On Wed, Dec 20, 2023 at 4:20 AM Chengming Zhou
<zhouchengming@xxxxxxxxxxxxx> wrote:
>
> On 2023/12/20 05:39, Yosry Ahmed wrote:
> > On Tue, Dec 19, 2023 at 10:43 AM Nhat Pham <nphamcs@xxxxxxxxx> wrote:
> >>
> >> On Tue, Dec 19, 2023 at 5:29 AM Chris Li <chrisl@xxxxxxxxxx> wrote:
> >>>
> >>> Hi Chengming and Yosry,
> >>>
> >>> On Mon, Dec 18, 2023 at 3:50 AM Chengming Zhou
> >>> <zhouchengming@xxxxxxxxxxxxx> wrote:
> >>>>
> >>>> Since the introduce of reusing the dstmem in the load path, it seems
> >>>> confusing that we are now using acomp_ctx->dstmem and acomp_ctx->mutex
> >>>> now for purposes other than what the naming suggests.
> >>>>
> >>>> Yosry suggested removing these two fields from acomp_ctx, and directly
> >>>> using zswap_dstmem and zswap_mutex in both the load and store paths,
> >>>> rename them, and add proper comments above their definitions that they
> >>>> are for generic percpu buffering on the load and store paths.
> >>>>
> >>>> So this patch remove dstmem and mutex from acomp_ctx, and rename the
> >>>> zswap_dstmem to zswap_buffer, using the percpu mutex and buffer on
> >>>> the load and store paths.
> >>>
> >>> Sorry joining this discussion late.
> >>>
> >>> I get the rename of "dstmem" to "buffer". Because the buffer is used
> >>> for both load and store as well. What I don't get is that, why do we
> >>> move it out of the acomp_ctx struct. Now we have 3 per cpu entry:
> >>> buffer, mutex and acomp_ctx. I think we should do the reverse, fold
> >>> this three per cpu entry into one struct the acomp_ctx. Each per_cpu
> >>> load() has a sequence of dance around the cpu id and disable preempt
> >>> etc, while each of the struct member load is just a plan memory load.
> >>> It seems to me it would be more optimal to combine this three per cpu
> >>> entry into acomp_ctx. Just do the per cpu for the acomp_ctx once.
> >>
> >> I agree with Chris. From a practicality POV, what Chris says here
> >> makes sense. From a semantic POV, this buffer is only used in
> >> (de)compression contexts - be it in store, load, or writeback - so it
> >> belonging to the orignal struct still makes sense to me. Why separate
> >> it out, without any benefits. Just rename the old field buffer or
> >> zswap_buffer and call it a day? It will be a smaller patch too!
> >>
> >
> > My main concern is that the struct name is specific for the crypto
> > acomp stuff, but that buffer and mutex are not.
> > How about we keep it in the struct, but refactor the struct as follows:
> >
> > struct zswap_ctx {
> > struct {
> > struct crypto_acomp *acomp;
> > struct acomp_req *req;
> > struct crypto_wait wait;
> > } acomp_ctx;
> > u8 *dstmem;
> > struct mutex *mutex;
> > };
> >
> > , and then rename zswap_pool.acomp_ctx to zswap_pool.ctx?
>
> I think there are two viewpoints here, both works ok to me.
>
> The first is from ownship or lifetime, these percpu mutex and dstmem
> are shared between all pools, so no one pool own the mutex and dstmem,
> nor does the percpu acomp_ctx in each pool.
>
> The second is from usage, these percpu mutex and dstmem are used by
> the percpu acomp_ctx in each pool, so it's easy to use to put pointers
> to them in the percpu acomp_ctx.
>
> Actually I think it's simpler to let the percpu acomp_ctx has its own
> mutex and dstmem, which in fact are the necessary parts when it use
> the acomp interfaces.
>
> This way, we could delete the percpu mutex and dstmem, and its hotplugs,
> and not shared anymore between all pools. Maybe we would have many pools
> at the same time in the future, like different compression algorithm or
> different zpool for different memcg workloads. Who knows? :-)
>
> So how about this patch below? Just RFC.
The general approach looks fine to me, although I still prefer we
reorganize the struct as Chris and I discussed: rename
crypto_acomp_ctx to a generic name, add a (anonymous) struct for the
crypto_acomp stuff, rename dstmem to buffer or so.
I think we can also make the mutex a static part of the struct, any
advantage to dynamically allocating it?
>
> Subject: [PATCH] mm/zswap: make each acomp_ctx has its own mutex and dstmem
>
> Signed-off-by: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>
> ---
> include/linux/cpuhotplug.h | 1 -
> mm/zswap.c | 86 ++++++++++++--------------------------
> 2 files changed, 26 insertions(+), 61 deletions(-)
>
> diff --git a/include/linux/cpuhotplug.h b/include/linux/cpuhotplug.h
> index efc0c0b07efb..c3e06e21766a 100644
> --- a/include/linux/cpuhotplug.h
> +++ b/include/linux/cpuhotplug.h
> @@ -124,7 +124,6 @@ enum cpuhp_state {
> CPUHP_ARM_BL_PREPARE,
> CPUHP_TRACE_RB_PREPARE,
> CPUHP_MM_ZS_PREPARE,
> - CPUHP_MM_ZSWP_MEM_PREPARE,
> CPUHP_MM_ZSWP_POOL_PREPARE,
> CPUHP_KVM_PPC_BOOK3S_PREPARE,
> CPUHP_ZCOMP_PREPARE,
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 2c349fd88904..37301f1a80a9 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -694,63 +694,31 @@ static void zswap_alloc_shrinker(struct zswap_pool *pool)
> /*********************************
> * per-cpu code
> **********************************/
> -static DEFINE_PER_CPU(u8 *, zswap_dstmem);
> -/*
> - * If users dynamically change the zpool type and compressor at runtime, i.e.
> - * zswap is running, zswap can have more than one zpool on one cpu, but they
> - * are sharing dtsmem. So we need this mutex to be per-cpu.
> - */
> -static DEFINE_PER_CPU(struct mutex *, zswap_mutex);
> -
> -static int zswap_dstmem_prepare(unsigned int cpu)
> -{
> - struct mutex *mutex;
> - u8 *dst;
> -
> - dst = kmalloc_node(PAGE_SIZE, GFP_KERNEL, cpu_to_node(cpu));
> - if (!dst)
> - return -ENOMEM;
> -
> - mutex = kmalloc_node(sizeof(*mutex), GFP_KERNEL, cpu_to_node(cpu));
> - if (!mutex) {
> - kfree(dst);
> - return -ENOMEM;
> - }
> -
> - mutex_init(mutex);
> - per_cpu(zswap_dstmem, cpu) = dst;
> - per_cpu(zswap_mutex, cpu) = mutex;
> - return 0;
> -}
> -
> -static int zswap_dstmem_dead(unsigned int cpu)
> -{
> - struct mutex *mutex;
> - u8 *dst;
> -
> - mutex = per_cpu(zswap_mutex, cpu);
> - kfree(mutex);
> - per_cpu(zswap_mutex, cpu) = NULL;
> -
> - dst = per_cpu(zswap_dstmem, cpu);
> - kfree(dst);
> - per_cpu(zswap_dstmem, cpu) = NULL;
> -
> - return 0;
> -}
> -
> static int zswap_cpu_comp_prepare(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);
> struct crypto_acomp *acomp;
> struct acomp_req *req;
> + int ret = 0;
> +
> + acomp_ctx->dstmem = kmalloc_node(PAGE_SIZE, GFP_KERNEL, cpu_to_node(cpu));
> + if (!acomp_ctx->dstmem)
> + return -ENOMEM;
> +
> + acomp_ctx->mutex = kmalloc_node(sizeof(struct mutex), GFP_KERNEL, cpu_to_node(cpu));
> + if (!acomp_ctx->mutex) {
> + ret = -ENOMEM;
> + goto mutex_fail;
> + }
> + mutex_init(acomp_ctx->mutex);
>
> acomp = crypto_alloc_acomp_node(pool->tfm_name, 0, 0, cpu_to_node(cpu));
> if (IS_ERR(acomp)) {
> pr_err("could not alloc crypto acomp %s : %ld\n",
> pool->tfm_name, PTR_ERR(acomp));
> - return PTR_ERR(acomp);
> + ret = PTR_ERR(acomp);
> + goto acomp_fail;
> }
> acomp_ctx->acomp = acomp;
>
> @@ -758,8 +726,8 @@ static int zswap_cpu_comp_prepare(unsigned int cpu, struct hlist_node *node)
> if (!req) {
> pr_err("could not alloc crypto acomp_request %s\n",
> pool->tfm_name);
> - crypto_free_acomp(acomp_ctx->acomp);
> - return -ENOMEM;
> + ret = -ENOMEM;
> + goto req_fail;
> }
> acomp_ctx->req = req;
>
> @@ -772,10 +740,15 @@ static int zswap_cpu_comp_prepare(unsigned int cpu, struct hlist_node *node)
> acomp_request_set_callback(req, CRYPTO_TFM_REQ_MAY_BACKLOG,
> crypto_req_done, &acomp_ctx->wait);
>
> - acomp_ctx->mutex = per_cpu(zswap_mutex, cpu);
> - acomp_ctx->dstmem = per_cpu(zswap_dstmem, cpu);
> -
> return 0;
> +req_fail:
> + crypto_free_acomp(acomp_ctx->acomp);
> +acomp_fail:
> + kfree(acomp_ctx->mutex);
> +mutex_fail:
> + kfree(acomp_ctx->dstmem);
> +
> + return ret;
> }
>
> static int zswap_cpu_comp_dead(unsigned int cpu, struct hlist_node *node)
> @@ -788,6 +761,8 @@ static int zswap_cpu_comp_dead(unsigned int cpu, struct hlist_node *node)
> acomp_request_free(acomp_ctx->req);
> if (!IS_ERR_OR_NULL(acomp_ctx->acomp))
> crypto_free_acomp(acomp_ctx->acomp);
> + kfree(acomp_ctx->mutex);
> + kfree(acomp_ctx->dstmem);
> }
>
> return 0;
> @@ -1901,13 +1876,6 @@ static int zswap_setup(void)
> goto cache_fail;
> }
>
> - ret = cpuhp_setup_state(CPUHP_MM_ZSWP_MEM_PREPARE, "mm/zswap:prepare",
> - zswap_dstmem_prepare, zswap_dstmem_dead);
> - if (ret) {
> - pr_err("dstmem alloc failed\n");
> - goto dstmem_fail;
> - }
> -
> ret = cpuhp_setup_state_multi(CPUHP_MM_ZSWP_POOL_PREPARE,
> "mm/zswap_pool:prepare",
> zswap_cpu_comp_prepare,
> @@ -1939,8 +1907,6 @@ static int zswap_setup(void)
> if (pool)
> zswap_pool_destroy(pool);
> hp_fail:
> - cpuhp_remove_state(CPUHP_MM_ZSWP_MEM_PREPARE);
> -dstmem_fail:
> kmem_cache_destroy(zswap_entry_cache);
> cache_fail:
> /* if built-in, we aren't unloaded on failure; don't allow use */
> --
> 2.20.1
>
