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SubjectRe: [PATCH/RFC] zswap: do not map same object twice
On Sun, Sep 15, 2019 at 5:46 PM Vitaly Wool <vitalywool@gmail.com> wrote:
>
> zswap_writeback_entry() maps a handle to read swpentry first, and
> then in the most common case it would map the same handle again.
> This is ok when zbud is the backend since its mapping callback is
> plain and simple, but it slows things down for z3fold.
>
> Since there's hardly a point in unmapping a handle _that_ fast as
> zswap_writeback_entry() does when it reads swpentry, the
> suggestion is to keep the handle mapped till the end.

LGTM

>
> Signed-off-by: Vitaly Wool <vitalywool@gmail.com>

Reviewed-by: Dan Streetman <ddstreet@ieee.org>

> ---
> mm/zswap.c | 7 +++----
> 1 file changed, 3 insertions(+), 4 deletions(-)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 0e22744a76cb..b35464bc7315 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -856,7 +856,6 @@ static int zswap_writeback_entry(struct zpool *pool, unsigned long handle)
> /* extract swpentry from data */
> zhdr = zpool_map_handle(pool, handle, ZPOOL_MM_RO);
> swpentry = zhdr->swpentry; /* here */
> - zpool_unmap_handle(pool, handle);
> tree = zswap_trees[swp_type(swpentry)];
> offset = swp_offset(swpentry);
>
> @@ -866,6 +865,7 @@ static int zswap_writeback_entry(struct zpool *pool, unsigned long handle)
> if (!entry) {
> /* entry was invalidated */
> spin_unlock(&tree->lock);
> + zpool_unmap_handle(pool, handle);
> return 0;
> }
> spin_unlock(&tree->lock);
> @@ -886,15 +886,13 @@ static int zswap_writeback_entry(struct zpool *pool, unsigned long handle)
> case ZSWAP_SWAPCACHE_NEW: /* page is locked */
> /* decompress */
> dlen = PAGE_SIZE;
> - src = (u8 *)zpool_map_handle(entry->pool->zpool, entry->handle,
> - ZPOOL_MM_RO) + sizeof(struct zswap_header);
> + src = (u8 *)zhdr + sizeof(struct zswap_header);
> dst = kmap_atomic(page);
> tfm = *get_cpu_ptr(entry->pool->tfm);
> ret = crypto_comp_decompress(tfm, src, entry->length,
> dst, &dlen);
> put_cpu_ptr(entry->pool->tfm);
> kunmap_atomic(dst);
> - zpool_unmap_handle(entry->pool->zpool, entry->handle);
> BUG_ON(ret);
> BUG_ON(dlen != PAGE_SIZE);
>
> @@ -940,6 +938,7 @@ static int zswap_writeback_entry(struct zpool *pool, unsigned long handle)
> spin_unlock(&tree->lock);
>
> end:
> + zpool_unmap_handle(pool, handle);
> return ret;
> }
>
> --
> 2.17.1

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