Re: [RFC PATCH v2 6/7] mm: zswap: simplify writeback function

[Yosry Ahmed <yosryahmed@google.com>]

On Fri, Jun 9, 2023 at 3:23 AM Domenico Cerasuolo
<cerasuolodomenico@gmail.com> wrote:
>
> On Wed, Jun 7, 2023 at 11:27 AM Yosry Ahmed <yosryahmed@google.com> wrote:
> >
> > On Tue, Jun 6, 2023 at 7:56 AM Domenico Cerasuolo
> > <cerasuolodomenico@gmail.com> wrote:
> > >
> > > Previously, in zswap, the writeback function was passed to zpool drivers
> > > for their usage in calling the writeback operation. However, since the
> > > drivers did not possess references to entries and the function was
> > > specifically designed to work with handles, the writeback process has
> > > been modified to occur directly within zswap. Consequently, this change
> > > allows for some simplification of the writeback function, taking into
> > > account the updated workflow.
> > >
> > > Signed-off-by: Domenico Cerasuolo <cerasuolodomenico@gmail.com>
> > > ---
> > >  mm/zswap.c | 69 ++++++++++++++----------------------------------------
> > >  1 file changed, 17 insertions(+), 52 deletions(-)
> > >
> > > diff --git a/mm/zswap.c b/mm/zswap.c
> > > index 2831bf56b168..ef8604812352 100644
> > > --- a/mm/zswap.c
> > > +++ b/mm/zswap.c
> > > @@ -250,7 +250,8 @@ static bool zswap_has_pool;
> > >         pr_debug("%s pool %s/%s\n", msg, (p)->tfm_name,         \
> > >                  zpool_get_type((p)->zpool))
> > >
> > > -static int zswap_writeback_entry(struct zpool *pool, unsigned long handle);
> > > +static int zswap_writeback_entry(struct zswap_entry *entry, struct zswap_header *zhdr,
> > > +                                struct zswap_tree *tree);
> > >  static int zswap_pool_get(struct zswap_pool *pool);
> > >  static void zswap_pool_put(struct zswap_pool *pool);
> > >
> > > @@ -632,15 +633,21 @@ static int zswap_shrink(struct zswap_pool *pool)
> > >         }
> > >         spin_unlock(&tree->lock);
> > >
> > > -       ret = zswap_writeback_entry(pool->zpool, lru_entry->handle);
> > > +       ret = zswap_writeback_entry(lru_entry, zhdr, tree);
> > >
> > >         spin_lock(&tree->lock);
> > >         if (ret) {
> > >                 spin_lock(&pool->lru_lock);
> > >                 list_move(&lru_entry->lru, &pool->lru);
> > >                 spin_unlock(&pool->lru_lock);
> > > +               zswap_entry_put(tree, tree_entry);
> > > +       } else {
> > > +               /* free the local reference */
> > > +               zswap_entry_put(tree, tree_entry);
> > > +               /* free the entry if it's not been invalidated*/
> > > +               if (lru_entry == zswap_rb_search(&tree->rbroot, swpoffset))
> > > +                       zswap_entry_put(tree, tree_entry);
> >
> > The comment that was here about the 2 possible cases was useful imo,
> > maybe keep it?
>
> I honestly found it brittle in that we're not interested in the refcount there,
> but rather in releasing the base reference that keeps the entry valid.
> There's not way to know which refcount value it should be though, consider
> that throughout this series the values can be 1 or 0, but also 2 or 1,
> depending on where this function is called (zpool_shrink or zswap_shrink).

Yeah looking at it with fresh eyes makes sense, we want to invalidate
the entry, not really caring about the refcount (see below).

>
> >
> > Also, I am not sure why we need to do a tree search vs. just reading
> > the refcount here before the first put. We can even make
> > zswap_entry_put() return the refcount after the put to know if we need
> > the additional put or not.
> >
> > Can anyone think of any reason why we need to explicitly search the tree here?
>
> I think that the reasoning here is that refcount > 0 doesn't necessarily mean
> that the entry is on the tree. I'm not sure if reading refcount directly here
> would cause an issue now, probably not, but I wonder if bugs could be
> introduced if the context in which this function is called changes.

Yeah I agree. As Johannes suggested, using zswap_invalidate_entry()
(from my exclusive loads patch in mm-unstable) would be best here imo.

>
> >
> > >         }
> > > -       zswap_entry_put(tree, tree_entry);
> > >         spin_unlock(&tree->lock);
> > >
> > >         return ret ? -EAGAIN : 0;
> > > @@ -1039,16 +1046,14 @@ static int zswap_get_swap_cache_page(swp_entry_t entry,
> > >   * the swap cache, the compressed version stored by zswap can be
> > >   * freed.
> > >   */
> > > -static int zswap_writeback_entry(struct zpool *pool, unsigned long handle)
> > > +static int zswap_writeback_entry(struct zswap_entry *entry, struct zswap_header *zhdr,
> > > +                                struct zswap_tree *tree)
> > >  {
> > > -       struct zswap_header *zhdr;
> > > -       swp_entry_t swpentry;
> > > -       struct zswap_tree *tree;
> > > -       pgoff_t offset;
> > > -       struct zswap_entry *entry;
> > > +       swp_entry_t swpentry = zhdr->swpentry;
> > >         struct page *page;
> > >         struct scatterlist input, output;
> > >         struct crypto_acomp_ctx *acomp_ctx;
> > > +       struct zpool *pool = entry->pool->zpool;
> > >
> > >         u8 *src, *tmp = NULL;
> > >         unsigned int dlen;
> > > @@ -1063,25 +1068,6 @@ static int zswap_writeback_entry(struct zpool *pool, unsigned long handle)
> > >                         return -ENOMEM;
> > >         }
> > >
> > > -       /* extract swpentry from data */
> > > -       zhdr = zpool_map_handle(pool, handle, ZPOOL_MM_RO);
> > > -       swpentry = zhdr->swpentry; /* here */
> > > -       tree = zswap_trees[swp_type(swpentry)];
> > > -       offset = swp_offset(swpentry);
> > > -       zpool_unmap_handle(pool, handle);
> > > -
> > > -       /* find and ref zswap entry */
> > > -       spin_lock(&tree->lock);
> > > -       entry = zswap_entry_find_get(&tree->rbroot, offset);
> > > -       if (!entry) {
> > > -               /* entry was invalidated */
> > > -               spin_unlock(&tree->lock);
> > > -               kfree(tmp);
> > > -               return 0;
> > > -       }
> > > -       spin_unlock(&tree->lock);
> > > -       BUG_ON(offset != entry->offset);
> > > -
> > >         /* try to allocate swap cache page */
> > >         switch (zswap_get_swap_cache_page(swpentry, &page)) {
> > >         case ZSWAP_SWAPCACHE_FAIL: /* no memory or invalidate happened */
> > > @@ -1115,12 +1101,12 @@ static int zswap_writeback_entry(struct zpool *pool, unsigned long handle)
> > >                 acomp_ctx = raw_cpu_ptr(entry->pool->acomp_ctx);
> > >                 dlen = PAGE_SIZE;
> > >
> > > -               zhdr = zpool_map_handle(pool, handle, ZPOOL_MM_RO);
> > > +               zhdr = zpool_map_handle(pool, entry->handle, ZPOOL_MM_RO);
> > >                 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);
> > > +                       zpool_unmap_handle(pool, entry->handle);
> > >                 }
> > >
> > >                 mutex_lock(acomp_ctx->mutex);
> > > @@ -1135,7 +1121,7 @@ static int zswap_writeback_entry(struct zpool *pool, unsigned long handle)
> > >                 if (!zpool_can_sleep_mapped(pool))
> > >                         kfree(tmp);
> > >                 else
> > > -                       zpool_unmap_handle(pool, handle);
> > > +                       zpool_unmap_handle(pool, entry->handle);
> > >
> > >                 BUG_ON(ret);
> > >                 BUG_ON(dlen != PAGE_SIZE);
> > > @@ -1152,23 +1138,7 @@ static int zswap_writeback_entry(struct zpool *pool, unsigned long handle)
> > >         put_page(page);
> > >         zswap_written_back_pages++;
> > >
> > > -       spin_lock(&tree->lock);
> > > -       /* drop local reference */
> > > -       zswap_entry_put(tree, entry);
> > > -
> > > -       /*
> > > -       * There are two possible situations for entry here:
> > > -       * (1) refcount is 1(normal case),  entry is valid and on the tree
> > > -       * (2) refcount is 0, entry is freed and not on the tree
> > > -       *     because invalidate happened during writeback
> > > -       *  search the tree and free the entry if find entry
> > > -       */
> > > -       if (entry == zswap_rb_search(&tree->rbroot, offset))
> > > -               zswap_entry_put(tree, entry);
> > > -       spin_unlock(&tree->lock);
> > > -
> > >         return ret;
> > > -
> > >  fail:
> > >         if (!zpool_can_sleep_mapped(pool))
> > >                 kfree(tmp);
> > > @@ -1177,13 +1147,8 @@ static int zswap_writeback_entry(struct zpool *pool, unsigned long handle)
> > >         * if we get here due to ZSWAP_SWAPCACHE_EXIST
> > >         * a load may be happening concurrently.
> > >         * it is safe and okay to not free the entry.
> > > -       * if we free the entry in the following put
> > >         * it is also okay to return !0
> > >         */
> > > -       spin_lock(&tree->lock);
> > > -       zswap_entry_put(tree, entry);
> > > -       spin_unlock(&tree->lock);
> > > -
> > >         return ret;
> > >  }
> > >
> > > --
> > > 2.34.1
> > >