linux-kernel - Re: [PATCH 2/3] Add swap slot free callback to block_device

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Date:	Fri, 07 May 2010 19:22:30 +1000
From:	Nigel Cunningham <nigel@...onice.net>
To:	Nitin Gupta <ngupta@...are.org>
CC:	Greg KH <greg@...ah.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Cyp <cyp561@...il.com>, Minchan Kim <minchan.kim@...il.com>,
	Al Viro <viro@...IV.linux.org.uk>,
	Christoph Hellwig <hch@...radead.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	driverdev <devel@...verdev.osuosl.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] Add swap slot free callback to block_device_operations

Hi.

On 07/05/10 17:25, Nitin Gupta wrote:
> This callback is required when RAM based devices are used as swap disks.
> One such device is ramzswap which is used as compressed in-memory swap
> disk.  For such devices, we need a callback as soon as a swap slot is no
> longer used to allow freeing memory allocated for this slot.  Without this
> callback, stale data can quickly accumulate in memory defeating the whole
> purpose of such devices.
>
> Signed-off-by: Nitin Gupta<ngupta@...are.org>
> ---
>   include/linux/blkdev.h |    2 ++
>   mm/swapfile.c          |    4 ++++
>   2 files changed, 6 insertions(+), 0 deletions(-)
>
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index 6690e8b..413284a 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -1287,6 +1287,8 @@ struct block_device_operations {
>   						unsigned long long);
>   	int (*revalidate_disk) (struct gendisk *);
>   	int (*getgeo)(struct block_device *, struct hd_geometry *);
> +	/* this callback is with swap_lock and sometimes page table lock held */
> +	void (*swap_slot_free_notify) (struct block_device *, unsigned long);
>   	struct module *owner;
>   };
>
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index ecb069e..f5ccc47 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -574,6 +574,7 @@ static unsigned char swap_entry_free(struct swap_info_struct *p,
>
>   	/* free if no reference */
>   	if (!usage) {
> +		struct gendisk *disk = p->bdev->bd_disk;
>   		if (offset<  p->lowest_bit)
>   			p->lowest_bit = offset;
>   		if (offset>  p->highest_bit)
> @@ -583,6 +584,9 @@ static unsigned char swap_entry_free(struct swap_info_struct *p,
>   			swap_list.next = p->type;
>   		nr_swap_pages++;
>   		p->inuse_pages--;
> +		if ((p->flags&  SWP_BLKDEV)&&
> +				disk->fops->swap_slot_free_notify)
> +			disk->fops->swap_slot_free_notify(p->bdev, offset);

Is this p->flags & SWP_BLKDEV logic reversed? (Don't you want the 
notifier called for devices that aren't backed by a block device?)

I also wonder whether leaving the p->flags & SWP_BLKDEV part out might 
be a good idea. Other potential notifier users?

Regards,

Nigel
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