Re: [PATCH 2/4] send callback when swap slot is freed

[Nitin Gupta <ngupta@vflare.org>]

Hi Pekka,

On 09/18/2009 03:03 PM, Pekka Enberg wrote:
>
> On Fri, 2009-09-18 at 08:59 +0100, Hugh Dickins wrote:
>> On Fri, 18 Sep 2009, Pekka Enberg wrote:
>>>
>>> The *hook* looks OK to me but set_swap_free_notify() looks like an ugly
>>> hack. I don't understand why we're setting up the hook lazily in
>>> ramzswap_read() nor do I understand why we need to look up struct
>>> swap_info_struct with a bdev. Surely there's a cleaner way to do all
>>> this? Probably somewhere in sys_swapon()?
>>
>> Sounds like you have something in mind, may well be better,
>> but please show us a patch...  (my mind is elsewhere!)
>
> Hey, you tricked me into a world of pain! Here's a totally untested
> patch that adds a "swapon" funciton to struct block_device_operations
> that the ramzswap driver can implement to setup ->swap_free_notify_fn.
>


It is understood that this swap notify callback is bit of a hack. I think
we will not gain much trying to beautify this hack. However, I agree with
Hugh's suggestion to rename this notify callback related function/variables
to make it explicit that its completely ramzswap related. I will send a path
that affects these renames as reply to patch 0/4.

A solution that is generic while satisfying ramzswap demands may be a bit more
involved. It will be easier to incrementally develop these additions.

Thanks,
Nitin


>
>> From 41827c57196f7eae66701a6ae6565c226c0b7674 Mon Sep 17 00:00:00 2001
> From: Nitin Gupta<ngupta@vflare.org>
> Date: Fri, 18 Sep 2009 04:13:30 +0530
> Subject: [PATCH] mm: swap slot free notifier
>
> Currently, we have "swap discard" mechanism which sends a discard bio request
> when we find a free cluster during scan_swap_map(). This callback can come a
> long time after swap slots are actually freed.
>
> This delay in callback is a great problem when (compressed) RAM [1] is used
> as a swap device. So, this change adds a callback which is called as
> soon as a swap slot becomes free. For above mentioned case of swapping
> over compressed RAM device, this is very useful since we can immediately
> free memory allocated for this swap page.
>
> This callback does not replace swap discard support. It is called with
> swap_lock held, so it is meant to trigger action that finishes quickly.
> However, swap discard is an I/O request and can be used for taking longer
> actions.
>
> It is preferred to use this callback for ramzswap case even if discard
> mechanism could be improved such that it can be called as often as required.
> This is because, allocation of 'bio'(s) is undesirable since ramzswap always
> operates under low memory conditions (its a swap device). Also, batching of
> discard bio requests is not optimal since stale data can accumulate very
> quickly in ramzswap devices, pushing system further into low memory state.
>
> Signed-off-by: Nitin Gupta<ngupta@vflare.org>
> Signed-off-by: Pekka Enberg<penberg@cs.helsinki.fi>
> ---
>   include/linux/blkdev.h |    2 ++
>   include/linux/swap.h   |   16 ++++++++++++++++
>   mm/swapfile.c          |    5 +++++
>   3 files changed, 23 insertions(+), 0 deletions(-)
>
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index e23a86c..688e024 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -27,6 +27,7 @@ struct scsi_ioctl_command;
>   struct request_queue;
>   struct elevator_queue;
>   struct request_pm_state;
> +struct swap_info_struct;
>   struct blk_trace;
>   struct request;
>   struct sg_io_hdr;
> @@ -1239,6 +1240,7 @@ struct block_device_operations {
>   						unsigned long long);
>   	int (*revalidate_disk) (struct gendisk *);
>   	int (*getgeo)(struct block_device *, struct hd_geometry *);
> +	int (*swapon)(struct block_device *, struct swap_info_struct *);
>   	struct module *owner;
>   };
>
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 7c15334..6603009 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -8,6 +8,7 @@
>   #include<linux/memcontrol.h>
>   #include<linux/sched.h>
>   #include<linux/node.h>
> +#include<linux/blkdev.h>
>
>   #include<asm/atomic.h>
>   #include<asm/page.h>
> @@ -20,6 +21,8 @@ struct bio;
>   #define SWAP_FLAG_PRIO_MASK	0x7fff
>   #define SWAP_FLAG_PRIO_SHIFT	0
>
> +typedef void (swap_free_notify_fn) (struct block_device *, unsigned long);
> +
>   static inline int current_is_kswapd(void)
>   {
>   	return current->flags&  PF_KSWAPD;
> @@ -155,6 +158,7 @@ struct swap_info_struct {
>   	unsigned int max;
>   	unsigned int inuse_pages;
>   	unsigned int old_block_size;
> +	swap_free_notify_fn *swap_free_notify_fn;
>   };
>
>   struct swap_list_t {
> @@ -297,6 +301,18 @@ extern int reuse_swap_page(struct page *);
>   extern int try_to_free_swap(struct page *);
>   struct backing_dev_info;
>
> +static inline int
> +blkdev_swapon(struct block_device *bdev, struct swap_info_struct *sis)
> +{
> +	struct gendisk *disk = bdev->bd_disk;
> +	int err = 0;
> +
> +	if (disk->fops->swapon)
> +		err = disk->fops->swapon(bdev, sis);
> +
> +	return err;
> +}
> +
>   /* linux/mm/thrash.c */
>   extern struct mm_struct *swap_token_mm;
>   extern void grab_swap_token(struct mm_struct *);
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 74f1102..9d10f02 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -585,6 +585,8 @@ static int swap_entry_free(struct swap_info_struct *p,
>   			swap_list.next = p - swap_info;
>   		nr_swap_pages++;
>   		p->inuse_pages--;
> +		if (p->swap_free_notify_fn)
> +			p->swap_free_notify_fn(p->bdev, offset);
>   	}
>   	if (!swap_count(count))
>   		mem_cgroup_uncharge_swap(ent);
> @@ -1845,6 +1847,9 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
>   		if (error<  0)
>   			goto bad_swap;
>   		p->bdev = bdev;
> +		error = blkdev_swapon(bdev, p);
> +		if (error<  0)
> +			goto bad_swap;
>   	} else if (S_ISREG(inode->i_mode)) {
>   		p->bdev = inode->i_sb->s_bdev;
>   		mutex_lock(&inode->i_mutex);