linux-kernel - Re: [PATCH v7 6/8] mm: zswap: Support mTHP swapout in zswap

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Message-ID: <20240925142741.GD875661@cmpxchg.org>
Date: Wed, 25 Sep 2024 10:27:41 -0400
From: Johannes Weiner <hannes@...xchg.org>
To: Kanchana P Sridhar <kanchana.p.sridhar@...el.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, yosryahmed@...gle.com,
	nphamcs@...il.com, chengming.zhou@...ux.dev, usamaarif642@...il.com,
	shakeel.butt@...ux.dev, ryan.roberts@....com, ying.huang@...el.com,
	21cnbao@...il.com, akpm@...ux-foundation.org, nanhai.zou@...el.com,
	wajdi.k.feghali@...el.com, vinodh.gopal@...el.com
Subject: Re: [PATCH v7 6/8] mm: zswap: Support mTHP swapout in zswap_store().

On Mon, Sep 23, 2024 at 06:17:07PM -0700, Kanchana P Sridhar wrote:
> zswap_store() will now store mTHP and PMD-size THP folios by compressing

The hugepage terminology throughout the patches is a bit convoluted.

There is no real distinction in this code between PMD-size THPs and
sub-PMD-sized mTHPs e.g. In particular, I think "mTHP" made sense when
they were added, to distinguish them from conventional THPs. But using
this term going forward just causes confusion, IMO.

We're going through a big effort in the codebase to call all of these
things simply "folios" - which stands for "one or more pages". If you
want to emphasize the "more than one page", the convention is to call
it a "large folio". (If you need to emphasize that it's PMD size -
which doesn't apply to these patches, but just for the record - the
convention is "pmd-mappable folio".)

So what this patch set does is "support large folios in zswap".

> @@ -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.
> + */

This is a changelog, not a code comment ;) Please delete it.

>  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;
>  
>  	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

Use "folio" and "large folio" as appropriate here and throughout.