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

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Message-ID: <CAKEwX=OHcZB8Uy5zj8Dhq-ieiwpJFcqRXN_7=mbM1FD_h_uOOA@mail.gmail.com>
Date: Wed, 8 Jan 2025 11:46:15 +0700
From: Nhat Pham <nphamcs@...il.com>
To: Yosry Ahmed <yosryahmed@...gle.com>
Cc: Johannes Weiner <hannes@...xchg.org>, Andrew Morton <akpm@...ux-foundation.org>, 
	Chengming Zhou <chengming.zhou@...ux.dev>, Vitaly Wool <vitalywool@...il.com>, 
	Barry Song <baohua@...nel.org>, Sam Sun <samsun1006219@...il.com>, 
	"linux-mm@...ck.org" <linux-mm@...ck.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
	"stable@...r.kernel.org" <stable@...r.kernel.org>, 
	"Sridhar, Kanchana P" <kanchana.p.sridhar@...el.com>
Subject: Re: [PATCH v2 2/2] mm: zswap: disable migration while using per-CPU acomp_ctx

On Wed, Jan 8, 2025 at 9:34 AM Yosry Ahmed <yosryahmed@...gle.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?

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);
>         /*