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Fix 'zpool remap' freeing race

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The dmu_objset_remap_indirects_impl() logic depends on dnode_hold()
returning ENOENT for dnodes which will be freed and should be skipped.

This behavior can only be relied upon when taking a new hold and
while the caller has an open transaction.  This ensures that the
open txg cannot advance and that a concurrent free will end up
in the same txg (which is critical).  Relying on an existing hold
will not prevent dnode_free() from succeeding.

The solution is to take an additional dnode_hold() after assigning
the transaction.  This ensures the remap will never dirty the dnode
if it was freed while we were waiting in dmu_tx_assign(, TXG_WAIT).

Randomly set zfs_object_remap_one_indirect_delay_ms in ztest.  This
increases the likelihood of an operation racing with the remap.
Converted from ticks to milliseconds.

Reviewed by: Matt Ahrens <mahrens@delphix.com>
Reviewed by: Tom Caputi <tcaputi@datto.com>
Reviewed by: Igor Kozhukhov <igor@dilos.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #8215
This commit is contained in:
Brian Behlendorf 2019-01-02 11:46:04 -08:00 committed by GitHub
parent 06f3fc2a4b
commit 65ca2c1eb9
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GPG Key ID: 4AEE18F83AFDEB23
2 changed files with 32 additions and 10 deletions
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8
cmd/ztest/ztest.c
View File

@ -215,6 +215,8 @@ extern int dmu_object_alloc_chunk_shift;
extern boolean_t zfs_force_some_double_word_sm_entries;
extern unsigned long zio_decompress_fail_fraction;
extern unsigned long zfs_reconstruct_indirect_damage_fraction;
extern int zfs_object_remap_one_indirect_delay_ms;
static ztest_shared_opts_t *ztest_shared_opts;
static ztest_shared_opts_t ztest_opts;
@ -6526,6 +6528,12 @@ ztest_resume_thread(void *arg)
*/
if (ztest_random(10) == 0)
zfs_abd_scatter_enabled = ztest_random(2);
/*
* Periodically inject remapping delays (10% of the time).
*/
zfs_object_remap_one_indirect_delay_ms =
ztest_random(10) == 0 ? ztest_random(1000) + 1 : 0;
}
thread_exit();

32
module/zfs/dmu.c
View File

@ -76,9 +76,9 @@ int zfs_dmu_offset_next_sync = 0;
/*
* This can be used for testing, to ensure that certain actions happen
* while in the middle of a remap (which might otherwise complete too
* quickly).
* quickly). Used by ztest(8).
*/
int zfs_object_remap_one_indirect_delay_ticks = 0;
int zfs_object_remap_one_indirect_delay_ms = 0;
const dmu_object_type_info_t dmu_ot[DMU_OT_NUMTYPES] = {
{DMU_BSWAP_UINT8, TRUE, FALSE, FALSE, "unallocated" },
@ -1075,6 +1075,7 @@ dmu_object_remap_one_indirect(objset_t *os, dnode_t *dn,
uint64_t last_removal_txg, uint64_t offset)
{
uint64_t l1blkid = dbuf_whichblock(dn, 1, offset);
dnode_t *dn_tx;
int err = 0;
rw_enter(&dn->dn_struct_rwlock, RW_READER);
@ -1093,7 +1094,9 @@ dmu_object_remap_one_indirect(objset_t *os, dnode_t *dn,
/*
* If this L1 was already written after the last removal, then we've
* already tried to remap it.
* already tried to remap it. An additional hold is taken after the
* dmu_tx_assign() to handle the case where the dnode is freed while
* waiting for the next open txg.
*/
if (birth <= last_removal_txg &&
dbuf_read(dbuf, NULL, DB_RF_MUST_SUCCEED) == 0 &&
@ -1101,8 +1104,12 @@ dmu_object_remap_one_indirect(objset_t *os, dnode_t *dn,
dmu_tx_t *tx = dmu_tx_create(os);
dmu_tx_hold_remap_l1indirect(tx, dn->dn_object);
err = dmu_tx_assign(tx, TXG_WAIT);
if (err == 0) {
err = dnode_hold(os, dn->dn_object, FTAG, &dn_tx);
if (err == 0) {
(void) dbuf_dirty(dbuf, tx);
dnode_rele(dn_tx, FTAG);
}
dmu_tx_commit(tx);
} else {
dmu_tx_abort(tx);
@ -1111,7 +1118,7 @@ dmu_object_remap_one_indirect(objset_t *os, dnode_t *dn,
dbuf_rele(dbuf, FTAG);
delay(zfs_object_remap_one_indirect_delay_ticks);
delay(MSEC_TO_TICK(zfs_object_remap_one_indirect_delay_ms));
return (err);
}
@ -1133,7 +1140,7 @@ dmu_object_remap_indirects(objset_t *os, uint64_t object,
{
uint64_t offset, l1span;
int err;
dnode_t *dn;
dnode_t *dn, *dn_tx;
err = dnode_hold(os, object, FTAG, &dn);
if (err != 0) {
@ -1148,13 +1155,20 @@ dmu_object_remap_indirects(objset_t *os, uint64_t object,
/*
* If the dnode has no indirect blocks, we cannot dirty them.
* We still want to remap the blkptr(s) in the dnode if
* appropriate, so mark it as dirty.
* appropriate, so mark it as dirty. An additional hold is
* taken after the dmu_tx_assign() to handle the case where
* the dnode is freed while waiting for the next open txg.
*/
if (err == 0 && dnode_needs_remap(dn)) {
dmu_tx_t *tx = dmu_tx_create(os);
dmu_tx_hold_bonus(tx, dn->dn_object);
if ((err = dmu_tx_assign(tx, TXG_WAIT)) == 0) {
dnode_setdirty(dn, tx);
dmu_tx_hold_bonus(tx, object);
err = dmu_tx_assign(tx, TXG_WAIT);
if (err == 0) {
err = dnode_hold(os, object, FTAG, &dn_tx);
if (err == 0) {
dnode_setdirty(dn_tx, tx);
dnode_rele(dn_tx, FTAG);
}
dmu_tx_commit(tx);
} else {
dmu_tx_abort(tx);