linux-kernel - Re: [PATCH 1/2] mm: store zero pages to be swapped out in a bitmap

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Date: Thu, 30 May 2024 12:58:33 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Usama Arif <usamaarif642@...il.com>
Cc: hannes@...xchg.org, yosryahmed@...gle.com, nphamcs@...il.com,
 chengming.zhou@...ux.dev, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 kernel-team@...a.com
Subject: Re: [PATCH 1/2] mm: store zero pages to be swapped out in a bitmap

On Thu, 30 May 2024 11:19:07 +0100 Usama Arif <usamaarif642@...il.com> wrote:

> Approximately 10-20% of pages to be swapped out are zero pages [1].
> Rather than reading/writing these pages to flash resulting
> in increased I/O and flash wear, a bitmap can be used to mark these
> pages as zero at write time, and the pages can be filled at
> read time if the bit corresponding to the page is set.
> With this patch, NVMe writes in Meta server fleet decreased
> by almost 10% with conventional swap setup (zswap disabled).

A little nitlet as you'll be altering the code...

> --- a/mm/page_io.c
> +++ b/mm/page_io.c
> @@ -172,6 +172,77 @@ int generic_swapfile_activate(struct swap_info_struct *sis,
>  	goto out;
>  }
>  
> +static bool is_folio_page_zero_filled(struct folio *folio, int i)
> +{
> +	unsigned long *page;
> +	unsigned int pos;
> +	bool ret = false;
> +
> +	page = kmap_local_folio(folio, i * PAGE_SIZE);

It's rather expected that a local variable called `page' has type
`struct page *'.  Can we use something like `addr' here?

> +	for (pos = 0; pos < PAGE_SIZE / sizeof(*page); pos++) {
> +		if (page[pos] != 0)
> +			goto out;
> +	}
> +	ret = true;
> +out:
> +	kunmap_local(page);
> +	return ret;
> +}