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

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

On Fri, Dec 25, 2020 at 12:02 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 |  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.
>   * @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);

This has escaped my attention, but this is obviously a bug. 'entry' is
not initialized at this point.
You either have to move memory allocation further down when
entry->length starts making sense, or allocate PAGE_SIZE.

Best regards,
   Vitaly

> +               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
>