From 4e1d1b4b927711db71e93d64f052aba6a0dbe6e4 Mon Sep 17 00:00:00 2001 From: Matthew Ahrens Date: Thu, 7 Jan 2021 20:06:32 -0800 Subject: [PATCH] assertion failed in arc_wait_for_eviction() If the system is very low on memory (specifically, `arc_free_memory() < arc_sys_free/2`, i.e. less than 1/16th of RAM free), `arc_evict_state_impl()` will defer wakups. In this case, the arc_evict_waiter_t's remain on the list, even though `arc_evict_count` has been incremented past their `aew_count`. The problem is that `arc_wait_for_eviction()` assumes that if there are waiters on the list, the count they are waiting for has not yet been reached. However, the deferred wakeups may violate this, causing `ASSERT(last->aew_count > arc_evict_count)` to fail. This commit resolves the issue by having new waiters use the greater of `arc_evict_count` and the last `aew_count`. Reviewed-by: Brian Behlendorf Reviewed-by: George Wilson Reviewed-by: George Amanakis Signed-off-by: Matthew Ahrens Closes #11285 Closes #11397 --- module/zfs/arc.c | 22 ++++++++++++++-------- 1 file changed, 14 insertions(+), 8 deletions(-) diff --git a/module/zfs/arc.c b/module/zfs/arc.c index af6f52e97..0d776832c 100644 --- a/module/zfs/arc.c +++ b/module/zfs/arc.c @@ -4050,7 +4050,7 @@ arc_evict_state_impl(multilist_t *ml, int idx, arc_buf_hdr_t *marker, mutex_enter(&arc_evict_lock); arc_evict_count += bytes_evicted; - if ((int64_t)(arc_free_memory() - arc_sys_free / 2) > 0) { + if (arc_free_memory() > arc_sys_free / 2) { arc_evict_waiter_t *aw; while ((aw = list_head(&arc_evict_waiters)) != NULL && aw->aew_count <= arc_evict_count) { @@ -5136,14 +5136,20 @@ arc_wait_for_eviction(uint64_t amount) list_link_init(&aw.aew_node); cv_init(&aw.aew_cv, NULL, CV_DEFAULT, NULL); - arc_evict_waiter_t *last = - list_tail(&arc_evict_waiters); - if (last != NULL) { - ASSERT3U(last->aew_count, >, arc_evict_count); - aw.aew_count = last->aew_count + amount; - } else { - aw.aew_count = arc_evict_count + amount; + uint64_t last_count = 0; + if (!list_is_empty(&arc_evict_waiters)) { + arc_evict_waiter_t *last = + list_tail(&arc_evict_waiters); + last_count = last->aew_count; } + /* + * Note, the last waiter's count may be less than + * arc_evict_count if we are low on memory in which + * case arc_evict_state_impl() may have deferred + * wakeups (but still incremented arc_evict_count). + */ + aw.aew_count = + MAX(last_count, arc_evict_count) + amount; list_insert_tail(&arc_evict_waiters, &aw);