Re: [RFC mm/zswap 1/2] mm/zswap: add the flag can_sleep_mapped

[Minchan Kim <minchan@kernel.org>]

On Fri, Dec 25, 2020 at 07:02:50PM +0800, Tian Tao wrote:
> add a flag to zpool, named is "can_sleep_mapped", and have it set true
> for zbud/z3fold, set false for zsmalloc. Then zswap could go the current
> path if the flag is true; and if it's false, copy data from src to a
> temporary buffer, then unmap the handle, take the mutex, process the
> buffer instead of src to avoid sleeping function called from atomic
> context.
> 
> Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
> ---
>  include/linux/zpool.h |  3 +++
>  mm/zpool.c            | 13 +++++++++++++
>  mm/zswap.c            | 50 +++++++++++++++++++++++++++++++++++++++++++++-----
>  3 files changed, 61 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/zpool.h b/include/linux/zpool.h
> index 51bf430..e899701 100644
> --- a/include/linux/zpool.h
> +++ b/include/linux/zpool.h
> @@ -73,6 +73,7 @@ u64 zpool_get_total_size(struct zpool *pool);
>   * @malloc:	allocate mem from a pool.
>   * @free:	free mem from a pool.
>   * @shrink:	shrink the pool.
> + * @sleep_mapped: whether zpool driver can sleep during map.

I don't think it's a good idea. It just breaks zpool abstraction
in that it exposes internal implementation to user to avoid issue
zswap recently introduced. It also conflicts zpool_map_handle's
semantic.

Rather than introducing another break in zpool due to the new
zswap feature recenlty introduced, zswap could introduce
CONFIG_ZSWAP_HW_COMPRESSOR. Once it's configured, zsmalloc could
be disabled. And with disabling CONFIG_ZSWAP_HW_COMPRESSOR, zswap
doesn't need to make any bounce buffer copy so that no existing
zsmalloc user will see performance regression.


>   * @map:	map a handle.
>   * @unmap:	unmap a handle.
>   * @total_size:	get total size of a pool.
> @@ -100,6 +101,7 @@ struct zpool_driver {
>  	int (*shrink)(void *pool, unsigned int pages,
>  				unsigned int *reclaimed);
>  
> +	bool sleep_mapped;
>  	void *(*map)(void *pool, unsigned long handle,
>  				enum zpool_mapmode mm);
>  	void (*unmap)(void *pool, unsigned long handle);
> @@ -112,5 +114,6 @@ void zpool_register_driver(struct zpool_driver *driver);
>  int zpool_unregister_driver(struct zpool_driver *driver);
>  
>  bool zpool_evictable(struct zpool *pool);
> +bool zpool_can_sleep_mapped(struct zpool *pool);
>  
>  #endif
> diff --git a/mm/zpool.c b/mm/zpool.c
> index 3744a2d..5ed7120 100644
> --- a/mm/zpool.c
> +++ b/mm/zpool.c
> @@ -23,6 +23,7 @@ struct zpool {
>  	void *pool;
>  	const struct zpool_ops *ops;
>  	bool evictable;
> +	bool can_sleep_mapped;
>  
>  	struct list_head list;
>  };
> @@ -183,6 +184,7 @@ struct zpool *zpool_create_pool(const char *type, const char *name, gfp_t gfp,
>  	zpool->pool = driver->create(name, gfp, ops, zpool);
>  	zpool->ops = ops;
>  	zpool->evictable = driver->shrink && ops && ops->evict;
> +	zpool->can_sleep_mapped = driver->sleep_mapped;
>  
>  	if (!zpool->pool) {
>  		pr_err("couldn't create %s pool\n", type);
> @@ -393,6 +395,17 @@ bool zpool_evictable(struct zpool *zpool)
>  	return zpool->evictable;
>  }
>  
> +/**
> + * zpool_can_sleep_mapped - Test if zpool can sleep when do mapped.
> + * @zpool:	The zpool to test
> + *
> + * Returns: true if zpool can sleep; false otherwise.
> + */
> +bool zpool_can_sleep_mapped(struct zpool *zpool)
> +{
> +	return zpool->can_sleep_mapped;
> +}
> +
>  MODULE_LICENSE("GPL");
>  MODULE_AUTHOR("Dan Streetman <ddstreet@ieee.org>");
>  MODULE_DESCRIPTION("Common API for compressed memory storage");
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 182f6ad..67d4555 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -935,13 +935,20 @@ static int zswap_writeback_entry(struct zpool *pool, unsigned long handle)
>  	struct scatterlist input, output;
>  	struct crypto_acomp_ctx *acomp_ctx;
>  
> -	u8 *src;
> +	u8 *src, *tmp;
>  	unsigned int dlen;
>  	int ret;
>  	struct writeback_control wbc = {
>  		.sync_mode = WB_SYNC_NONE,
>  	};
>  
> +	if (!zpool_can_sleep_mapped(pool)) {
> +
> +		tmp = kmalloc(entry->length, GFP_ATOMIC);
> +		if (!tmp)
> +			return -ENOMEM;
> +	}
> +
>  	/* extract swpentry from data */
>  	zhdr = zpool_map_handle(pool, handle, ZPOOL_MM_RO);
>  	swpentry = zhdr->swpentry; /* here */
> @@ -979,6 +986,14 @@ static int zswap_writeback_entry(struct zpool *pool, unsigned long handle)
>  		dlen = PAGE_SIZE;
>  		src = (u8 *)zhdr + sizeof(struct zswap_header);
>  
> +		if (!zpool_can_sleep_mapped(pool)) {
> +
> +			memcpy(tmp, src, entry->length);
> +			src = tmp;
> +
> +			zpool_unmap_handle(pool, handle);
> +		}
> +
>  		mutex_lock(acomp_ctx->mutex);
>  		sg_init_one(&input, src, entry->length);
>  		sg_init_table(&output, 1);
> @@ -1033,7 +1048,11 @@ static int zswap_writeback_entry(struct zpool *pool, unsigned long handle)
>  	spin_unlock(&tree->lock);
>  
>  end:
> -	zpool_unmap_handle(pool, handle);
> +	if (zpool_can_sleep_mapped(pool))
> +		zpool_unmap_handle(pool, handle);
> +	else
> +		kfree(tmp);
> +
>  	return ret;
>  }
>  
> @@ -1235,7 +1254,7 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
>  	struct zswap_entry *entry;
>  	struct scatterlist input, output;
>  	struct crypto_acomp_ctx *acomp_ctx;
> -	u8 *src, *dst;
> +	u8 *src, *dst, *tmp;
>  	unsigned int dlen;
>  	int ret;
>  
> @@ -1256,12 +1275,29 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
>  		goto freeentry;
>  	}
>  
> +	if (!zpool_can_sleep_mapped(entry->pool->zpool)) {
> +
> +		tmp = kmalloc(entry->length, GFP_ATOMIC);
> +		if (!tmp) {
> +			ret = -ENOMEM;
> +			goto freeentry;
> +		}
> +	}
> +
>  	/* decompress */
>  	dlen = PAGE_SIZE;
>  	src = zpool_map_handle(entry->pool->zpool, entry->handle, ZPOOL_MM_RO);
>  	if (zpool_evictable(entry->pool->zpool))
>  		src += sizeof(struct zswap_header);
>  
> +	if (!zpool_can_sleep_mapped(entry->pool->zpool)) {
> +
> +		memcpy(tmp, src, entry->length);
> +		src = tmp;
> +
> +		zpool_unmap_handle(entry->pool->zpool, entry->handle);
> +	}
> +
>  	acomp_ctx = raw_cpu_ptr(entry->pool->acomp_ctx);
>  	mutex_lock(acomp_ctx->mutex);
>  	sg_init_one(&input, src, entry->length);
> @@ -1271,7 +1307,11 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
>  	ret = crypto_wait_req(crypto_acomp_decompress(acomp_ctx->req), &acomp_ctx->wait);
>  	mutex_unlock(acomp_ctx->mutex);
>  
> -	zpool_unmap_handle(entry->pool->zpool, entry->handle);
> +	if (zpool_can_sleep_mapped(entry->pool->zpool))
> +		zpool_unmap_handle(entry->pool->zpool, entry->handle);
> +	else
> +		kfree(tmp);
> +
>  	BUG_ON(ret);
>  
>  freeentry:
> @@ -1279,7 +1319,7 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
>  	zswap_entry_put(tree, entry);
>  	spin_unlock(&tree->lock);
>  
> -	return 0;
> +	return ret;
>  }
>  
>  /* frees an entry in zswap */
> -- 
> 2.7.4
> 
>