Re: [RFC PATCH 9/9] mm: zswap: use zswap_entry_free() for partially initialized entries

[Yosry Ahmed <yosryahmed@google.com>]

On Thu, Mar 28, 2024 at 1:31 AM Chengming Zhou <chengming.zhou@linux.dev> wrote:
>
> On 2024/3/26 07:50, Yosry Ahmed wrote:
> > zswap_entry_free() performs four types of cleanups before freeing a
> > zswap_entry:
> > - Deletes the entry from the LRU.
> > - Frees compressed memory.
> > - Puts the pool reference.
> > - Uncharges the compressed memory and puts the objcg.
> >
> > zswap_entry_free() always expects a fully initialized entry. Allow
> > zswap_entry_free() to handle partially initialized entries by making it
> > possible to identify what cleanups are needed as follows:
> > - Allocate entries with __GFP_ZERO and initialize zswap_entry.lru when
> >   the entry is allocated. Points are NULL and length is zero upon
> >   initialization.
> > - Use zswap_entry.length to identify if there is compressed memory to
> >   free. This is possible now that zero-filled pages are handled
> >   separately, so a length of zero means we did not successfully compress
> >   the page.
> > - Only initialize entry->objcg after the memory is charged in
> >   zswap_store().
> >
> > With this in place, use zswap_entry_free() in the failure path of
> > zswap_store() to cleanup partially initialized entries. This simplifies
> > the cleanup code in zswap_store(). While we are at it, rename the
> > remaining cleanup labels to more meaningful names.
>
> Not sure if it's worthwhile to clean up the fail path with the normal path
> gets a little verbose.

I was on the fence about this, so I  thought I would just send it and
see what others think.

On one hand it makes the initialization more robust because the
zswap_entry is always in a clean identifiable state, but on the other
hand it adds churn to the normal path as you noticed. Also after
removing handling zero-length entries from the failure path it isn't
that bad without this patch anyway.

So if no one else thinks this is useful, I will drop the patch in the
next version.

Thanks!

>
> >
> > Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> > ---
> >  mm/zswap.c | 62 ++++++++++++++++++++++++++----------------------------
> >  1 file changed, 30 insertions(+), 32 deletions(-)
> >
> > diff --git a/mm/zswap.c b/mm/zswap.c
> > index 9357328d940af..c50f9df230ca3 100644
> > --- a/mm/zswap.c
> > +++ b/mm/zswap.c
> > @@ -774,12 +774,13 @@ void zswap_memcg_offline_cleanup(struct mem_cgroup *memcg)
> >  **********************************/
> >  static struct kmem_cache *zswap_entry_cache;
> >
> > -static struct zswap_entry *zswap_entry_cache_alloc(gfp_t gfp, int nid)
> > +static struct zswap_entry *zswap_entry_cache_alloc(int nid)
> >  {
> >       struct zswap_entry *entry;
> > -     entry = kmem_cache_alloc_node(zswap_entry_cache, gfp, nid);
> > -     if (!entry)
> > -             return NULL;
> > +     entry = kmem_cache_alloc_node(zswap_entry_cache,
> > +                                   GFP_KERNEL | __GFP_ZERO, nid);
> > +     if (entry)
> > +             INIT_LIST_HEAD(&entry->lru);
> >       return entry;
> >  }
> >
> > @@ -795,9 +796,12 @@ static struct zpool *zswap_find_zpool(struct zswap_entry *entry)
> >
> >  static void zswap_entry_free(struct zswap_entry *entry)
> >  {
> > -     zswap_lru_del(&zswap_list_lru, entry);
> > -     zpool_free(zswap_find_zpool(entry), entry->handle);
> > -     zswap_pool_put(entry->pool);
> > +     if (!list_empty(&entry->lru))
> > +             zswap_lru_del(&zswap_list_lru, entry);
> > +     if (entry->length)
> > +             zpool_free(zswap_find_zpool(entry), entry->handle);
> > +     if (entry->pool)
> > +             zswap_pool_put(entry->pool);
> >       if (entry->objcg) {
> >               obj_cgroup_uncharge_zswap(entry->objcg, entry->length);
> >               obj_cgroup_put(entry->objcg);
> > @@ -1447,7 +1451,7 @@ bool zswap_store(struct folio *folio)
> >               return false;
> >
> >       if (!zswap_enabled)
> > -             goto check_old;
> > +             goto erase_old;
> >
> >       /* Check cgroup limits */
> >       objcg = get_obj_cgroup_from_folio(folio);
> > @@ -1455,54 +1459,52 @@ bool zswap_store(struct folio *folio)
> >               memcg = get_mem_cgroup_from_objcg(objcg);
> >               if (shrink_memcg(memcg)) {
> >                       mem_cgroup_put(memcg);
> > -                     goto reject;
> > +                     goto put_objcg;
> >               }
> >               mem_cgroup_put(memcg);
> >       }
> >
> >       if (zswap_is_folio_zero_filled(folio)) {
> >               if (zswap_store_zero_filled(tree, offset, objcg))
> > -                     goto reject;
> > +                     goto put_objcg;
> >               goto stored;
> >       }
> >
> >       if (!zswap_non_zero_filled_pages_enabled)
> > -             goto reject;
> > +             goto put_objcg;
> >
> >       if (!zswap_check_limit())
> > -             goto reject;
> > +             goto put_objcg;
> >
> > -     entry = zswap_entry_cache_alloc(GFP_KERNEL, folio_nid(folio));
> > +     entry = zswap_entry_cache_alloc(folio_nid(folio));
> >       if (!entry) {
> >               zswap_reject_kmemcache_fail++;
> > -             goto reject;
> > +             goto put_objcg;
> >       }
> >
> > -     /* if entry is successfully added, it keeps the reference */
> >       entry->pool = zswap_pool_current_get();
> >       if (!entry->pool)
> > -             goto freepage;
> > +             goto free_entry;
> >
> >       if (objcg) {
> >               memcg = get_mem_cgroup_from_objcg(objcg);
> >               if (memcg_list_lru_alloc(memcg, &zswap_list_lru, GFP_KERNEL)) {
> >                       mem_cgroup_put(memcg);
> > -                     goto put_pool;
> > +                     goto free_entry;
> >               }
> >               mem_cgroup_put(memcg);
> >       }
> >
> >       if (!zswap_compress(folio, entry))
> > -             goto put_pool;
> > -
> > -     entry->swpentry = swp;
> > -     entry->objcg = objcg;
> > +             goto free_entry;
> >
> >       if (zswap_tree_store(tree, offset, entry))
> > -             goto store_failed;
> > +             goto free_entry;
> >
> > -     if (objcg)
> > +     if (objcg) {
> >               obj_cgroup_charge_zswap(objcg, entry->length);
> > +             entry->objcg = objcg;
> > +     }
> >
> >       /*
> >        * We finish initializing the entry while it's already in xarray.
> > @@ -1514,7 +1516,7 @@ bool zswap_store(struct folio *folio)
> >        *    The publishing order matters to prevent writeback from seeing
> >        *    an incoherent entry.
> >        */
> > -     INIT_LIST_HEAD(&entry->lru);
> > +     entry->swpentry = swp;
> >       zswap_lru_add(&zswap_list_lru, entry);
> >
> >  stored:
> > @@ -1525,17 +1527,13 @@ bool zswap_store(struct folio *folio)
> >
> >       return true;
> >
> > -store_failed:
> > -     zpool_free(zswap_find_zpool(entry), entry->handle);
> > -put_pool:
> > -     zswap_pool_put(entry->pool);
> > -freepage:
> > -     zswap_entry_cache_free(entry);
> > -reject:
> > +free_entry:
> > +     zswap_entry_free(entry);
> > +put_objcg:
> >       obj_cgroup_put(objcg);
> >       if (zswap_pool_reached_full)
> >               queue_work(shrink_wq, &zswap_shrink_work);
> > -check_old:
> > +erase_old:
> >       /*
> >        * If the zswap store fails or zswap is disabled, we must invalidate the
> >        * possibly stale entry which was previously stored at this offset.