Message-ID: <SJ0PR11MB567887AF292EC178BEF1CF43C9682@SJ0PR11MB5678.namprd11.prod.outlook.com>
Date: Tue, 24 Sep 2024 22:45:17 +0000
From: "Sridhar, Kanchana P" <kanchana.p.sridhar@...el.com>
To: Yosry Ahmed <yosryahmed@...gle.com>
CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>, "hannes@...xchg.org"
	<hannes@...xchg.org>, "nphamcs@...il.com" <nphamcs@...il.com>,
	"chengming.zhou@...ux.dev" <chengming.zhou@...ux.dev>,
	"usamaarif642@...il.com" <usamaarif642@...il.com>, "shakeel.butt@...ux.dev"
	<shakeel.butt@...ux.dev>, "ryan.roberts@....com" <ryan.roberts@....com>,
	"Huang, Ying" <ying.huang@...el.com>, "21cnbao@...il.com"
	<21cnbao@...il.com>, "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"Zou, Nanhai" <nanhai.zou@...el.com>, "Feghali, Wajdi K"
	<wajdi.k.feghali@...el.com>, "Gopal, Vinodh" <vinodh.gopal@...el.com>,
	"Sridhar, Kanchana P" <kanchana.p.sridhar@...el.com>
Subject: RE: [PATCH v7 5/8] mm: zswap: Compress and store a specific page in a
 folio.

> -----Original Message-----
> From: Yosry Ahmed <yosryahmed@...gle.com>
> Sent: Tuesday, September 24, 2024 12:29 PM
> To: Sridhar, Kanchana P <kanchana.p.sridhar@...el.com>
> Cc: linux-kernel@...r.kernel.org; linux-mm@...ck.org;
> hannes@...xchg.org; nphamcs@...il.com; chengming.zhou@...ux.dev;
> usamaarif642@...il.com; shakeel.butt@...ux.dev; ryan.roberts@....com;
> Huang, Ying <ying.huang@...el.com>; 21cnbao@...il.com; akpm@...ux-
> foundation.org; Zou, Nanhai <nanhai.zou@...el.com>; Feghali, Wajdi K
> <wajdi.k.feghali@...el.com>; Gopal, Vinodh <vinodh.gopal@...el.com>
> Subject: Re: [PATCH v7 5/8] mm: zswap: Compress and store a specific page
> in a folio.
> 
> On Mon, Sep 23, 2024 at 6:17 PM Kanchana P Sridhar
> <kanchana.p.sridhar@...el.com> wrote:
> >
> > For zswap_store() to handle mTHP folios, we need to iterate through each
> > page in the mTHP, compress it and store it in the zswap pool. This patch
> > introduces an auxiliary function zswap_store_page() that provides this
> > functionality.
> >
> > The function signature reflects the design intent, namely, for it
> > to be invoked by zswap_store() per-page in an mTHP. Hence, the folio's
> > objcg and the zswap_pool to use are input parameters for sake of
> > efficiency and consistency.
> >
> > The functionality in zswap_store_page() is reused and adapted from
> > Ryan Roberts' RFC patch [1]:
> >
> >   "[RFC,v1] mm: zswap: Store large folios without splitting"
> >
> >   [1] https://lore.kernel.org/linux-mm/20231019110543.3284654-1-
> ryan.roberts@....com/T/#u
> >
> > Co-developed-by: Ryan Roberts
> > Signed-off-by:
> > Signed-off-by: Kanchana P Sridhar <kanchana.p.sridhar@...el.com>
> > ---
> >  mm/zswap.c | 88
> ++++++++++++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 88 insertions(+)
> >
> > diff --git a/mm/zswap.c b/mm/zswap.c
> > index 9bea948d653e..8f2e0ab34c84 100644
> > --- a/mm/zswap.c
> > +++ b/mm/zswap.c
> > @@ -1463,6 +1463,94 @@ static void zswap_delete_stored_offsets(struct
> xarray *tree,
> >         }
> >  }
> >
> > +/*
> > + * Stores the page at specified "index" in a folio.
> > + *
> > + * @folio: The folio to store in zswap.
> > + * @index: Index into the page in the folio that this function will store.
> > + * @objcg: The folio's objcg.
> > + * @pool:  The zswap_pool to store the compressed data for the page.
> > + */
> > +static bool __maybe_unused zswap_store_page(struct folio *folio, long
> index,
> > +                                           struct obj_cgroup *objcg,
> > +                                           struct zswap_pool *pool)
> 
> Why are we adding an unused function that duplicates code in
> zswap_store(), then using it in the following patch? This makes it
> difficult to see that the function does the same thing. This patch
> should be refactoring the per-page code out of zswap_store() into
> zswap_store_page(), and directly calling zswap_store_page() from
> zswap_store().

Sure, thanks Yosry for this suggestion. Will fix in v8.

> 
> > +{
> > +       swp_entry_t swp = folio->swap;
> > +       int type = swp_type(swp);
> > +       pgoff_t offset = swp_offset(swp) + index;
> > +       struct page *page = folio_page(folio, index);
> > +       struct xarray *tree = swap_zswap_tree(swp);
> > +       struct zswap_entry *entry;
> > +
> > +       if (objcg)
> > +               obj_cgroup_get(objcg);
> > +
> > +       if (zswap_check_limits())
> > +               goto reject;
> > +
> > +       /* allocate entry */
> > +       entry = zswap_entry_cache_alloc(GFP_KERNEL, folio_nid(folio));
> > +       if (!entry) {
> > +               zswap_reject_kmemcache_fail++;
> > +               goto reject;
> > +       }
> > +
> > +       /* if entry is successfully added, it keeps the reference */
> > +       if (!zswap_pool_get(pool))
> > +               goto freepage;
> 
> I think we can batch this for all pages in zswap_store(), maybe first
> add zswap_pool_get_many().
> 
> I am also wondering if it would be better to batch the limit checking
> and allocating the entries, to front load any failures before we start
> compression. Not sure if that's overall better though.
> 
> To batch allocate entries we will have to also allocate an array to
> hold them. To batch the limit checking we will have to either allow
> going further over limit for mTHPs, or check if there is enough
> clearance to allow for compressing all the pages. Using the
> uncompressed size will lead to false negatives though, so maybe we can
> start tracking the average compression ratio for better limit
> checking.
> 
> Nhat, Johannes, any thoughts here? I need someone to tell me if I am
> overthinking this :)

These are all good points. I suppose I was thinking along the same lines
of what Nhat mentioned in an earlier comment. I was trying the
incremental zswap_pool_get() and limit checks and shrinker invocations
in case of (all) error conditions to allow different concurrent stores to make
progress, without favoring only one process's mTHP store. I was thinking
this would have minimal impact on the process(es) that see the zswap
limit being exceeded, and that this would be better than preemptively
checking for the entire mTHP and failing (this could also complicate things
where no one makes progress because multiple processes run the batch
checks and fail, when realistically one/many could have triggered
the shrinker before erroring out, and at least one could have made
progress).

Would appreciate your perspectives on how this should be handled,
and will implement a solution in v8 accordingly.

Thanks,
Kanchana

> 
> > +
> > +       entry->pool = pool;
> > +
> > +       if (!zswap_compress(page, entry))
> > +               goto put_pool;
> > +
> > +       entry->swpentry = swp_entry(type, offset);
> > +       entry->objcg = objcg;
> > +       entry->referenced = true;
> > +
> > +       if (!zswap_store_entry(tree, entry))
> > +               goto store_failed;
> > +
> > +       if (objcg) {
> > +               obj_cgroup_charge_zswap(objcg, entry->length);
> > +               count_objcg_event(objcg, ZSWPOUT);
> > +       }
> > +
> > +       /*
> > +        * We finish initializing the entry while it's already in xarray.
> > +        * This is safe because:
> > +        *
> > +        * 1. Concurrent stores and invalidations are excluded by folio lock.
> > +        *
> > +        * 2. Writeback is excluded by the entry not being on the LRU yet.
> > +        *    The publishing order matters to prevent writeback from seeing
> > +        *    an incoherent entry.
> > +        */
> > +       if (entry->length) {
> > +               INIT_LIST_HEAD(&entry->lru);
> > +               zswap_lru_add(&zswap_list_lru, entry);
> > +       }
> > +
> > +       /* update stats */
> > +       atomic_inc(&zswap_stored_pages);
> > +       count_vm_event(ZSWPOUT);
> 
> We should probably also batch updating the stats. It actually seems
> like now we don't handle rolling them back upon failure.

Good point! I assume you are referring only to the "ZSWPOUT" vm event stats
updates and not the "zswap_stored_pages" (since latter is used in limit checking)?

I will fix this in v8.

Thanks,
Kanchana

> 
> 
> > +
> > +       return true;
> > +
> > +store_failed:
> > +       zpool_free(entry->pool->zpool, entry->handle);
> > +put_pool:
> > +       zswap_pool_put(pool);
> > +freepage:
> > +       zswap_entry_cache_free(entry);
> > +reject:
> > +       obj_cgroup_put(objcg);
> > +       if (zswap_pool_reached_full)
> > +               queue_work(shrink_wq, &zswap_shrink_work);
> > +
> > +       return false;
> > +}
> > +
> >  bool zswap_store(struct folio *folio)
> >  {
> >         long nr_pages = folio_nr_pages(folio);
> > --
> > 2.27.0
> >

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