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    SubjectRe: [PATCH resend] memcg: introduce per-memcg reclaim interface
    On Thu, Mar 31, 2022 at 8:38 PM Wei Xu <weixugc@google.com> wrote:
    >
    > On Thu, Mar 31, 2022 at 5:33 PM Andrew Morton <akpm@linux-foundation.org> wrote:
    > >
    > > On Thu, 31 Mar 2022 08:41:51 +0000 Yosry Ahmed <yosryahmed@google.com> wrote:
    > >
    > > > --- a/mm/memcontrol.c
    > > > +++ b/mm/memcontrol.c
    > > > @@ -6355,6 +6355,38 @@ static ssize_t memory_oom_group_write(struct kernfs_open_file *of,
    > > > return nbytes;
    > > > }
    > > >
    > > > +static ssize_t memory_reclaim(struct kernfs_open_file *of, char *buf,
    > > > + size_t nbytes, loff_t off)
    > > > +{
    > > > + struct mem_cgroup *memcg = mem_cgroup_from_css(of_css(of));
    > > > + unsigned int nr_retries = MAX_RECLAIM_RETRIES;
    > > > + unsigned long nr_to_reclaim, nr_reclaimed = 0;
    > > > + int err;
    > > > +
    > > > + buf = strstrip(buf);
    > > > + err = page_counter_memparse(buf, "", &nr_to_reclaim);
    > > > + if (err)
    > > > + return err;
    > > > +
    > > > + while (nr_reclaimed < nr_to_reclaim) {
    > > > + unsigned long reclaimed;
    > > > +
    > > > + if (signal_pending(current))
    > > > + break;
    > > > +
    > > > + reclaimed = try_to_free_mem_cgroup_pages(memcg,
    > > > + nr_to_reclaim - nr_reclaimed,
    > > > + GFP_KERNEL, true);
    > > > +
    > > > + if (!reclaimed && !nr_retries--)
    > > > + break;
    > > > +
    > > > + nr_reclaimed += reclaimed;
    > > > + }
    > >
    > > Is there any way in which this can be provoked into triggering the
    > > softlockup detector?
    >
    > memory.reclaim is similar to memory.high w.r.t. reclaiming memory,
    > except that memory.reclaim is stateless, while the kernel remembers
    > the state set by memory.high. So memory.reclaim should not bring in
    > any new risks of triggering soft lockup, if any.
    >
    > > Is it optimal to do the MAX_RECLAIM_RETRIES loop in the kernel?
    > > Would additional flexibility be gained by letting userspace handle
    > > retrying?
    >
    > I agree it is better to retry from the userspace.

    Thanks Andrew and Wei for looking at this. IIUC the
    MAX_RECLAIM_RETRIES loop was modeled after the loop in memory.high as
    well. Is there a reason why it should be different here?

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