On 7/2/20 9:54 AM, Qu Wenruo wrote:
On 2020/7/2 下午9:43, Josef Bacik wrote:
On 7/1/20 8:14 PM, Qu Wenruo wrote:
[PROBLEM]
There are known problem related to how btrfs handles qgroup reserved
space.
One of the most obvious case is the the test case btrfs/153, which do
fallocate, then write into the preallocated range.
btrfs/153 1s ... - output mismatch (see
xfstests-dev/results//btrfs/153.out.bad)
--- tests/btrfs/153.out 2019-10-22 15:18:14.068965341 +0800
+++ xfstests-dev/results//btrfs/153.out.bad 2020-07-01
20:24:40.730000089 +0800
@@ -1,2 +1,5 @@
QA output created by 153
+pwrite: Disk quota exceeded
+/mnt/scratch/testfile2: Disk quota exceeded
+/mnt/scratch/testfile2: Disk quota exceeded
Silence is golden
...
(Run 'diff -u xfstests-dev/tests/btrfs/153.out
xfstests-dev/results//btrfs/153.out.bad' to see the entire diff)
[CAUSE]
Since commit c6887cd11149 ("Btrfs: don't do nocow check unless we have
to"),
we always reserve space no matter if it's COW or not.
Such behavior change is mostly for performance, and reverting it is not
a good idea anyway.
For preallcoated extent, we reserve qgroup data space for it already,
and since we also reserve data space for qgroup at buffered write time,
it needs twice the space for us to write into preallocated space.
This leads to the -EDQUOT in buffered write routine.
And we can't follow the same solution, unlike data/meta space check,
qgroup reserved space is shared between data/meta.
The EDQUOT can happen at the metadata reservation, so doing NODATACOW
check after qgroup reservation failure is not a solution.
Why not? I get that we don't know for sure how we failed, but in the
case of a write we're way more likely to have failed for data reasons
right?
Nope, mostly we failed at metadata reservation, as that would return
EDQUOT to user space.
We may have some cases which get EDQUOT at data reservation part, but
that's what we excepted.
(And already what we're doing)
The problem is when the metadata reservation failed with EDQUOT.
So why not just fall back to the NODATACOW check and then do the
metadata reservation. Then if it fails again you know its a real EDQUOT
and your done.
Or if you want to get super fancy you could even break up the metadata
and data reservations here so that we only fall through to the NODATACOW
check if we fail the data reservation. Thanks,
The problem is, qgroup doesn't split metadata and data (yet).
Currently data and meta shares the same limit.
So when we hit EDQUOT, you have no guarantee it would happen only in
qgroup data reservation.
Sure, but if you are able to do the nocow thing, then presumably your quota
reservation is less now? So on failure you go do the complicated nocow check,
and if it succeeds you retry your quota reservation with just the metadata
portion, right? Thanks,
Josef
