RE: [PATCH v6 2/3] mm: zswap: zswap_store() extended to handle mTHP folios.

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

Hi Chengming,

> -----Original Message-----
> From: Chengming Zhou <chengming.zhou@linux.dev>
> Sent: Monday, September 2, 2024 4:38 AM
> To: Sridhar, Kanchana P <kanchana.p.sridhar@intel.com>; linux-
> kernel@vger.kernel.org; linux-mm@kvack.org; hannes@cmpxchg.org;
> yosryahmed@google.com; nphamcs@gmail.com; usamaarif642@gmail.com;
> ryan.roberts@arm.com; Huang, Ying <ying.huang@intel.com>;
> 21cnbao@gmail.com; akpm@linux-foundation.org
> Cc: Zou, Nanhai <nanhai.zou@intel.com>; Feghali, Wajdi K
> <wajdi.k.feghali@intel.com>; Gopal, Vinodh <vinodh.gopal@intel.com>
> Subject: Re: [PATCH v6 2/3] mm: zswap: zswap_store() extended to handle
> mTHP folios.
> 
> On 2024/8/30 05:27, Kanchana P Sridhar wrote:
> > zswap_store() will now process and store mTHP and PMD-size THP folios.
> >
> > A new config variable CONFIG_ZSWAP_STORE_THP_DEFAULT_ON (off by
> default)
> > will enable/disable zswap storing of (m)THP.
> >
> > 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@arm.com/T/#u
> >
> > 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).
> >
> > Also, addressed some of the RFC comments from the discussion in [1].
> >
> > Made a minor edit in the comments for "struct zswap_entry" to delete
> > the comments related to "value" since same-filled page handling has
> > been removed from zswap.
> >
> > Co-developed-by: Ryan Roberts
> > Signed-off-by:
> > Signed-off-by: Kanchana P Sridhar <kanchana.p.sridhar@intel.com>
> 
> The code looks ok, but I also find this patch is a little hard to
> review, maybe it's better to split it into small patches as Yosry suggested.

Definitely, will do so and submit a v7.

Thanks,
Kanchana

> 
> Thanks!
> 
> [...]
> > +
> > +/*
> > + * 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 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));
> > +
> > +	/* Storing large folios isn't enabled */
> > +	if (!zswap_mthp_enabled && folio_test_large(folio))
> > +		return false;
> > +
> > +	if (!zswap_enabled)
> > +		goto 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.
> > +	 * 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.
> >   	 */
> > -	entry = xa_erase(tree, offset);
> > -	if (entry)
> > -		zswap_entry_free(entry);
> > -	return false;
> > +	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 put_objcg;
> > +		}
> > +		mem_cgroup_put(memcg);
> > +	}
> > +
> > +	if (zswap_check_limits())
> > +		goto put_objcg;
> > +
> > +	pool = zswap_pool_current_get();
> > +	if (!pool)
> > +		goto put_objcg;
> > +
> > +	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;
> > +		}
> > +		mem_cgroup_put(memcg);
> > +	}
> > +
> > +	/*
> > +	 * 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.
> > +	 */
> > +	for (index = 0; index < nr_pages; ++index) {
> > +		if (!zswap_store_page(folio, index, objcg, pool))
> > +			goto put_pool;
> > +	}
> > +
> > +	ret = true;
> > +
> > +put_pool:
> > +	zswap_pool_put(pool);
> > +put_objcg:
> > +	obj_cgroup_put(objcg);
> > +	if (zswap_pool_reached_full)
> > +		queue_work(shrink_wq, &zswap_shrink_work);
> > +reject:
> > +	/*
> > +	 * 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.
> > +	 */
> > +	if (!ret)
> > +		zswap_delete_stored_offsets(tree, offset, nr_pages);
> > +
> > +	return ret;
> >   }
> >
> >   bool zswap_load(struct folio *folio)