Re: [PATCH] mm/zswap: Fix the compatibility of zsmalloc and zswap

[Vitaly Wool <vitaly.wool@konsulko.com>]

On Thu, Dec 24, 2020 at 1:16 PM Tian Tao <tiantao6@hisilicon.com> 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 |  2 ++
>  mm/z3fold.c           |  1 +
>  mm/zbud.c             |  1 +
>  mm/zpool.c            | 13 +++++++++++++
>  mm/zsmalloc.c         |  1 +
>  mm/zswap.c            | 23 +++++++++++++++++++++--
>  6 files changed, 39 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/zpool.h b/include/linux/zpool.h
> index 51bf430..a354f4fe 100644
> --- a/include/linux/zpool.h
> +++ b/include/linux/zpool.h
> @@ -82,6 +82,7 @@ u64 zpool_get_total_size(struct zpool *pool);
>   */
>  struct zpool_driver {
>         char *type;
> +       bool sleep_mapped;

Could you please add this somewhat further down, close to  map / unmap
callbacks, and add a description of the field above, too?
>         struct module *owner;
>         atomic_t refcount;
>         struct list_head list;
> @@ -112,5 +113,6 @@ 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/z3fold.c b/mm/z3fold.c
> index dacb0d7..234b46f 100644
> --- a/mm/z3fold.c
> +++ b/mm/z3fold.c
> @@ -1778,6 +1778,7 @@ static u64 z3fold_zpool_total_size(void *pool)
>
>  static struct zpool_driver z3fold_zpool_driver = {
>         .type =         "z3fold",
> +       .sleep_mapped = true,
>         .owner =        THIS_MODULE,
>         .create =       z3fold_zpool_create,
>         .destroy =      z3fold_zpool_destroy,
> diff --git a/mm/zbud.c b/mm/zbud.c
> index c49966e..7ec5f27 100644
> --- a/mm/zbud.c
> +++ b/mm/zbud.c
> @@ -203,6 +203,7 @@ static u64 zbud_zpool_total_size(void *pool)
>
>  static struct zpool_driver zbud_zpool_driver = {
>         .type =         "zbud",
> +       .sleep_mapped = true,
>         .owner =        THIS_MODULE,
>         .create =       zbud_zpool_create,
>         .destroy =      zbud_zpool_destroy,
> 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/zsmalloc.c b/mm/zsmalloc.c
> index 7289f50..a991d3c 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -440,6 +440,7 @@ static u64 zs_zpool_total_size(void *pool)
>
>  static struct zpool_driver zs_zpool_driver = {
>         .type =                   "zsmalloc",
> +       .sleep_mapped =           false,

You don't have to add this, it will be false if not explicitly specified.

>         .owner =                  THIS_MODULE,
>         .create =                 zs_zpool_create,
>         .destroy =                zs_zpool_destroy,
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 182f6ad..51b033a 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -1235,7 +1235,7 @@ 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 +1256,27 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
>                 goto freeentry;
>         }
>
> +       if (!zpool_can_sleep_mapped(entry->pool->zpool)) {
> +
> +               tmp = kzalloc(entry->length, GFP_KERNEL);

Do we really need kzalloc here? the buffer will be written to in a few
moments anyway.
Also, we allocate a small buffer for a short while, and this is likely
to be a performance critical path, so GFP_ATOMIC looks more
appropriate.

> +               if (!tmp)
> +                       return -ENOMEM;

You can't just return here, you need to call zswap_entry_put() first.

Since this is obviously not the final version of your patch, could you
please split it in 2 the next time? I'd prefer setting flags to
specific backends in a separate patch.

Best regards,
   Vitaly

> +       }
> +
>         /* 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 +1286,11 @@ 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:
> --
> 2.7.4
>