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?
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);
> /*
