linux-kernel - Re: [PATCH v2 5/5] mm: zswap: remove same

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Date: Wed, 10 Apr 2024 10:31:04 +0800
From: Chengming Zhou <chengming.zhou@...ux.dev>
To: Yosry Ahmed <yosryahmed@...gle.com>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: Johannes Weiner <hannes@...xchg.org>, Nhat Pham <nphamcs@...il.com>,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 5/5] mm: zswap: remove same_filled module params

On 2024/4/5 13:35, Yosry Ahmed wrote:
> These knobs offer more fine-grained control to userspace than needed and
> directly expose/influence kernel implementation; remove them.
> 
> For disabling same_filled handling, there is no logical reason to refuse
> storing same-filled pages more efficiently and opt for compression.
> Scanning pages for patterns may be an argument, but the page contents
> will be read into the CPU cache anyway during compression. Also,
> removing the same_filled handling code does not move the needle
> significantly in terms of performance anyway [1].
> 
> For disabling non_same_filled handling, it was added when the compressed
> pages in zswap were not being properly charged to memcgs, as workloads
> could escape the accounting with compression [2]. This is no longer the
> case after commit f4840ccfca25 ("zswap: memcg accounting"), and using
> zswap without compression does not make much sense.
> 
> [1]https://lore.kernel.org/lkml/CAJD7tkaySFP2hBQw4pnZHJJwe3bMdjJ1t9VC2VJd=khn1_TXvA@mail.gmail.com/
> [2]https://lore.kernel.org/lkml/19d5cdee-2868-41bd-83d5-6da75d72e940@maciej.szmigiero.name/
> 
> Signed-off-by: Yosry Ahmed <yosryahmed@...gle.com>
> Acked-by: Johannes Weiner <hannes@...xchg.org>

Nice cleanup!

Reviewed-by: Chengming Zhou <chengming.zhou@...ux.dev>

Thanks.

> ---
>  mm/zswap.c | 19 -------------------
>  1 file changed, 19 deletions(-)
> 
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 13869d18c13bd..b738435215218 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -140,19 +140,6 @@ static const struct kernel_param_ops zswap_accept_thr_param_ops = {
>  module_param_cb(accept_threshold_percent, &zswap_accept_thr_param_ops,
>  		&zswap_accept_thr_percent, 0644);
>  
> -/*
> - * Enable/disable handling same-value filled pages (enabled by default).
> - * If disabled every page is considered non-same-value filled.
> - */
> -static bool zswap_same_filled_pages_enabled = true;
> -module_param_named(same_filled_pages_enabled, zswap_same_filled_pages_enabled,
> -		   bool, 0644);
> -
> -/* Enable/disable handling non-same-value filled pages (enabled by default) */
> -static bool zswap_non_same_filled_pages_enabled = true;
> -module_param_named(non_same_filled_pages_enabled, zswap_non_same_filled_pages_enabled,
> -		   bool, 0644);
> -
>  /* Number of zpools in zswap_pool (empirically determined for scalability) */
>  #define ZSWAP_NR_ZPOOLS 32
>  
> @@ -1421,9 +1408,6 @@ static bool zswap_is_folio_same_filled(struct folio *folio, unsigned long *value
>  	unsigned int pos, last_pos = PAGE_SIZE / sizeof(*page) - 1;
>  	bool ret = false;
>  
> -	if (!zswap_same_filled_pages_enabled)
> -		return false;
> -
>  	page = kmap_local_folio(folio, 0);
>  	val = page[0];
>  
> @@ -1512,9 +1496,6 @@ bool zswap_store(struct folio *folio)
>  		goto store_entry;
>  	}
>  
> -	if (!zswap_non_same_filled_pages_enabled)
> -		goto freepage;
> -
>  	/* if entry is successfully added, it keeps the reference */
>  	entry->pool = zswap_pool_current_get();
>  	if (!entry->pool)