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Message-ID: <SJ0PR11MB56785712C0EF98B7BE558720C9682@SJ0PR11MB5678.namprd11.prod.outlook.com>
Date: Tue, 24 Sep 2024 20:51:28 +0000
From: "Sridhar, Kanchana P" <kanchana.p.sridhar@...el.com>
To: Nhat Pham <nphamcs@...il.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>, "yosryahmed@...gle.com" <yosryahmed@...gle.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: Nhat Pham <nphamcs@...il.com>
> Sent: Tuesday, September 24, 2024 10:34 AM
> To: Sridhar, Kanchana P <kanchana.p.sridhar@...el.com>
> Cc: linux-kernel@...r.kernel.org; linux-mm@...ck.org;
> hannes@...xchg.org; yosryahmed@...gle.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);
> > +
>
> Hmm, so this is a runtime knob. Also, should this be zswap_thp_enabled? :)
Agreed, zswap_thp_enabled is a better name. I will make the change in v8.
More comments below as to the runtime knob.
>
> > 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)
> > {
> > 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 */
> > + if (!zswap_mthp_enabled && folio_test_large(folio))
> > return false;
>
> Hmm can this go wrong somehow? Can we have a case where we enable
> zswap_mthp_enabled, have a large folio written to zswap, disable
> zswap_mthp_enabled, and attempt to store that folio to zswap again.
>
> Now, we have a stale copy in zswap that is not invalidated...?
>
> Or am I missing something here :)
This is an excellent point. Thanks Nhat for catching this! I can see two
options to solving this:
Option 1: If zswap_mthp_enabled is "false", delete all stored offsets
for the mTHP in zswap before exiting. This could race with writeback
(either one or more subpages could be written back before zswap_store
acquires the tree lock), however, I don't think it will cause data inconsistencies.
Any offsets for subpages not written back will be deleted from zswap,
zswap_store() will return false, and the backing swap device's subsequent
swapout will over-write the zswap write-back data. Could anything go wrong
with this?
Option 2: Only provide a build config option,
CONFIG_ZSWAP_STORE_THP_DEFAULT_ON, that cannot be dynamically changed.
Would appreciate suggestions on these, and other potential solutions.
Thanks,
Kanchana
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