[#LU-10902] mdd_changelog_user_purge() must check current number of users safer and earlier to disable ChangeLogs recording if none

During my analysis of a situation where a ChangeLogs user's purge was taking a very long time (due to single user never being unregistered and idle since ages), I have found that the current code (still true in master) in mdd_changelog_user_purge() only disable ChangeLogs recording when the last user's purge has been synchronously done, meaning that more records will unnecessary continue to be recorded (and compete adding overhead) until the full purge completes :

int mdd_changelog_user_purge(const struct lu_env *env,
                             struct mdd_device *mdd, __u32 id)
{
        struct mdd_changelog_user_purge mcup = {
                .mcup_id = id,
                .mcup_found = false,
                .mcup_usercount = 0,
                .mcup_minrec = ULLONG_MAX,
        };
        struct llog_ctxt *ctxt;
        int rc;

        ENTRY;

        CDEBUG(D_IOCTL, "%s: Purge request: id=%u\n",
               mdd2obd_dev(mdd)->obd_name, id);

        ctxt = llog_get_context(mdd2obd_dev(mdd),
                                LLOG_CHANGELOG_USER_ORIG_CTXT);
        if (ctxt == NULL ||
            (ctxt->loc_handle->lgh_hdr->llh_flags & LLOG_F_IS_CAT) == 0)
                GOTO(out, rc = -ENXIO);

        rc = llog_cat_process(env, ctxt->loc_handle,
                              mdd_changelog_user_purge_cb, &mcup,
                              0, 0);

        if ((rc == 0) && mcup.mcup_found) {
                CDEBUG(D_IOCTL, "%s: Purging changelog entries for user %d "
                       "record=%llu\n",
                       mdd2obd_dev(mdd)->obd_name, id, mcup.mcup_minrec);
                /* Cancelling record 0 destroys the entire changelog, make sure
                   we don't do that unless we mean it. */
                if (mcup.mcup_minrec != 0 || mcup.mcup_usercount == 0) {
                        rc = mdd_changelog_llog_cancel(env, mdd, <<<<<< sync, and can take a long 
                                                       mcup.mcup_minrec); <<<<<<<<< time if huge backlog
                }
        } else {
                CWARN("%s: No changelog for user %u; rc=%d\n",
                      mdd2obd_dev(mdd)->obd_name, id, rc);
                GOTO(out, rc = -ENOENT);
        }

        if ((rc == 0) && (mcup.mcup_usercount == 0)) {
                /* No more users; turn changelogs off */
                CDEBUG(D_IOCTL, "turning off changelogs\n");
                rc = mdd_changelog_off(env, mdd); <<<<<<<<< ChangeLog recording being stopped
        }

        EXIT;
out:
        if (ctxt != NULL)
                llog_ctxt_put(ctxt);

        return rc;
}

Also, ChangeLog recording is being stopped without any regard nor protection against the fact a new user may have registered in the interval, leading to a situation where a user can be registered but not ChangeLog recording will happen.

[LU-10902] mdd_changelog_user_purge() must check current number of users safer and earlier to disable ChangeLogs recording if none Created: 11/Apr/18 Updated: 06/May/18 Resolved: 06/May/18
Status:	Resolved
Project:	Lustre
Component/s:	None
Affects Version/s:	None
Fix Version/s:	Lustre 2.12.0

[LU-10902] mdd_changelog_user_purge() must check current number of users safer and earlier to disable ChangeLogs recording if none Created: 11/Apr/18 Updated: 06/May/18 Resolved: 06/May/18