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Message-ID: <SJ0PR11MB5678BC4F2D060F760D3AF802C9682@SJ0PR11MB5678.namprd11.prod.outlook.com>
Date: Tue, 24 Sep 2024 23:02:06 +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 6/8] mm: zswap: Support mTHP swapout in zswap_store().
> -----Original Message-----
> From: Yosry Ahmed <yosryahmed@...gle.com>
> Sent: Tuesday, September 24, 2024 12:39 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 6/8] mm: zswap: Support mTHP swapout in
> zswap_store().
>
> On Mon, Sep 23, 2024 at 6:17 PM Kanchana P Sridhar
> <kanchana.p.sridhar@...el.com> wrote:
> >
> > zswap_store() will now store mTHP and PMD-size THP folios by compressing
> > them page by page.
> >
> > This patch provides a sequential implementation of storing an mTHP in
> > zswap_store() by iterating through each page in the folio to compress
> > and store it in the zswap zpool.
> >
> > Towards this goal, zswap_compress() is modified to take a page instead
> > of a folio as input.
> >
> > Each page's swap offset is stored as a separate zswap entry.
> >
> > If an error is encountered during the store of any page in the mTHP,
> > all previous pages/entries stored will be invalidated. Thus, an mTHP
> > is either entirely stored in ZSWAP, or entirely not stored in ZSWAP.
> >
> > This forms the basis for building batching of pages during zswap store
> > of large folios by compressing batches of up to say, 8 pages in an
> > mTHP in parallel in hardware, with the Intel In-Memory Analytics
> > Accelerator (Intel IAA).
> >
> > A new config variable CONFIG_ZSWAP_STORE_THP_DEFAULT_ON (off by
> default)
> > will enable/disable zswap storing of (m)THP. The corresponding tunable
> > zswap module parameter is "mthp_enabled".
> >
> > This change reuses and adapts the functionality in 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
> >
> > Also, addressed some of the RFC comments from the discussion in [1].
> >
> > Co-developed-by: Ryan Roberts
> > Signed-off-by:
> > Signed-off-by: Kanchana P Sridhar <kanchana.p.sridhar@...el.com>
> > ---
> > mm/Kconfig | 8 ++++
> > mm/zswap.c | 122 +++++++++++++++++++++++++----------------------------
> > 2 files changed, 66 insertions(+), 64 deletions(-)
> >
> > diff --git a/mm/Kconfig b/mm/Kconfig
> > index 09aebca1cae3..c659fb732ec4 100644
> > --- a/mm/Kconfig
> > +++ b/mm/Kconfig
> > @@ -59,6 +59,14 @@ config ZSWAP_SHRINKER_DEFAULT_ON
> > reducing the chance that cold pages will reside in the zswap pool
> > and consume memory indefinitely.
> >
> > +config ZSWAP_STORE_THP_DEFAULT_ON
> > + bool "Store mTHP and THP folios in zswap"
> > + depends on ZSWAP
> > + default n
> > + help
> > + If selected, zswap will process mTHP and THP folios by
> > + compressing and storing each 4K page in the large folio.
> > +
> > choice
> > prompt "Default compressor"
> > depends on ZSWAP
> > diff --git a/mm/zswap.c b/mm/zswap.c
> > index 8f2e0ab34c84..16ab770546d6 100644
> > --- a/mm/zswap.c
> > +++ b/mm/zswap.c
> > @@ -127,6 +127,14 @@ static bool zswap_shrinker_enabled =
> IS_ENABLED(
> > CONFIG_ZSWAP_SHRINKER_DEFAULT_ON);
> > module_param_named(shrinker_enabled, zswap_shrinker_enabled, bool,
> 0644);
> >
> > +/*
> > + * Enable/disable zswap processing of mTHP folios.
> > + * For now, only zswap_store will process mTHP folios.
> > + */
> > +static bool zswap_mthp_enabled = IS_ENABLED(
> > + CONFIG_ZSWAP_STORE_THP_DEFAULT_ON);
> > +module_param_named(mthp_enabled, zswap_mthp_enabled, bool,
> 0644);
> > +
> > bool zswap_is_enabled(void)
> > {
> > return zswap_enabled;
> > @@ -1471,9 +1479,9 @@ static void zswap_delete_stored_offsets(struct
> xarray *tree,
> > * @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)
> > +static bool zswap_store_page(struct folio *folio, long index,
> > + struct obj_cgroup *objcg,
> > + struct zswap_pool *pool)
>
> As I mentioned earlier, the patch that introduced zswap_store_page()
> should have directly used it in zswap_store(). This would make this
> patch much clearer.
Sure. I will fix this in v8.
>
> > {
> > swp_entry_t swp = folio->swap;
> > int type = swp_type(swp);
> > @@ -1551,51 +1559,63 @@ static bool __maybe_unused
> zswap_store_page(struct folio *folio, long index,
> > return false;
> > }
> >
> > +/*
> > + * Modified to store mTHP folios. Each page in the mTHP will be
> compressed
> > + * and stored sequentially.
> > + */
> > bool zswap_store(struct folio *folio)
> > {
> > long nr_pages = folio_nr_pages(folio);
> > swp_entry_t swp = folio->swap;
> > pgoff_t offset = swp_offset(swp);
> > struct xarray *tree = swap_zswap_tree(swp);
> > - struct zswap_entry *entry;
> > struct obj_cgroup *objcg = NULL;
> > struct mem_cgroup *memcg = NULL;
> > + struct zswap_pool *pool;
> > + bool ret = false;
> > + long index;
> >
> > VM_WARN_ON_ONCE(!folio_test_locked(folio));
> > VM_WARN_ON_ONCE(!folio_test_swapcache(folio));
> >
> > - /* Large folios aren't supported */
> > - if (folio_test_large(folio))
> > + /* Storing large folios isn't enabled */
>
> The comment is now stating the obvious, please remove it.
Ok. I suppose this check will also no longer be needed based on the
config knob not being needed.
>
> > + if (!zswap_mthp_enabled && folio_test_large(folio))
> > return false;
> >
> > if (!zswap_enabled)
> > - goto check_old;
> > + goto reject;
> >
> > - /* Check cgroup limits */
> > + /*
> > + * Check cgroup limits:
> > + *
> > + * The cgroup zswap limit check is done once at the beginning of an
> > + * mTHP store, and not within zswap_store_page() for each page
> > + * in the mTHP. We do however check the zswap pool limits at the
> > + * start of zswap_store_page(). What this means is, the cgroup
> > + * could go over the limits by at most (HPAGE_PMD_NR - 1) pages.
> > + * However, the per-store-page zswap pool limits check should
> > + * hopefully trigger the cgroup aware and zswap LRU aware global
> > + * reclaim implemented in the shrinker. If this assumption holds,
> > + * the cgroup exceeding the zswap limits could potentially be
> > + * resolved before the next zswap_store, and if it is not, the next
> > + * zswap_store would fail the cgroup zswap limit check at the start.
> > + */
>
> I do not really like this. Allowing going one page above the limit is
> one thing, but one THP above the limit seems too much. I also don't
> like relying on the repeated limit checking in zswap_store_page(), if
> anything I think that should be batched too.
>
> Is it too unreasonable to maintain the average compression ratio and
> use that to estimate limit checking for both memcg and global limits?
> Johannes, Nhat, any thoughts on this?
I see that Nhat has responded. Hopefully we can discuss this
in the follow-up to Nhat's comments.
Thanks,
Kanchana
>
> > objcg = get_obj_cgroup_from_folio(folio);
> > if (objcg && !obj_cgroup_may_zswap(objcg)) {
> > 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_check_limits())
> > - goto reject;
> > -
> > - /* allocate entry */
> > - entry = zswap_entry_cache_alloc(GFP_KERNEL, 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;
> > + pool = zswap_pool_current_get();
> > + if (!pool)
> > + goto put_objcg;
> >
> > if (objcg) {
> > memcg = get_mem_cgroup_from_objcg(objcg);
> > @@ -1606,60 +1626,34 @@ bool zswap_store(struct folio *folio)
> > mem_cgroup_put(memcg);
> > }
> >
> > - if (!zswap_compress(&folio->page, entry))
> > - goto put_pool;
> > -
> > - entry->swpentry = swp;
> > - 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.
> > + * Store each page of the folio as a separate entry. If we fail to store
> > + * a page, unwind by removing all the previous pages we stored.
> > */
> > - if (entry->length) {
> > - INIT_LIST_HEAD(&entry->lru);
> > - zswap_lru_add(&zswap_list_lru, entry);
> > + for (index = 0; index < nr_pages; ++index) {
> > + if (!zswap_store_page(folio, index, objcg, pool))
> > + goto put_pool;
> > }
> >
> > - /* update stats */
> > - atomic_inc(&zswap_stored_pages);
> > - count_vm_event(ZSWPOUT);
> > -
> > - return true;
> > + ret = true;
> >
> > -store_failed:
> > - zpool_free(entry->pool->zpool, entry->handle);
> > put_pool:
> > - zswap_pool_put(entry->pool);
> > -freepage:
> > - zswap_entry_cache_free(entry);
> > -reject:
> > + zswap_pool_put(pool);
> > +put_objcg:
> > obj_cgroup_put(objcg);
> > if (zswap_pool_reached_full)
> > queue_work(shrink_wq, &zswap_shrink_work);
> > -check_old:
> > +reject:
> > /*
> > - * If the zswap store fails or zswap is disabled, we must invalidate the
> > - * possibly stale entry which was previously stored at this offset.
> > - * Otherwise, writeback could overwrite the new data in the swapfile.
> > + * If the zswap store fails or zswap is disabled, we must invalidate
> > + * the possibly stale entries which were previously stored at the
> > + * offsets corresponding to each page of the folio. Otherwise,
> > + * writeback could overwrite the new data in the swapfile.
> > */
> > - zswap_delete_stored_offsets(tree, offset, nr_pages);
> > - return false;
> > + if (!ret)
> > + zswap_delete_stored_offsets(tree, offset, nr_pages);
> > +
> > + return ret;
> > }
> >
> > bool zswap_load(struct folio *folio)
> > --
> > 2.27.0
> >
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