From: Johannes Weiner hannes@cmpxchg.org Subject: Revert "mm: memcontrol: avoid workload stalls when lowering memory.high"

This reverts commit 536d3bf261a2fc3b05b3e91e7eef7383443015cf, as it can cause writers to memory.high to get stuck in the kernel forever, performing page reclaim and consuming excessive amounts of CPU cycles.

Before the patch, a write to memory.high would first put the new limit in place for the workload, and then reclaim the requested delta. After the patch, the kernel tries to reclaim the delta before putting the new limit into place, in order to not overwhelm the workload with a sudden, large excess over the limit. However, if reclaim is actively racing with new allocations from the uncurbed workload, it can keep the write() working inside the kernel indefinitely.

This is causing problems in Facebook production. A privileged system-level daemon that adjusts memory.high for various workloads running on a host can get unexpectedly stuck in the kernel and essentially turn into a sort of involuntary kswapd for one of the workloads. We've observed that daemon busy-spin in a write() for minutes at a time, neglecting its other duties on the system, and expending privileged system resources on behalf of a workload.

To remedy this, we have first considered changing the reclaim logic to break out after a couple of loops - whether the workload has converged to the new limit or not - and bound the write() call this way. However, the root cause that inspired the sequence change in the first place has been fixed through other means, and so a revert back to the proven limit-setting sequence, also used by memory.max, is preferable.

The sequence was changed to avoid extreme latencies in the workload when the limit was lowered: the sudden, large excess created by the limit lowering would erroneously trigger the penalty sleeping code that is meant to throttle excessive growth from below. Allocating threads could end up sleeping long after the write() had already reclaimed the delta for which they were being punished.

However, erroneous throttling also caused problems in other scenarios at around the same time. This resulted in commit b3ff92916af3 ("mm, memcg: reclaim more aggressively before high allocator throttling"), included in the same release as the offending commit. When allocating threads now encounter large excess caused by a racing write() to memory.high, instead of entering punitive sleeps, they will simply be tasked with helping reclaim down the excess, and will be held no longer than it takes to accomplish that. This is in line with regular limit enforcement - i.e. if the workload allocates up against or over an otherwise unchanged limit from below.

With the patch breaking userspace, and the root cause addressed by other means already, revert it again.

Link: https://lkml.kernel.org/r/20210122184341.292461-1-hannes@cmpxchg.org Fixes: 536d3bf261a2 ("mm: memcontrol: avoid workload stalls when lowering memory.high") Signed-off-by: Johannes Weiner hannes@cmpxchg.org Reported-by: Tejun Heo tj@kernel.org Acked-by: Chris Down chris@chrisdown.name Acked-by: Michal Hocko mhocko@suse.com Cc: Roman Gushchin guro@fb.com Cc: Shakeel Butt shakeelb@google.com Cc: Michal Koutný mkoutny@suse.com Cc: stable@vger.kernel.org [5.8+] Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/memcontrol.c | 5 ++--- 1 file changed, 2 insertions(+), 3 deletions(-)

--- a/mm/memcontrol.c~revert-mm-memcontrol-avoid-workload-stalls-when-lowering-memoryhigh +++ a/mm/memcontrol.c @@ -6271,6 +6271,8 @@ static ssize_t memory_high_write(struct if (err) return err;

+ page_counter_set_high(&memcg->memory, high); + for (;;) { unsigned long nr_pages = page_counter_read(&memcg->memory); unsigned long reclaimed; @@ -6294,10 +6296,7 @@ static ssize_t memory_high_write(struct break; }

- page_counter_set_high(&memcg->memory, high); - memcg_wb_domain_size_changed(memcg); - return nbytes; }

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