RE: [PATCH v7 3/8] mm: zswap: Refactor code to store an entry in zswap xarray.

["Sridhar, Kanchana P" <kanchana.p.sridhar@intel.com>]

> -----Original Message-----
> From: Yosry Ahmed <yosryahmed@google.com>
> Sent: Tuesday, September 24, 2024 12:15 PM
> To: Sridhar, Kanchana P <kanchana.p.sridhar@intel.com>
> Cc: linux-kernel@vger.kernel.org; linux-mm@kvack.org;
> hannes@cmpxchg.org; nphamcs@gmail.com; chengming.zhou@linux.dev;
> usamaarif642@gmail.com; shakeel.butt@linux.dev; ryan.roberts@arm.com;
> Huang, Ying <ying.huang@intel.com>; 21cnbao@gmail.com; akpm@linux-
> foundation.org; Zou, Nanhai <nanhai.zou@intel.com>; Feghali, Wajdi K
> <wajdi.k.feghali@intel.com>; Gopal, Vinodh <vinodh.gopal@intel.com>
> Subject: Re: [PATCH v7 3/8] mm: zswap: Refactor code to store an entry in
> zswap xarray.
> 
> On Mon, Sep 23, 2024 at 6:17 PM Kanchana P Sridhar
> <kanchana.p.sridhar@intel.com> wrote:
> >
> > Added a new procedure zswap_store_entry() that refactors the code
> > currently in zswap_store() to store an entry in the zswap xarray.
> > This will allow us to call this procedure for each storing the swap
> > offset of each page in an mTHP in the xarray, as part of zswap_store()
> > supporting mTHP.
> >
> > Also, made a minor edit in the comments for 'struct zswap_entry' to delete
> > the description of the 'value' member that was deleted in commit
> > 20a5532ffa53d6ecf41ded920a7b0ff9c65a7dcf ("mm: remove code to
> handle
> > same filled pages").
> >
> > Signed-off-by: Kanchana P Sridhar <kanchana.p.sridhar@intel.com>
> > ---
> >  mm/zswap.c | 51 ++++++++++++++++++++++++++++++++++-----------------
> >  1 file changed, 34 insertions(+), 17 deletions(-)
> >
> > diff --git a/mm/zswap.c b/mm/zswap.c
> > index 59b7733a62d3..fd35a81b6e36 100644
> > --- a/mm/zswap.c
> > +++ b/mm/zswap.c
> > @@ -190,7 +190,6 @@ static struct shrinker *zswap_shrinker;
> >   *              section for context.
> >   * pool - the zswap_pool the entry's data is in
> >   * handle - zpool allocation handle that stores the compressed page data
> > - * value - value of the same-value filled pages which have same content
> >   * objcg - the obj_cgroup that the compressed memory is charged to
> >   * lru - handle to the pool's lru used to evict pages.
> >   */
> > @@ -1404,12 +1403,44 @@ static void shrink_worker(struct work_struct
> *w)
> >  /*********************************
> >  * main API
> >  **********************************/
> > +
> > +/*
> > + * Returns true if the entry was successfully
> > + * stored in the xarray, and false otherwise.
> > + */
> > +static bool zswap_store_entry(struct xarray *tree,
> > +                             struct zswap_entry *entry)
> 
> 
> I think zswap_tree_store() is a more descriptive name.

Thanks Yosry for the code review comments!
Sure, will change this to zswap_tree_store() in v8. 

> 
> >
> > +{
> > +       struct zswap_entry *old;
> > +       pgoff_t offset = swp_offset(entry->swpentry);
> 
> 
> Reverse xmas tree where possible please (longest to shortest declarations).
> 
> >
> > +
> > +       old = xa_store(tree, offset, entry, GFP_KERNEL);
> > +
> 
> No need for the blank line here.

Ok, will fix in v8.

Thanks,
Kanchana

> 
> > +       if (xa_is_err(old)) {
> > +               int err = xa_err(old);
> > +
> > +               WARN_ONCE(err != -ENOMEM, "unexpected xarray error: %d\n",
> err);
> > +               zswap_reject_alloc_fail++;
> > +               return false;
> > +       }
> > +
> > +       /*
> > +        * We may have had an existing entry that became stale when
> > +        * the folio was redirtied and now the new version is being
> > +        * swapped out. Get rid of the old.
> > +        */
> > +       if (old)
> > +               zswap_entry_free(old);
> > +
> > +       return true;
> > +}
> > +
> >  bool zswap_store(struct folio *folio)
> >  {
> >         swp_entry_t swp = folio->swap;
> >         pgoff_t offset = swp_offset(swp);
> >         struct xarray *tree = swap_zswap_tree(swp);
> > -       struct zswap_entry *entry, *old;
> > +       struct zswap_entry *entry;
> >         struct obj_cgroup *objcg = NULL;
> >         struct mem_cgroup *memcg = NULL;
> >
> > @@ -1465,22 +1496,8 @@ bool zswap_store(struct folio *folio)
> >         entry->objcg = objcg;
> >         entry->referenced = true;
> >
> > -       old = xa_store(tree, offset, entry, GFP_KERNEL);
> > -       if (xa_is_err(old)) {
> > -               int err = xa_err(old);
> > -
> > -               WARN_ONCE(err != -ENOMEM, "unexpected xarray error: %d\n",
> err);
> > -               zswap_reject_alloc_fail++;
> > +       if (!zswap_store_entry(tree, entry))
> >                 goto store_failed;
> > -       }
> > -
> > -       /*
> > -        * We may have had an existing entry that became stale when
> > -        * the folio was redirtied and now the new version is being
> > -        * swapped out. Get rid of the old.
> > -        */
> > -       if (old)
> > -               zswap_entry_free(old);
> >
> >         if (objcg) {
> >                 obj_cgroup_charge_zswap(objcg, entry->length);
> > --
> > 2.27.0
> >