On 12/12/24 12:21, Rik van Riel wrote: > On Thu, 2024-12-12 at 10:15 +1100, Balbir Singh wrote: >> On 12/12/24 02:53, Rik van Riel wrote: >>> >>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c >>> index 7b3503d12aaf..03d77e93087e 100644 >>> --- a/mm/memcontrol.c >>> +++ b/mm/memcontrol.c >>> @@ -5371,6 +5371,15 @@ bool >>> mem_cgroup_zswap_writeback_enabled(struct mem_cgroup *memcg) >>> if (!zswap_is_enabled()) >>> return true; >>> >>> + /* >>> + * Always allow exiting tasks to push data to swap. A >>> process in >>> + * the middle of exit cannot get OOM killed, but may need >>> to push >>> + * uncompressible data to swap in order to get the cgroup >>> memory >>> + * use below the limit, and make progress with the exit. >>> + */ >>> + if ((current->flags & PF_EXITING) && memcg == >>> mem_cgroup_from_task(current)) >>> + return true; >>> + >>> for (; memcg; memcg = parent_mem_cgroup(memcg)) >>> if (!READ_ONCE(memcg->zswap_writeback)) >>> return false; >> >> Rik, >> >> I am unable to understand the motivation here, so we want >> mem_cgroup_zswap_writeback_enabled() to return true, it only >> returns false if a memcg in the hierarchy has zswap_writeback >> set to 0 (false). In my git-grep I can't seem to find how/why >> that may be the case. I can see memcg starts of with the value >> set to true, if CONFIG_ZSWAP is enabled. >> >> Your changelog above makes sense, but I am unable to map it to >> the code changes. >> > > Wait, are you asking about the code that I'm > adding, or about the code that was already > there? > > I want to add the code that allows zswap > writeback if the reclaiming task is exiting, > and in the same cgroup as the to be written > back memory. > I was asking about this change (this patch), I know that the return true will help avoid the PAGE_ACTIVATE path, but I am not sure why this function will return false if CONFIG_ZSWAP is enabled (unless zswap_writeback is turned off in one of the groups) Balbir
