Message-ID: <SA3PR11MB8120AF70EF3C24DA43C2699BC9CC2@SA3PR11MB8120.namprd11.prod.outlook.com>
Date: Fri, 28 Feb 2025 10:00:04 +0000
From: "Sridhar, Kanchana P" <kanchana.p.sridhar@...el.com>
To: Yosry Ahmed <yosry.ahmed@...ux.dev>
CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>, "hannes@...xchg.org"
	<hannes@...xchg.org>, "nphamcs@...il.com" <nphamcs@...il.com>,
	"chengming.zhou@...ux.dev" <chengming.zhou@...ux.dev>,
	"usamaarif642@...il.com" <usamaarif642@...il.com>, "ryan.roberts@....com"
	<ryan.roberts@....com>, "21cnbao@...il.com" <21cnbao@...il.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
	"herbert@...dor.apana.org.au" <herbert@...dor.apana.org.au>,
	"davem@...emloft.net" <davem@...emloft.net>, "clabbe@...libre.com"
	<clabbe@...libre.com>, "ardb@...nel.org" <ardb@...nel.org>,
	"ebiggers@...gle.com" <ebiggers@...gle.com>, "surenb@...gle.com"
	<surenb@...gle.com>, "Accardi, Kristen C" <kristen.c.accardi@...el.com>,
	"Feghali, Wajdi K" <wajdi.k.feghali@...el.com>, "Gopal, Vinodh"
	<vinodh.gopal@...el.com>, "Sridhar, Kanchana P"
	<kanchana.p.sridhar@...el.com>
Subject: RE: [PATCH v6 12/16] mm: zswap: Allocate pool batching resources if
 the compressor supports batching.

Hi Yosry,

> -----Original Message-----
> From: Yosry Ahmed <yosry.ahmed@...ux.dev>
> Sent: Thursday, February 6, 2025 10:55 AM
> To: Sridhar, Kanchana P <kanchana.p.sridhar@...el.com>
> Cc: linux-kernel@...r.kernel.org; linux-mm@...ck.org;
> hannes@...xchg.org; nphamcs@...il.com; chengming.zhou@...ux.dev;
> usamaarif642@...il.com; ryan.roberts@....com; 21cnbao@...il.com;
> akpm@...ux-foundation.org; linux-crypto@...r.kernel.org;
> herbert@...dor.apana.org.au; davem@...emloft.net;
> clabbe@...libre.com; ardb@...nel.org; ebiggers@...gle.com;
> surenb@...gle.com; Accardi, Kristen C <kristen.c.accardi@...el.com>;
> Feghali, Wajdi K <wajdi.k.feghali@...el.com>; Gopal, Vinodh
> <vinodh.gopal@...el.com>
> Subject: Re: [PATCH v6 12/16] mm: zswap: Allocate pool batching resources if
> the compressor supports batching.
> 
> On Wed, Feb 05, 2025 at 11:20:58PM -0800, Kanchana P Sridhar wrote:
> > This patch adds support for the per-CPU acomp_ctx to track multiple
> > compression/decompression requests. The zswap_cpu_comp_prepare() cpu
> 
> nit: s/cpu/CPU

Thanks, this is fixed in v7.

> 
> > onlining code will check if the compressor supports batching. If so, it
> > will allocate the necessary batching resources.
> >
> > However, zswap does not use more than one request yet. Follow-up
> patches
> > will actually utilize the multiple acomp_ctx requests/buffers for batch
> > compression/decompression of multiple pages.
> >
> > The newly added ZSWAP_MAX_BATCH_SIZE limits the amount of extra
> memory used
> > for batching. There is no extra memory usage for compressors that do not
> > support batching.
> 
> That's not entirely accurate, there's a tiny bit of extra overhead to
> allocate the arrays. It can be avoided, but I am not sure it's worth the
> complexity.

You're right. The v7 commit log has been amended accordingly.

> 
> >
> > Signed-off-by: Kanchana P Sridhar <kanchana.p.sridhar@...el.com>
> > ---
> >  mm/zswap.c | 132 +++++++++++++++++++++++++++++++++++++++--------
> ------
> >  1 file changed, 98 insertions(+), 34 deletions(-)
> >
> > diff --git a/mm/zswap.c b/mm/zswap.c
> > index a2baceed3bf9..dc7d1ff04b22 100644
> > --- a/mm/zswap.c
> > +++ b/mm/zswap.c
> > @@ -78,6 +78,16 @@ static bool zswap_pool_reached_full;
> >
> >  #define ZSWAP_PARAM_UNSET ""
> >
> > +/*
> > + * For compression batching of large folios:
> > + * Maximum number of acomp compress requests that will be processed
> > + * in a batch, iff the zswap compressor supports batching.
> > + * This limit exists because we preallocate enough requests and buffers
> > + * in the per-cpu acomp_ctx accordingly. Hence, a higher limit means
> higher
> > + * memory usage.
> > + */
> > +#define ZSWAP_MAX_BATCH_SIZE 8U
> > +
> >  static int zswap_setup(void);
> >
> >  /* Enable/disable zswap */
> > @@ -143,9 +153,10 @@ bool zswap_never_enabled(void)
> >
> >  struct crypto_acomp_ctx {
> >  	struct crypto_acomp *acomp;
> > -	struct acomp_req *req;
> > +	struct acomp_req **reqs;
> > +	u8 **buffers;
> > +	unsigned int nr_reqs;
> >  	struct crypto_wait wait;
> > -	u8 *buffer;
> >  	struct mutex mutex;
> >  	bool is_sleepable;
> >  };
> > @@ -821,15 +832,13 @@ 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 = NULL;
> > -	struct acomp_req *req = NULL;
> > -	u8 *buffer = NULL;
> > -	int ret;
> > +	unsigned int nr_reqs = 1;
> > +	int ret = -ENOMEM;
> > +	int i;
> >
> > -	buffer = kmalloc_node(PAGE_SIZE * 2, GFP_KERNEL,
> cpu_to_node(cpu));
> > -	if (!buffer) {
> > -		ret = -ENOMEM;
> > -		goto fail;
> > -	}
> > +	acomp_ctx->buffers = NULL;
> > +	acomp_ctx->reqs = NULL;
> > +	acomp_ctx->nr_reqs = 0;
> >
> >  	acomp = crypto_alloc_acomp_node(pool->tfm_name, 0, 0,
> cpu_to_node(cpu));
> >  	if (IS_ERR(acomp)) {
> > @@ -839,12 +848,30 @@ static int zswap_cpu_comp_prepare(unsigned int
> cpu, struct hlist_node *node)
> >  		goto fail;
> >  	}
> >
> > -	req = acomp_request_alloc(acomp);
> > -	if (!req) {
> > -		pr_err("could not alloc crypto acomp_request %s\n",
> > -		       pool->tfm_name);
> > -		ret = -ENOMEM;
> > +	if (acomp_has_async_batching(acomp))
> > +		nr_reqs = min(ZSWAP_MAX_BATCH_SIZE,
> crypto_acomp_batch_size(acomp));
> 
> Do we need to check acomp_has_async_batching() here? Shouldn't
> crypto_acomp_batch_size() just return 1 if batching is not supported?

Good catch. This is fixed in v7.

> 
> > +
> > +	acomp_ctx->buffers = kcalloc_node(nr_reqs, sizeof(u8 *),
> GFP_KERNEL, cpu_to_node(cpu));
> > +	if (!acomp_ctx->buffers)
> > +		goto fail;
> > +
> > +	for (i = 0; i < nr_reqs; ++i) {
> > +		acomp_ctx->buffers[i] = kmalloc_node(PAGE_SIZE * 2,
> GFP_KERNEL, cpu_to_node(cpu));
> > +		if (!acomp_ctx->buffers[i])
> > +			goto fail;
> > +	}
> > +
> > +	acomp_ctx->reqs = kcalloc_node(nr_reqs, sizeof(struct acomp_req *),
> GFP_KERNEL, cpu_to_node(cpu));
> > +	if (!acomp_ctx->reqs)
> >  		goto fail;
> > +
> > +	for (i = 0; i < nr_reqs; ++i) {
> > +		acomp_ctx->reqs[i] = acomp_request_alloc(acomp);
> > +		if (!acomp_ctx->reqs[i]) {
> > +			pr_err("could not alloc crypto acomp_request
> reqs[%d] %s\n",
> > +			       i, pool->tfm_name);
> > +			goto fail;
> > +		}
> >  	}
> >
> >  	/*
> > @@ -853,6 +880,13 @@ static int zswap_cpu_comp_prepare(unsigned int
> cpu, struct hlist_node *node)
> >  	 * again resulting in a deadlock.
> >  	 */
> >  	mutex_lock(&acomp_ctx->mutex);
> 
> I had moved all the acomp_ctx initializations under the mutex to keep
> its state always fully initialized or uninitialized for anyone holding
> the lock. With this change, acomp_ctx->reqs will be set to non-NULL
> before the mutex is held and the acomp_ctx is fully initialized.
> 
> The code in the compression/decompression path uses acomp_ctx->reqes to
> check if the acomp_ctx can be used. While I don't believe it's currently
> possible for zswap_cpu_comp_prepare() to race with these paths, I did
> this to be future proof. I don't want the code to end up initializing
> some of the struct under the lock and some of it without it.
> 
> So I think there's two options here:
> - Do the due diligence check that holding the mutex is not required when
>   initializing acomp_ctx here, and remove the mutex locking here
>   completely.
> - Keep the initializations in the lock critical section (i.e. allocate
>   everything first, then initialize under the lock).

Thanks again, another great catch! In v7, I have attempted to simplify
the acomp_ctx resources allocation/deallocation vis-à-vis CPU onlining
and offlining. The main idea I have implemented is that the lifetime of
the acomp_ctx resources should be from pool creation to pool deletion.
The v7 code comments and commit log provide more details. I would greatly
appreciate your review comments on this approach.

> 
> > +
> > +	/*
> > +	 * The crypto_wait is used only in fully synchronous, i.e., with scomp
> > +	 * or non-poll mode of acomp, hence there is only one "wait" per
> > +	 * acomp_ctx, with callback set to reqs[0], under the assumption that
> > +	 * there is at least 1 request per acomp_ctx.
> > +	 */
> >  	crypto_init_wait(&acomp_ctx->wait);
> >
> >  	/*
> > @@ -860,20 +894,33 @@ static int zswap_cpu_comp_prepare(unsigned int
> cpu, struct hlist_node *node)
> >  	 * crypto_wait_req(); if the backend of acomp is scomp, the callback
> >  	 * won't be called, crypto_wait_req() will return without blocking.
> >  	 */
> > -	acomp_request_set_callback(req,
> CRYPTO_TFM_REQ_MAY_BACKLOG,
> > +	acomp_request_set_callback(acomp_ctx->reqs[0],
> CRYPTO_TFM_REQ_MAY_BACKLOG,
> >  				   crypto_req_done, &acomp_ctx->wait);
> >
> > -	acomp_ctx->buffer = buffer;
> > +	acomp_ctx->nr_reqs = nr_reqs;
> >  	acomp_ctx->acomp = acomp;
> >  	acomp_ctx->is_sleepable = acomp_is_async(acomp);
> > -	acomp_ctx->req = req;
> >  	mutex_unlock(&acomp_ctx->mutex);
> >  	return 0;
> >
> >  fail:
> > +	if (acomp_ctx->buffers) {
> > +		for (i = 0; i < nr_reqs; ++i)
> > +			kfree(acomp_ctx->buffers[i]);
> > +		kfree(acomp_ctx->buffers);
> > +		acomp_ctx->buffers = NULL;
> > +	}
> > +
> > +	if (acomp_ctx->reqs) {
> > +		for (i = 0; i < nr_reqs; ++i)
> > +			if (!IS_ERR_OR_NULL(acomp_ctx->reqs[i]))
> > +				acomp_request_free(acomp_ctx->reqs[i]);
> > +		kfree(acomp_ctx->reqs);
> > +		acomp_ctx->reqs = NULL;
> > +	}
> > +
> >  	if (acomp)
> >  		crypto_free_acomp(acomp);
> > -	kfree(buffer);
> >  	return ret;
> >  }
> >
> > @@ -883,14 +930,31 @@ static int zswap_cpu_comp_dead(unsigned int
> cpu, struct hlist_node *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;
> > +		int i;
> > +
> > +		if (acomp_ctx->reqs) {
> > +			for (i = 0; i < acomp_ctx->nr_reqs; ++i)
> > +				if (!IS_ERR_OR_NULL(acomp_ctx->reqs[i]))
> > +					acomp_request_free(acomp_ctx-
> >reqs[i]);
> > +			kfree(acomp_ctx->reqs);
> > +			acomp_ctx->reqs = NULL;
> > +		}
> > +
> > +		if (acomp_ctx->buffers) {
> > +			for (i = 0; i < acomp_ctx->nr_reqs; ++i)
> > +				kfree(acomp_ctx->buffers[i]);
> > +			kfree(acomp_ctx->buffers);
> > +			acomp_ctx->buffers = NULL;
> > +		}
> > +
> 
> The code here seems to be almost exactly like the failure path in
> zswap_cpu_comp_prepare(), would it be better to put it in a helper?

Makes sense, and addressed in v7.

> 
> >  		if (!IS_ERR_OR_NULL(acomp_ctx->acomp))
> >  			crypto_free_acomp(acomp_ctx->acomp);
> > -		kfree(acomp_ctx->buffer);
> > +
> > +		acomp_ctx->nr_reqs = 0;
> >  	}
> > +
> >  	mutex_unlock(&acomp_ctx->mutex);
> >
> >  	return 0;
> > @@ -903,7 +967,7 @@ static struct crypto_acomp_ctx
> *acomp_ctx_get_cpu_lock(struct zswap_pool *pool)
> >  	for (;;) {
> >  		acomp_ctx = raw_cpu_ptr(pool->acomp_ctx);
> >  		mutex_lock(&acomp_ctx->mutex);
> > -		if (likely(acomp_ctx->req))
> > +		if (likely(acomp_ctx->reqs))
> >  			return acomp_ctx;
> >  		/*
> >  		 * It is possible that we were migrated to a different CPU
> after
> > @@ -935,7 +999,7 @@ static bool zswap_compress(struct page *page,
> struct zswap_entry *entry,
> >  	u8 *dst;
> >
> >  	acomp_ctx = acomp_ctx_get_cpu_lock(pool);
> > -	dst = acomp_ctx->buffer;
> > +	dst = acomp_ctx->buffers[0];
> >  	sg_init_table(&input, 1);
> >  	sg_set_page(&input, page, PAGE_SIZE, 0);
> >
> > @@ -945,7 +1009,7 @@ static bool zswap_compress(struct page *page,
> struct zswap_entry *entry,
> >  	 * giving the dst buffer with enough length to avoid buffer overflow.
> >  	 */
> >  	sg_init_one(&output, dst, PAGE_SIZE * 2);
> > -	acomp_request_set_params(acomp_ctx->req, &input, &output,
> PAGE_SIZE, dlen);
> > +	acomp_request_set_params(acomp_ctx->reqs[0], &input, &output,
> PAGE_SIZE, dlen);
> >
> >  	/*
> >  	 * it maybe looks a little bit silly that we send an asynchronous
> request,
> > @@ -959,8 +1023,8 @@ static bool zswap_compress(struct page *page,
> struct zswap_entry *entry,
> >  	 * but in different threads running on different cpu, we have different
> >  	 * acomp instance, so multiple threads can do (de)compression in
> parallel.
> >  	 */
> > -	comp_ret = crypto_wait_req(crypto_acomp_compress(acomp_ctx-
> >req), &acomp_ctx->wait);
> > -	dlen = acomp_ctx->req->dlen;
> > +	comp_ret = crypto_wait_req(crypto_acomp_compress(acomp_ctx-
> >reqs[0]), &acomp_ctx->wait);
> > +	dlen = acomp_ctx->reqs[0]->dlen;
> >  	if (comp_ret)
> >  		goto unlock;
> >
> > @@ -1011,19 +1075,19 @@ static void zswap_decompress(struct
> zswap_entry *entry, struct folio *folio)
> >  	 */
> >  	if ((acomp_ctx->is_sleepable && !zpool_can_sleep_mapped(zpool))
> ||
> >  	    !virt_addr_valid(src)) {
> > -		memcpy(acomp_ctx->buffer, src, entry->length);
> > -		src = acomp_ctx->buffer;
> > +		memcpy(acomp_ctx->buffers[0], src, entry->length);
> > +		src = acomp_ctx->buffers[0];
> >  		zpool_unmap_handle(zpool, entry->handle);
> >  	}
> >
> >  	sg_init_one(&input, src, entry->length);
> >  	sg_init_table(&output, 1);
> >  	sg_set_folio(&output, folio, PAGE_SIZE, 0);
> > -	acomp_request_set_params(acomp_ctx->req, &input, &output,
> entry->length, PAGE_SIZE);
> > -	BUG_ON(crypto_wait_req(crypto_acomp_decompress(acomp_ctx-
> >req), &acomp_ctx->wait));
> > -	BUG_ON(acomp_ctx->req->dlen != PAGE_SIZE);
> > +	acomp_request_set_params(acomp_ctx->reqs[0], &input, &output,
> entry->length, PAGE_SIZE);
> > +	BUG_ON(crypto_wait_req(crypto_acomp_decompress(acomp_ctx-
> >reqs[0]), &acomp_ctx->wait));
> > +	BUG_ON(acomp_ctx->reqs[0]->dlen != PAGE_SIZE);
> >
> > -	if (src != acomp_ctx->buffer)
> > +	if (src != acomp_ctx->buffers[0])
> >  		zpool_unmap_handle(zpool, entry->handle);
> >  	acomp_ctx_put_unlock(acomp_ctx);
> >  }
> > --
> > 2.27.0
> >

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