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Date: Wed, 12 Aug 2009 14:56:17 +0200
From: Ingo Molnar <mingo@...e.hu>
To: Nitin Gupta <ngupta@...are.org>
Cc: linux-kernel@...r.kernel.org, linux-mm@...r.kernel.org
Subject: Re: [RFC][PATCH] swap: send callback when swap slot is freed
* Nitin Gupta <ngupta@...are.org> wrote:
> (Re-sending since I didn't get any reply last time)
>
> 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.
>
> This patch is simply a quick hack but it works (tested on x86 and
> x64). Can anyone please suggest a cleaner and proper way to do
> this? Any feedback would be really helpful. Thanks.
>
>
> Links:
> [1] http://code.google.com/p/compcache/
>
> Signed-off-by: Nitin Gupta <ngupta@...are.org>
> ---
>
> include/linux/swap.h | 14 ++++++++++++++
> init/Kconfig | 9 +++++++++
> mm/swapfile.c | 20 ++++++++++++++++++++
> 3 files changed, 43 insertions(+), 0 deletions(-)
>
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 7c15334..483d0b9 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 +158,9 @@ struct swap_info_struct {
> unsigned int max;
> unsigned int inuse_pages;
> unsigned int old_block_size;
> +#ifdef CONFIG_SWAP_FREE_NOTIFY
> + swap_free_notify_fn *swap_free_notify_fn;
> +#endif
> };
>
> struct swap_list_t {
> @@ -297,6 +303,14 @@ extern int reuse_swap_page(struct page *);
> extern int try_to_free_swap(struct page *);
> struct backing_dev_info;
>
> +#ifdef CONFIG_SWAP_FREE_NOTIFY
> +extern void set_swap_free_notify(unsigned, swap_free_notify_fn *);
> +#else
> +void set_swap_free_notify(unsigned, swap_free_notify_fn *)
> +{
> +}
> +#endif
> +
> /* linux/mm/thrash.c */
> extern struct mm_struct *swap_token_mm;
> extern void grab_swap_token(struct mm_struct *);
> diff --git a/init/Kconfig b/init/Kconfig
> index 3f7e609..b0112c6 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -176,6 +176,15 @@ config SWAP
> used to provide more virtual memory than the actual RAM present
> in your computer. If unsure say Y.
>
> +config SWAP_FREE_NOTIFY
> + bool "Enable swap free notification"
> + depends on SWAP
> + default y
> + help
> + This causes swap subsystem to send a callback to swap device when
> + a swap block becomes free. This is useful in cases such as when
> + compressed RAM device is used as swap disk.
> +
> config SYSVIPC
> bool "System V IPC"
> ---help---
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 8ffdc0d..25bacfd 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -552,6 +552,22 @@ out:
> return NULL;
> }
>
> +#ifdef CONFIG_SWAP_FREE_NOTIFY
> +/*
> + * Sets callback for event when swap_map[offset] == 0
> + * i.e. page at this swap offset is not longer used.
> + */
> +void set_swap_free_notify(unsigned type, swap_free_notify_fn *notify_fn)
> +{
> + struct swap_info_struct *sis;
> + sis = get_swap_info_struct(type);
> + BUG_ON(!sis);
> + sis->swap_free_notify_fn = notify_fn;
> + return;
> +}
> +EXPORT_SYMBOL(set_swap_free_notify);
> +#endif
> +
> static int swap_entry_free(struct swap_info_struct *p,
> swp_entry_t ent, int cache)
> {
> @@ -583,6 +599,10 @@ static int swap_entry_free(struct swap_info_struct *p,
> swap_list.next = p - swap_info;
> nr_swap_pages++;
> p->inuse_pages--;
> +#ifdef CONFIG_SWAP_FREE_NOTIFY
> + if (p->swap_free_notify_fn)
> + p->swap_free_notify_fn(p->bdev, offset);
> +#endif
> }
> if (!swap_count(count))
> mem_cgroup_uncharge_swap(ent);
Shouldnt this be all be unconditional? Sounds like a reasonable VM
callback. The patch would become a lot smaller as well.
Ingo
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