Re: [PATCH v2 2/2] mm: zswap: disable migration while using per-CPU acomp_ctx

[Yosry Ahmed <yosryahmed@google.com>]

On Tue, Jan 7, 2025 at 9:34 PM Yosry Ahmed <yosryahmed@google.com> wrote:
>
> On Tue, Jan 7, 2025 at 9:00 PM Chengming Zhou <chengming.zhou@linux.dev> wrote:
> >
> > On 2025/1/8 12:46, Nhat Pham wrote:
> > > On Wed, Jan 8, 2025 at 9:34 AM Yosry Ahmed <yosryahmed@google.com> wrote:
> > >>
> > >>
> > >> Actually, using the mutex to protect against CPU hotunplug is not too
> > >> complicated. The following diff is one way to do it (lightly tested).
> > >> Johannes, Nhat, any preferences between this patch (disabling
> > >> migration) and the following diff?
> > >
> > > I mean if this works, this over migration diasbling any day? :)
> > >
> > >>
> > >> diff --git a/mm/zswap.c b/mm/zswap.c
> > >> index f6316b66fb236..4d6817c679a54 100644
> > >> --- a/mm/zswap.c
> > >> +++ b/mm/zswap.c
> > >> @@ -869,17 +869,40 @@ static int zswap_cpu_comp_dead(unsigned int cpu,
> > >> struct hlist_node *node)
> > >>          struct zswap_pool *pool = hlist_entry(node, struct zswap_pool, node);
> > >>          struct crypto_acomp_ctx *acomp_ctx = per_cpu_ptr(pool->acomp_ctx, cpu);
> > >>
> > >> +       mutex_lock(&acomp_ctx->mutex);
> > >>          if (!IS_ERR_OR_NULL(acomp_ctx)) {
> > >>                  if (!IS_ERR_OR_NULL(acomp_ctx->req))
> > >>                          acomp_request_free(acomp_ctx->req);
> > >> +               acomp_ctx->req = NULL;
> > >>                  if (!IS_ERR_OR_NULL(acomp_ctx->acomp))
> > >>                          crypto_free_acomp(acomp_ctx->acomp);
> > >>                  kfree(acomp_ctx->buffer);
> > >>          }
> > >> +       mutex_unlock(&acomp_ctx->mutex);
> > >>
> > >>          return 0;
> > >>   }
> > >>
> > >> +static struct crypto_acomp_ctx *acomp_ctx_get_cpu_locked(
> > >> +               struct crypto_acomp_ctx __percpu *acomp_ctx)
> > >> +{
> > >> +       struct crypto_acomp_ctx *ctx;
> > >> +
> > >> +       for (;;) {
> > >> +               ctx = raw_cpu_ptr(acomp_ctx);
> > >> +               mutex_lock(&ctx->mutex);
> > >
> > > I'm a bit confused. IIUC, ctx is per-cpu right? What's protecting this
> > > cpu-local data (including the mutex) from being invalidated under us
> > > while we're sleeping and waiting for the mutex?
>
> Please correct me if I am wrong, but my understanding is that memory
> allocated with alloc_percpu() is allocated for each *possible* CPU,
> and does not go away when CPUs are offlined. We allocate the per-CPU
> crypto_acomp_ctx structs with alloc_percpu() (including the mutex), so
> they should not go away with CPU offlining.
>
> OTOH, we allocate the crypto_acomp_ctx.acompx, crypto_acomp_ctx.req,
> and crypto_acomp_ctx.buffer only for online CPUs through the CPU
> hotplug notifiers (i.e. zswap_cpu_comp_prepare() and
> zswap_cpu_comp_dead()). These are the resources that can go away with
> CPU offlining, and what we need to protect about.

..and now that I explain all of this I am wondering if the complexity
is warranted in the first place. It goes back all the way to the first
zswap commit, so I can't tell the justification for it.

I am not sure if they are setups that have significantly different
numbers of online and possible CPUs. Maybe we should just bite the
bullet and just allocate everything with alloc_percpu() and rip out
the hotplug callbacks completely.