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

[Chengming Zhou <chengming.zhou@xxxxxxxxx>]

On 2025/1/8 12:46, Nhat Pham wrote:
> On Wed, Jan 8, 2025 at 9:34 AM Yosry Ahmed <yosryahmed@xxxxxxxxxx> 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?

Yeah, it's not safe, we can only use this_cpu_ptr(), which will disable
preempt (so cpu offline can't kick in), and get refcount of ctx. Since
we can't mutex_lock in the preempt disabled section.

Thanks.

> If it is somehow protected, then yeah this seems quite elegant :)
>
> > +               if (likely(ctx->req))
> > +                       return ctx;
> > +               /* Raced with zswap_cpu_comp_dead() on CPU hotunplug */
> > +               mutex_unlock(&ctx->mutex);
> > +       }
> > +}
> > +
> > +static void acomp_ctx_put_unlock(struct crypto_acomp_ctx *ctx)
> > +{
> > +       mutex_unlock(&ctx->mutex);
> > +}
> > +
> >   static bool zswap_compress(struct page *page, struct zswap_entry *entry,
> >                             struct zswap_pool *pool)
> >   {
> > @@ -893,10 +916,7 @@ static bool zswap_compress(struct page *page,
> > struct zswap_entry *entry,
> >          gfp_t gfp;
> >          u8 *dst;
> >
> > -       acomp_ctx = raw_cpu_ptr(pool->acomp_ctx);
> > -
> > -       mutex_lock(&acomp_ctx->mutex);
> > -
> > +       acomp_ctx = acomp_ctx_get_cpu_locked(pool->acomp_ctx);
> >          dst = acomp_ctx->buffer;
> >          sg_init_table(&input, 1);
> >          sg_set_page(&input, page, PAGE_SIZE, 0);
> > @@ -949,7 +969,7 @@ static bool zswap_compress(struct page *page,
> > struct zswap_entry *entry,
> >          else if (alloc_ret)
> >                  zswap_reject_alloc_fail++;
> >
> > -       mutex_unlock(&acomp_ctx->mutex);
> > +       acomp_ctx_put_unlock(acomp_ctx);
> >          return comp_ret == 0 && alloc_ret == 0;
> >   }
> >
> > @@ -960,9 +980,7 @@ static void zswap_decompress(struct zswap_entry
> > *entry, struct folio *folio)
> >          struct crypto_acomp_ctx *acomp_ctx;
> >          u8 *src;
> >
> > -       acomp_ctx = raw_cpu_ptr(entry->pool->acomp_ctx);
> > -       mutex_lock(&acomp_ctx->mutex);
> > -
> > +       acomp_ctx = acomp_ctx_get_cpu_locked(entry->pool->acomp_ctx);
> >          src = zpool_map_handle(zpool, entry->handle, ZPOOL_MM_RO);
> >          /*