PROBLEM
When `dmu_tx_assign` is called from `zil_lwb_write_issue`, it's possible
for either `ERESTART` or `EIO` to be returned.
If `ERESTART` is returned, this will cause an assertion to fail directly
in `zil_lwb_write_issue`, where the code assumes the return value is
`EIO` if `dmu_tx_assign` returns a non-zero value. This can occur if the
SPA is suspended when `dmu_tx_assign` is called, and most often occurs
when running `zloop`.
If `EIO` is returned, this can cause assertions to fail elsewhere in the
ZIL code. For example, `zil_commit_waiter_timeout` contains the
following logic:
lwb_t *nlwb = zil_lwb_write_issue(zilog, lwb);
ASSERT3S(lwb->lwb_state, !=, LWB_STATE_OPENED);
In this case, if `dmu_tx_assign` returned `EIO` from within
`zil_lwb_write_issue`, the `lwb` variable passed in will not be issued
to disk. Thus, it's `lwb_state` field will remain `LWB_STATE_OPENED` and
this assertion will fail. `zil_commit_waiter_timeout` assumes that after
it calls `zil_lwb_write_issue`, the `lwb` will be issued to disk, and
doesn't handle the case where this is not true; i.e. it doesn't handle
the case where `dmu_tx_assign` returns `EIO`.
SOLUTION
This change modifies the `dmu_tx_assign` function such that `txg_how` is
a bitmask, rather than of the `txg_how_t` enum type. Now, `TXG_WAITED`
can be specified with either `TXG_NOWAIT` or `TXG_WAIT` semantics.
Previously, when `TXG_WAITED` was specified, `TXG_NOWAIT` semantics was
automatically invoked. This was not ideal when using `TXG_WAITED` within
`zil_lwb_write_issued`, leading the problem described above. Rather, we
want to achieve the semantics of `TXG_WAIT`, while also preventing the
`tx` from being penalized via the dirty delay throttling.
With this change, `zil_lwb_write_issued` can acheive the semtantics that
it requires by passing in the value `TXG_WAIT | TXG_WAITED` to
`dmu_tx_assign`.
Further, consumers of `dmu_tx_assign` wishing to achieve the old
`TXG_WAITED` semantics can pass in the value `TXG_NOWAIT | TXG_WAITED`.
You can view, comment on, or merge this pull request online at:
https://github.com/openzfs/openzfs/pull/514
-- Commit Summary --
* 0000 ztest assertion failure in zil_lwb_write_issue (v3)
-- File Changes --
M usr/src/uts/common/fs/zfs/dmu_tx.c (40)
M usr/src/uts/common/fs/zfs/sys/dmu.h (15)
M usr/src/uts/common/fs/zfs/sys/dmu_tx.h (2)
M usr/src/uts/common/fs/zfs/zfs_vnops.c (19)
M usr/src/uts/common/fs/zfs/zil.c (21)
-- Patch Links --
https://github.com/openzfs/openzfs/pull/514.patch
https://github.com/openzfs/openzfs/pull/514.diff
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