linux-kernel - Re: [RESEND v8 1/2] mm: store zero pages to be swapped out in a bitmap

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Message-ID: <70a3d71d-b0f9-48b8-bb00-497cd17da764@wanadoo.fr>
Date: Sat, 24 Aug 2024 08:55:25 +0200
From: Christophe JAILLET <christophe.jaillet@...adoo.fr>
To: Usama Arif <usamaarif642@...il.com>, akpm@...ux-foundation.org
Cc: hannes@...xchg.org, shakeel.butt@...ux.dev, david@...hat.com,
 ying.huang@...el.com, hughd@...gle.com, willy@...radead.org,
 yosryahmed@...gle.com, nphamcs@...il.com, chengming.zhou@...ux.dev,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org, kernel-team@...a.com,
 Andi Kleen <ak@...ux.intel.com>
Subject: Re: [RESEND v8 1/2] mm: store zero pages to be swapped out in a
 bitmap

Le 23/08/2024 à 21:04, Usama Arif a écrit :
> 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).
> 
> [1] https://lore.kernel.org/all/20171018104832epcms5p1b2232e2236258de3d03d1344dde9fce0@epcms5p1/
> 

...

> @@ -3428,6 +3444,17 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
>   		goto bad_swap_unlock_inode;
>   	}
>   
> +	/*
> +	 * Use kvmalloc_array instead of bitmap_zalloc as the allocation order might
> +	 * be above MAX_PAGE_ORDER incase of a large swap file.
> +	 */
> +	zeromap = kvmalloc_array(BITS_TO_LONGS(maxpages), sizeof(long),
> +				    GFP_KERNEL | __GFP_ZERO);

Nitpick: kvcalloc() maybe, to be slightly less verbose?

> +	if (!zeromap) {
> +		error = -ENOMEM;
> +		goto bad_swap_unlock_inode;
> +	}
> +
>   	if (si->bdev && bdev_stable_writes(si->bdev))
>   		si->flags |= SWP_STABLE_WRITES;
>   

...

CJ