overcommit+0x1e/0x110 [btrfs]
So we do identify the proper namespace in the Oops already.
Which somehow I missed...
Again, apologies for the confusion, I'm not used to reading an OOPS out
of a picture of a CRT, and less so when trying to get someone help as
quick as possible.
3. In general, it's a good idea to keep an eye on space usage on your
filesystems. If it's getting to be more than about 95% full, you should
be looking at getting some more storage space. This is especially true
for BTRFS, as a 100% full BTRFS filesystem functionally becomes
permanently read-only because there's nowhere for the copy-on-write
updates to write to.
The entire point of having the global metadata reserve is to avoid that
situation.
Except that the global metadata reserve is usually only just barely big
enough, and it only works for metadata. While I get that this issue is
what it's supposed to fix, it doesn't do so in a way that makes it easy
to get out of that situation. The reserve itself is often not big
enough to do anything in any reasonable amount of time once the FS gets
beyond about a hundred GB and yous tart talking about very large files.
Why would it need to apply to data? The reserve is used to meet the
reservation requirements to CoW metadata blocks needed to release the
data blocks. The data blocks themselves aren't touched; they're only
released. The size of the file really should only matter in terms of
how many extent items need to be released but it shouldn't matter at all
in terms of how many blocks the file's data occupies. E.g. a 100 GB
file that uses a handful of extents would be essentially free in this
context.
I'm not saying it needs to apply to data, but it would be nice if things
didn't blow up such that you immediately have to start deleting files or
add more space when the data chunks become full.
As far as the sizing, I have had multiple times where the largest file
in the filesystem couldn't be deleted because of the number of extents
when the rest of the FS was full and GlobalReserve was being used for
metadata operations (I don't know if it's significant, but I only saw
this on filesystems with compress=lzo).
--
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
